Prenatal and Newborn Parent Handout week 2

CHAPTER 2 GENETIC AND ENVIRONMENTAL FOUNDATIONS

Save Our Environment

F. N. Mithila, 12 years, Bangladesh

Children and adults enjoy themselves, embedded in the supportive context of an idyllic urban landscape. Chapter 2 considers how heredity and multiple layers of the surrounding environment jointly influence child development.

Reprinted with permission from The International Museum of Children’s Art, Oslo, Norway

WHAT’S AHEAD IN CHAPTER 2

2.1 Genetic Foundations

The Genetic Code • The Sex Cells • Sex Determination • Multiple Offspring • Patterns of Gene–Gene Interaction • Chromosomal Abnormalities

2.2 Reproductive Choices

Genetic Counseling • Prenatal Diagnosis • Adoption

■ Social Issues: health: The Pros and Cons of Reproductive Technologies

2.3 Environmental Contexts for Development

The Family • Socioeconomic Status and Family Functioning • Affluence • Poverty • Beyond the Family: Neighborhoods and Schools • The Cultural Context

■ Social Issues: education: Worldwide Education of Girls: Transforming Current and Future Generations

■ Cultural Influences: Familism Promotes Competence in Hispanic Children and Youths

2.4 Understanding the Relationship Between Heredity and Environment

The Question, “How Much?” • The Question, “How?”

■ Biology and Environment: The Tutsi Genocide and Epigenetic Transmission of Maternal Stress to Children

It’s a girl!” announces the doctor, holding up the squalling newborn baby as her parents gaze with amazement at their miraculous creation.

“A girl! We’ve named her Sarah!” exclaims the proud father to eager relatives waiting for news of their new family member.

As we join these parents in thinking about how this wondrous being came into existence and imagining her future, we are struck by many questions. How did this baby, equipped with everything necessary for life outside the womb, develop from the union of two tiny cells? What ensures that Sarah will, in due time, roll over, reach for objects, walk, talk, make friends, learn, imagine, and create—just like other typical children born before her? Why is she a girl and not a boy, dark-haired rather than blond, calm and patient rather than energetic and distractible? What difference will it make that Sarah is given a name and place in one family, community, nation, and culture rather than another?

To answer these questions, this chapter takes a close look at the foundations of development: heredity and environment. Because nature has prepared us for survival, all humans have features in common. Yet each of us is also unique. Think about several children you know well, and jot down the most obvious physical and behavioral similarities between them and their parents. Did you find that one child shows combined features of both parents, another resembles just one parent, whereas a third is not like either parent? These directly observable characteristics are called phenotypes. They depend in part on the individual’s genotype—the complex blend of genetic information that determines our species and influences all our unique characteristics. Yet phenotypes are also affected by each person’s lifelong history of experiences.

We begin our discussion with a review of basic genetic principles that help explain similarities and differences among children in appearance and behavior. Then we turn to aspects of the environment that play powerful roles in children’s lives. As our discussion proceeds, some findings may surprise you. For example, many people are convinced that when children inherit unfavorable characteristics, little can be done to help them. Others believe that the damage done to children by a harmful environment can easily be corrected. As we will see, neither of these assumptions is accurate. Rather, heredity and environment continuously collaborate, each modifying—for better or for worse—the power of the other to influence the course of development. ■

2.1 GENETIC FOUNDATIONS

2.1a Explain what genes are and how they are transmitted from one generation to the next.

2.1b Describe various patterns of gene–gene interaction.

2.1c Describe major chromosomal abnormalities, and explain how they occur.

Within each of the trillions of cells in the human body (except red blood cells) is a control center, or nucleus, that contains rodlike structures called chromosomes, which store and transmit genetic information. Human chromosomes come in 23 matching pairs; an exception is the XY pair in males, which we will discuss shortly. Each member of a pair corresponds to the other in size, shape, and genetic functions. One chromosome is inherited from the mother and one from the father (see Figure 2.1 on page 52).

2.1.1 The Genetic Code

Chromosomes are made up of a chemical substance called deoxyribonucleic acid, or DNA. As Figure 2.2 on page 52 shows, DNA is a long, double-stranded molecule that looks like a twisted ladder. Each rung of the ladder consists of a specific pair of chemical substances called bases. It is this sequence of base pairs that provides genetic instructions. Although the bases always pair up in the same way across the ladder rungs—A with T and C with G—they can occur in any order along its sides. A gene is a segment of DNA along the length of the chromosome. Genes can be of different lengths—perhaps 100 to several thousand ladder rungs long. An estimated 19,000 to 20,000 protein-coding genes, which directly affect our body’s characteristics, lie along the human chromosomes (Ezkurdia et al., 2014). They send instructions for making a rich assortment of proteins to the cytoplasm, the area surrounding the cell nucleus. Proteins, which trigger chemical reactions throughout the body, are the biological foundation on which our characteristics are built. An additional 18,000 regulator genes modify the instructions given by protein-coding genes, greatly complicating their genetic impact (Pennisi, 2012).

Figure 2.1 A karyotype, or photograph, of human chromosomes. The 46 chromosomes shown on the left were isolated from a human cell, stained, greatly magnified, and arranged in pairs according to decreasing size of the upper “arm” of each chromosome. The twenty-third pair, XY, reveals that the cell donor is a genetic male. In a genetic female, this pair would be XX.

© CNRI/SCIENCE SOURCE

We share some of our DNA with even the simplest organisms, such as bacteria and molds, and most of it with other mammals, especially primates. About 99 percent of chimpanzee and human DNA is identical. And the genetic variation from one human to the next is even less: Individuals around the world are about 99.6 percent genetically identical (Tishkoff & Kidd, 2004; Wong, 2014). But these straightforward comparisons are misleading. Many human DNA segments that appear like those of chimpanzees have undergone duplications and rearrangements with other segments. So in actuality, the species-specific genetic material responsible for the attributes that make us human, from our upright gait to our extraordinary language and cognitive capacities, is extensive (Sudmant et al., 2015). Furthermore, it takes a change in only a single DNA base pair to influence human traits. And such tiny changes generally combine in unique ways across multiple genes, amplifying human variability.

Figure 2.2 DNA’s ladderlike structure. A gene is a segment of DNA along the length of the chromosome, varying from perhaps 100 to several thousand ladder rungs long. The pairings of bases across the rungs of the ladder are very specific: Adenine (A) always appears with thymine (T), and cytosine (C) always appears with guanine (G).

How do humans, with far fewer genes than scientists once thought, manage to develop into such complex beings? The answer lies in the proteins our genes make, which break up and reassemble in staggering variety—about 10 to 20 million altogether. Simpler species have far fewer proteins. Furthermore, the communication system between the cell nucleus and cytoplasm, which fine-tunes gene activity, is more intricate in humans than in simpler organisms. Finally, within the cell, environmental factors modify gene expression. Many such effects are unique to humans and influence brain development (Lussier, Islam, & Kobor, 2018). So even at this microscopic level, biological events of profound developmental significance are the result of both genetic and nongenetic forces.

2.1.2 The Sex Cells

New individuals are created when two special cells called gametes, or sex cells—the sperm and ovum—combine. A gamete contains only 23 chromosomes, half as many as a regular body cell. Gametes are formed through a cell division process called meiosis, which halves the number of chromosomes normally present in body cells. When sperm and ovum unite at conception, the resulting cell, called a zygote, will again have 46 chromosomes. Meiosis ensures that a constant quantity of genetic material is transmitted from one generation to the next.

In meiosis, the chromosomes pair up and exchange segments, so that genes from one are replaced by genes from another. This shuffling of genes creates new hereditary combinations. Then chance determines which member of each pair will gather with others and end up in the same gamete. These events make the likelihood that nontwin siblings will be genetically identical about 1 in 700 trillion, or virtually nil. The genetic variability produced by meiosis is adaptive: It increases the chances that at least some members of a species will cope with ever-changing environments and will survive.

In the male, the cells from which sperm arise are produced continuously throughout life, so a healthy man can father a child at any age after sexual maturity. The female is born with a bank of ova already present in her ovaries, though recent findings suggest that new ova may arise from ovarian stem cells later on (Virant-Klun, 2015). Still, there are plenty of female sex cells. About 1 to 2 million are present at birth, 40,000 remain at adolescence, and approximately 350 to 450 female sex cells will mature during a woman’s childbearing years (Moore, Persaud, & Torchia, 2016).

2.1.3 Sex Determination

Return to Figure 2.1 and note that 22 of the 23 pairs of chromosomes are matching pairs, called autosomes (meaning not sex chromosomes). The twenty-third pair consists of sex chromosomes. In females, this pair is called XX; in males, it is called XY. The X is a relatively long chromosome, whereas the Y is short and carries little genetic material. When gametes form in males, the X and Y chromosomes separate into different sperm cells. The gametes that form in females all carry an X chromosome. Therefore, the genetic sex of the new organism is determined by whether an X-bearing or a Y-bearing sperm fertilizes the ovum. In fact, scientists have isolated a gene on the Y chromosome that initiates the formation of male sex organs during the prenatal period (Sekido & Lovell-Badge, 2009). Additional genes, some yet to be identified, are involved in the development of sexual characteristics.

Biologists caution that human sexual diversity is much wider than a simple male–female dichotomy. As a result of variations in genes or chance events in development, some individuals’ sex chromosomes do not match their sexual anatomy. An estimated 1 in every 100 people are affected, usually mildly but occasionally substantially (Ainsworth, 2015). The existence of people with intersex traits, many of whom go through life unaware of their condition unless they seek treatment for infertility or another medical issue, is redefining sex as a spectrum.

2.1.4 Multiple Offspring

Ruth and Peter, a couple I know well, tried for several years to have a child, without success. Eventually, Ruth’s doctor prescribed a fertility drug, and twins—Jeannie and Jason—were born. Jeannie and Jason are fraternal, or dizygotic, twins, the most common type of multiple offspring, resulting from the release and fertilization of two ova. Genetically, they are no more alike than ordinary siblings. Table 2.1 on page 254 summarizes genetic and environmental factors that increase the chances of giving birth to fraternal twins. Older maternal age, fertility drugs, and in vitro fertilization are major causes of the dramatic rise in fraternal twinning and other multiple births in industrialized nations over the past several decades. Currently, fraternal twins account for 1 in about every 33 births in the United States (Martin et al., 2017).

Table 2.1 Maternal Factors Linked to Fraternal Twinning

Factor

Description

Heredity

Occurs more often among women whose families contain fraternal twins, suggesting a genetic influence. Two recently identified genes, one that augments hormone levels and another that may heighten the ovaries’ responsiveness to hormones, increase the chances of fraternal twinning.

Geographic region

Occurs in 6 per 1,000 births in Asia and Latin America, 9 to 12 per 1,000 births in White Europeans, and 40 per 1,000 births among Black Africansa

Age

Rises with maternal age, peaking between 35 and 39 years, and then rapidly falls

Body build

Occurs more often among women who are tall and overweight or of normal weight as opposed to slight body build

Number of births

Is more likely with each additional birth

Fertility drugs and in vitro fertilization

Is more likely with fertility hormones and in vitro fertilization (see page 61), which also increase the chances of bearing higher-order multiples

a Worldwide rates, not including multiple births resulting from use of fertility drugs.

Sources: Hoekstra et al., 2008, 2010; Kulkarni et al., 2013; Smits & Monden, 2011; Mbarek et al., 2016.

Twins can also be created when a zygote that has started to duplicate separates into two clusters of cells that develop into two individuals. These are called identical, or monozygotic, twins because they have the same genetic makeup. The frequency of identical twins is the same around the world—about 3 to 4 per 1,000 births (Kulkarni et al., 2013). Animal research has uncovered environmental influences that prompt this type of twinning, including temperature changes, variation in oxygen levels, and late fertilization of the ovum (Lashley, 2007). In a minority of cases, identical twinning runs in families, but this occurs so rarely that it is likely due to chance rather than heredity.

During their early years, children of single births often are healthier and develop more rapidly than twins. Jeannie and Jason, like most twins, were born several weeks prematurely and required special care in the hospital. When the twins came home, Ruth and Peter had to divide time between them. Perhaps because neither baby received as much attention as the average single infant, Jeannie and Jason walked and talked several months later than most children their age, though like most twins they caught up in development by middle childhood (Lytton & Gallagher, 2002; Nan et al., 2013; Raz et al., 2016). Parental energies are further strained after the birth of triplets, whose early development is slower than that of twins (Feldman, Eidelman, & Rotenberg, 2004).

These identical, or monozygotic, twins were created when a duplicating zygote separated into two clusters of cells, which developed into two individuals with the same genetic makeup.

© RAY EVANS/ALAMY STOCK PHOTO

2.1.5 Patterns of Gene–Gene Interaction

Jeannie has her parents’ dark, straight hair; Jason is curly-haired and blond. The way genes from each parent interact helps explain these outcomes. Recall that except for the XY pair in males, all chromosomes come in matching pairs. Two forms of each gene occur at the same place on the chromosomes, one inherited from the mother and one from the father. Each form of a gene is called an allele. If the alleles from both parents are alike, the child is homozygous and will display the inherited trait. If the alleles differ, then the child is heterozygous, and relationships between the alleles influence the phenotype.

Dominant–Recessive Pattern

In many heterozygous pairings, dominant–recessive inheritance occurs: Only one allele affects the child’s characteristics. It is called dominant; the second allele, which has no effect, is called recessive. Hair color is an example. The allele for dark hair is dominant (we can represent it with a capital D), whereas the one for blond hair is recessive (symbolized by a lowercase b). Both a child who inherits a homozygous pair of dominant alleles (DD) and a child who inherits a heterozygous pair (Db) will be dark-haired, even though their genotypes differ. Blond hair (like Jason’s) can result only from having two recessive alleles (bb). Still, heterozygous individuals with just one recessive allele (Db) can pass that trait to their children. Therefore, they are called carriers of the trait.

Most recessive alleles—like those for blond hair, pattern baldness, or nearsightedness—are of little developmental importance. But some cause serious disabilities and diseases. One well-known recessive disorder is phenylketonuria, or PKU, which affects the way the body breaks down proteins contained in many foods. Infants born with two recessive alleles lack an enzyme that converts one of the basic amino acids that make up proteins (phenylalanine) into a byproduct essential for body functioning (tyrosine). Without this enzyme, phenylalanine quickly builds to toxic levels that damage the central nervous system, causing permanent intellectual disability.

Despite its potentially damaging effects, PKU illustrates that inheriting unfavorable genes does not always lead to an untreatable condition. All U.S. states require that each newborn be given a blood test for PKU. If the disease is found, doctors place the baby on a diet low in phenylalanine. Children who receive this treatment nevertheless show mild deficits in control of attention, memory, planning, decision making, and problem solving, because even small amounts of phenylalanine interfere with brain functioning (Fonnesbeck et al., 2013; Jahja et al. 2014). But as long as dietary treatment begins early and continues, children with PKU usually attain an average level of intelligence and have a normal lifespan.

In dominant–recessive inheritance, if we know the genetic makeup of the parents, we can predict the percentage of children in a family who are likely to display or carry a trait. Figure 2.3 illustrates this for PKU. For a child to inherit the condition, each parent must have a recessive allele. But because of the action of regulator genes, children vary in the degree to which phenylalanine accumulates in their tissues and in the extent to which they respond to treatment.

Description

Figure 2.3 Dominant–recessive mode of inheritance, as illustrated by PKU. When both parents are heterozygous carriers of the recessive gene (p), we can predict that 25 percent of their offspring are likely to be normal (NN), 50 percent are likely to be carriers (Np), and 25 percent are likely to inherit the disorder (pp). Notice that the child with PKU, in contrast to his siblings, has light hair. The recessive gene for PKU affects more than one trait. It also leads to fair coloring.

Only rarely are serious diseases due to dominant alleles. Think about why this is so. Children who inherit the dominant allele always develop the disorder. They seldom live long enough to reproduce, so the harmful dominant allele is eliminated from the family’s heredity in a single generation. Some dominant disorders, however, do persist. One is Huntington disease, a condition in which the central nervous system degenerates. Its symptoms usually do not appear until age 35 or later, after the person may have passed the dominant allele to his or her children.

Incomplete-Dominance Pattern

In some heterozygous circumstances, the dominant–recessive relationship does not hold completely. Instead, we see incomplete dominance, a pattern of inheritance in which both alleles are expressed in the phenotype, resulting in a combined trait, or one that is intermediate between the two.

The sickle cell trait, a heterozygous condition present in many Black Africans, provides an example. Sickle cell anemia occurs in full form when a child inherits two recessive alleles. They cause the usually round red blood cells to become sickle (crescent-moon) shaped, especially under low-oxygen conditions. The sickled cells clog the blood vessels and block the flow of blood, causing intense pain, swelling, and tissue damage. Despite medical advances that today allow 85 percent of affected children to survive to adulthood, North Americans with sickle cell anemia have an average life expectancy of only 55 years (Chakravorty & Williams, 2015). Heterozygous individuals are protected from the disease under most circumstances. However, when they experience oxygen deprivation—for example, at high altitudes or after intense physical exercise—the single recessive allele asserts itself, and a temporary, mild form of the illness occurs.

The sickle cell allele is common among Black Africans for a special reason. Carriers of it are more resistant to malaria than are individuals with two alleles for normal red blood cells. In Africa, where malaria is common, these carriers survived and reproduced more frequently than others, leading the gene to be maintained in the Black population. But in regions of the world where the risk of malaria is low, the frequency of the gene is declining. For example, only 8 percent of African Americans are carriers, compared with 20 percent of Black Africans (Centers for Disease Control and Prevention, 2017h).

X-Linked Pattern

Males and females have an equal chance of inheriting recessive disorders carried on the autosomes. When a harmful allele is carried on the X chromosome, however, X-linked inheritance applies, making males more likely to be affected because their sex chromosomes do not match. In females, any recessive allele on one X chromosome has a good chance of being suppressed by a dominant allele on the other X. But the Y chromosome is only about one-third as long and therefore lacks many corresponding alleles to override those on the X.

A well-known example of X-linked inheritance is hemophilia, a disorder in which the blood fails to clot normally. Figure 2.4 shows its greater likelihood of inheritance by male children whose mothers carry the abnormal allele. Another example is fragile X syndrome, the most common inherited cause of intellectual disability. In this disorder, which affects about 1 in 2,000 males and 1 in 6,000 females, an abnormal repetition of a sequence of DNA bases occurs on the X chromosome, damaging a particular gene. In addition to cognitive impairments, the majority of individuals with fragile X syndrome suffer from attention deficits and high anxiety, and about 30 to 35 percent also have symptoms of autism (Wadell, Hagerman, & Hessl, 2013). Because the disorder is X-linked, males are more often affected.

Besides X-linked disorders, many sex differences reveal the male to be at a disadvantage. Rates of miscarriage, infant and childhood deaths, birth defects, learning disabilities, behavior disorders, and intellectual disability all are higher for boys (Boyle et al., 2011; MacDorman & Gregory, 2015). It is possible that these sex differences can be traced to the genetic code. The female, with two X chromosomes, benefits from a greater variety of genes. Nature, however, seems to have adjusted for the male’s disadvantage. Worldwide, about 103 boys are born for every 100 girls, and an even greater number of males are conceived (United Nations, 2017).

In cultures with strong gender-biased attitudes that induce expectant parents to prefer a male child, the male-to-female birth sex ratio is often much larger. In China, for example, the spread of ultrasound technology (which enables prenatal sex determination) and enforcement of a one-child family policy to control population growth—both of which began in the 1980s—led to a dramatic increase in sex-selective abortion. In 2015, China ended its one-child policy, substituting a two-child policy. Nevertheless, many Chinese couples continue to say they desire just one child (Basten & Jiang, 2015; Jiang, Li, & Sanchez-Barricarte, 2016). Today, China’s birth sex ratio is 117 boys for every 100 girls—a gender imbalance with adverse social consequences, such as rising crime rates and male competition for marriage partners.

Figure 2.4 X-linked inheritance. In the example shown here, the allele on the father’s X chromosome is normal. The mother has one normal and one abnormal recessive allele on her X chromosomes. By looking at the possible combinations of the parents’ alleles, we can predict that 50 percent of these parents’ male children are likely to have the disorder and 50 percent of their female children are likely to be carriers of it.

In contrast, in Europe, the Middle East, and North America, the proportion of male births has declined in recent decades. Some researchers attribute this trend to a rise in stressful living conditions, which heighten spontaneous abortions, especially of male fetuses (Catalano et al., 2010). In support of this hypothesis, dips in the male-to-female birth ratio have been documented after armed conflicts, environmental disasters, and financial crises, such as the economic recession of 2007–2009 (Catalano et al., 2009; Grech, 2014).

In sum, social and cultural factors can modify the male-to-female birth sex ratio, in either direction. And they can readily undermine the ratio’s assumed evolutionary role: compensating for males’ greater genetic vulnerability.

Genomic Imprinting

More than 1,000 human characteristics follow the rules of dominant–recessive and incomplete-dominance inheritance (McKusick-Nathans Institute of Genetic Medicine, 2018). For these traits, whichever parent contributes a gene to the new individual, the gene responds similarly. Geneticists, however, have identified some exceptions. In genomic imprinting, alleles are imprinted, or chemically marked, within the ovum or sperm in such a way that one pair member (either the mother’s or the father’s) is silenced, leaving the other to be expressed regardless of its makeup (Perez, Rubinstein, & Dulac, 2016). The imprint may be passed to the next generation or be temporary, erased in the next generation.

A 9-year-old who has fragile X syndrome participates in an art class with typical students. This disorder—the most common inherited cause of intellectual disability—results from an abnormal repetition of a sequence of DNA bases that damages a gene on the X chromosome.

© LAURA DWIGHT PHOTOGRAPHY

The number of genes subjected to genomic imprinting is believed to be small—less than 1 percent. Nevertheless, these genes have a significant impact on brain development and physical health, as disruptions in imprinting reveal. For example, imprinting is involved in several childhood cancers and in Prader-Willi syndrome, a rare disorder with symptoms of intellectual disability, delays in language motor development, small stature, and severe obesity (Zoghbi & Beaudet, 2016). Imprinting also may explain why children are more likely to develop diabetes if their father, rather than their mother, suffers from it, and why people with asthma or hay fever tend to have mothers, not fathers, with the illness (Ishida & Moore, 2013).

Mutation

Although less than 3 percent of pregnancies result in the birth of a baby with a hereditary abnormality, these children account for about 20 percent of infant deaths and contribute substantially to lifelong impaired physical and mental functioning (Martin et al., 2017). How are harmful genes created in the first place? The answer is mutation, a sudden but permanent change in a segment of DNA. A mutation may affect only one or two genes, or it may involve many genes, as in the chromosomal disorders we will discuss shortly. Some mutations occur spontaneously, simply by chance. Others are caused by hazardous environmental agents.

Ionizing (high-energy) radiation is an established cause of mutation. Women who receive repeated doses before conception are more likely to miscarry or give birth to children with hereditary defects. The incidence of genetic abnormalities, such as physical malformations and childhood cancer, is also higher in children whose fathers are exposed to radiation in their occupations. However, infrequent and mild exposure to radiation generally does not cause genetic damage (Adelstein, 2014). Rather, moderate to high doses over an extended time can impair DNA.

The examples just given illustrate germline mutation, which takes place in the cells that give rise to gametes. When the affected individual mates, the defective DNA is passed on to the next generation. In a second type, called somatic mutation, normal body cells mutate, an event that can occur at any time of life. The DNA defect appears in every cell derived from the affected body cell, eventually causing disease (such as cancer) or disability.

It is easy to see how disorders that run in families can result from germline mutation. But somatic mutation may be involved in these disorders as well. Some people harbor a genetic susceptibility that causes certain body cells to mutate easily in the presence of triggering events (Insel, 2014). This helps explain why certain individuals develop serious illnesses (such as cancer) as a result of smoking, exposure to pollutants, or psychological stress, while others do not.

Although virtually all mutations that have been studied are harmful, some spontaneous ones (such as the sickle cell allele in malaria-ridden regions of the world) are necessary and desirable. By increasing genetic variation, they help individuals adapt to unexpected environmental challenges. Scientists, however, seldom go looking for mutations that contribute to favorable traits, such as an exceptional talent or sturdy immune system. They are far more concerned with identifying and eliminating unfavorable genes that threaten health and survival.

Polygenic Inheritance

So far, we have discussed patterns of gene–gene interaction in which people either display a particular trait or do not. These cut-and-dried individual differences are much easier to trace to their genetic origins than are characteristics that vary on a continuum among people, such as height, weight, intelligence, and personality. These traits are due to polygenic inheritance, in which many genes affect the characteristic in question. Polygenic inheritance is complex, and much about it is still unknown. In the final section of this chapter, we will discuss how researchers infer the influence of heredity on human attributes when they do not know the precise patterns of inheritance.

2.1.6 Chromosomal Abnormalities

Besides harmful recessive alleles, abnormalities of the chromosomes are a major cause of serious developmental problems. Most chromosomal defects result from mistakes during meiosis, when the ovum and sperm are formed. A chromosome pair does not separate properly, or part of a chromosome breaks off. Because these errors involve far more DNA than problems due to single genes, they usually produce many physical and mental symptoms.

Down Syndrome

The most common chromosomal disorder, occurring in 1 out of every 700 live births, is Down syndrome. In 95 percent of cases, it results from a failure of the twenty-first pair of chromosomes to separate during meiosis, so the new individual receives three of these chromosomes rather than the normal two. For this reason, Down syndrome is sometimes called trisomy 21. In other, less frequent forms, an extra broken piece of a twenty-first chromosome is attached to another chromosome (called translocation pattern). Or an error occurs during early prenatal cell duplication, causing some but not all body cells to have the defective chromosomal makeup (called mosaic pattern) (U.S. Department of Health and Human Services, 2017). Because the mosaic type involves less genetic material, symptoms may be less extreme.

The consequences of Down syndrome include intellectual disability, memory and speech problems, limited vocabulary, and slow motor development. EEG measures of brain activity reveal substantial disruption in connectivity among brain regions. This indicates that the brains of individuals with Down syndrome function in a less coordinated fashion than the brains of typical individuals (Ahmadlou et al., 2013). The disorder is also associated with distinct physical features—a short, stocky build, a flattened face, a protruding tongue, almond-shaped eyes, and (in 50 percent of cases) an unusual crease running across the palm of the hand. In addition, infants with Down syndrome are often born with eye cataracts, hearing loss, and heart and intestinal defects (U.S. Department of Health and Human Services, 2017).

Because of medical advances, life expectancy of individuals with Down syndrome has increased greatly: Today, it is about 60 years. However, about 70 percent of affected people who live past age 40 show symptoms of Alzheimer’s disease, the most common form of dementia (Hartley et al., 2015). Genes on chromosome 21 are linked to this disorder.

An 8-year-old with Down syndrome, at right, plays with a typically developing classmate. Despite impaired intellectual development, this child benefits from exposure to stimulating environments and from opportunities to interact with peers.

© LAURA DWIGHT PHOTOGRAPHY

Infants with Down syndrome smile less readily, show poor eye-to-eye contact, have weak muscle tone, and explore objects less persistently (Slonims & McConachie, 2006). But when parents encourage them to engage with their surroundings, children with Down syndrome develop more favorably. They also benefit from infant and preschool intervention programs, although emotional, social, and motor skills improve more than intellectual performance (Roizen, 2013). Clearly, environmental factors affect how well children with Down syndrome fare.

As Figure 2.5 shows, the risk of bearing a baby with Down syndrome, as well as other chromosomal abnormalities, rises dramatically with maternal age. Chromosomal analyses of ova from older women reveal errors during meiosis in the pairing up of chromosomes and exchange of segments between the pairs (Herbert et al., 2015). In about 5 percent of cases, the extra genetic material originates with the father (Vranekovic et al., 2012).

Abnormalities of the Sex Chromosomes

Other disorders of the autosomes usually disrupt development so severely that miscarriage occurs. When such babies are born, they rarely survive beyond early childhood. In contrast, sex chromosome disorders often are not recognized until adolescence when, in some deviations, puberty is delayed. The most common problems involve the presence of an extra chromosome (either X or Y) or the absence of one X in females.

Figure 2.5 Risk of Down syndrome and all chromosomal abnormalities by maternal age. Risk rises sharply after age 35. (From Batshaw, M. L., Roizen, N. J., & Pellegrino, L. (2012) Children with Disabilities, Seventh Edition. (p. 433): Reprinted with permission of Paul H. Brookes Publishing Co., Inc. Batshaw, M. L., Roizen, N. J., & Pellegrino, L. (2019) Children with Disabilities, Eighth Edition, available at www.brookespublishing.com.)

Research has discredited a variety of myths about individuals with sex chromosome disorders. For example, males with XYY syndrome are not necessarily more aggressive and antisocial than XY males (Re & Birkhoff, 2015). And most children with sex chromosome disorders do not suffer from intellectual disability but, rather, have specific cognitive challenges. Verbal difficulties—for example, with reading and vocabulary—are common among girls with triple X syndrome and boys with Klinefelter syndrome, both of whom inherit an extra X chromosome. In contrast, girls with Turner syndrome, who are missing an X, have trouble with spatial relationships—for example, drawing pictures, following travel directions, and noticing changes in facial expressions (Otter et al., 2013; Ross et al., 2012; Temple & Shephard, 2012). Brain-imaging evidence confirms that adding to or subtracting from the usual number of X chromosomes alters the development of certain brain structures, yielding particular intellectual deficits (Hong et al., 2014).

Ask Yourself

Connect ■ Referring to ecological systems theory (Chapter 1, pages 25–27), explain why parents of children with genetic disorders often experience increased stress. What factors, within and beyond the family, can help these parents support their children’s development?

Apply ■ Gilbert’s genetic makeup is homozygous for dark hair. Jan’s is homozygous for blond hair. What proportion of their children are likely to be dark-haired? Explain.

Reflect ■ Provide illustrations from our discussion, and from individuals you know with genetic disorders, of environmental influences on development.

2.2 REPRODUCTIVE CHOICES

2.2 Discuss counseling, medical procedures, and reproductive options that can assist prospective parents in having healthy children.

In the past, many couples with genetic disorders in their families chose not to bear a child at all rather than risk the birth of a baby with abnormalities. Today, genetic counseling and prenatal diagnosis help people make informed decisions about conceiving, carrying a pregnancy to term, or adopting a child.

Social Issues: HealthThe Pros and Cons of Reproductive Technologies

Some people decide not to risk pregnancy because of a history of genetic disease. Many others—12 percent of all couples who try to conceive—discover that they are infertile (Centers for Disease Control and Prevention, 2016). And some never-married adults and lesbian and gay couples want to bear children. Today, increasing numbers of individuals are turning to alternative methods of conception—technologies that have become the subject of heated debate.

Donor Insemination and In Vitro Fertilization

Donor insemination—injection of sperm from an anonymous man into a woman—is often used to overcome male reproductive difficulties. It also permits women without a male partner to become pregnant. Donor insemination is 70 percent successful, resulting in about 40,000 deliveries and 52,000 newborn babies in the United States each year (Rossi, 2014).

In vitro fertilization is another commonly used reproductive technology. About 1 percent of all children in developed countries—65,000 babies in the United States—are conceived through this technique annually (Sunderam et al., 2015). A woman is given hormones that stimulate ripening of several ova. These are removed surgically and placed in a dish of nutrients, to which sperm are added. Once an ovum is fertilized and duplicates into several cells, it is injected into the woman’s uterus.

By mixing and matching gametes, pregnancies can be brought about when either or both partners have a reproductive problem. Usually, in vitro fertilization is used to treat women whose fallopian tubes are permanently damaged. But a single sperm can now be injected directly into an ovum, thereby overcoming most male fertility problems. And a “sex sorter” method helps ensure that couples who carry X-linked diseases (which usually affect males) have a daughter.

Nevertheless, the success of assisted reproduction declines steadily with age, from 55 percent in women ages 31 to 35 to 8 percent in women age 43 (Cetinkaya, Siano, & Benadiva, 2013; Gnoth et al., 2011). Furthermore, assisted reproduction is associated with an elevated risk of pregnancy complications, miscarriage, and birth defects, due to the biological effects of in vitro techniques and the older age of many people seeking treatment.

Children conceived through these methods may be genetically unrelated to one or both of their parents. Does lack of genetic ties or secrecy interfere with parent–child relationships? Perhaps because of a strong desire for parenthood, caregiving is actually somewhat warmer for young children conceived through donor insemination or in vitro fertilization. Also, these children and adolescents are as well-adjusted as their naturally conceived counterparts (Punamaki, 2006; Wagenaar et al., 2011). Children whose parents feel comfortable telling them about their gamete-donor origins are particularly advantaged in parent–child relationship quality and psychological well-being. Telling children early, by age 7, appears most beneficial (Ilioi et al., 2017; Rueter et al., 2016). Perhaps older children’s more complex appreciation of the meaning of being genetically unrelated to at least one parent leads them to be less accepting.

Although reproductive technologies have many benefits, serious questions have arisen about their use. In many countries, including the United States, doctors are not required to keep records of donor characteristics, though information about the child’s genetic background might be critical in the case of serious disease (Murphy, 2013). Another concern is that the in vitro “sex sorter” method enables parental sex selection, thereby eroding the moral value that boys and girls are equally precious.

In vitro fertilization poses greater risks than natural conception to infant survival and healthy development. About 26 percent of in vitro procedures result in multiple births. Most are twins, but 3 percent are triplets and higher-order multiples. Consequently, among in vitro babies, the rate of low birth weight is nearly four times as high as in the general population. In response, doctors have reduced the number of fertilized ova injected into a woman’s uterus, typically to no more than two (Kulkarni et al., 2013; Sunderam et al., 2015). Risk of pregnancy complications, miscarriage, and major birth defects also rises, due to the biological effects of in vitro techniques and the older age of many people seeking treatment.

Surrogate Motherhood

An even more controversial form of medically assisted conception is surrogate motherhood. In this procedure, in vitro fertilization may be used to impregnate a woman (called a surrogate) with a couple’s fertilized ovum. Alternatively, sperm from a man whose partner is infertile may be used to inseminate the surrogate, who agrees to turn the baby over to the father. The child is then adopted by his partner. In both cases, the surrogate is paid a fee for her childbearing services.

Most surrogate arrangements proceed smoothly, and the limited evidence available suggests that families usually function well and stay in touch with the surrogate, especially if she is genetically related to the child (Golombok et al., 2011, 2013; Jadva, Casey, & Golombok, 2012). The small number of children who have been studied are generally well-adjusted. Nevertheless, because surrogacy typically involves the wealthy as contractors for infants and the less economically advantaged as surrogates, it may promote exploitation of financially needy women (Frankford, Bennington, & Ryan, 2015).

Reproductive Frontiers

Experts are debating the ethics of other reproductive options. Doctors have used donor ova from younger women in combination with in vitro fertilization to help postmenopausal women become pregnant. Most recipients are in their forties, but some in their fifties and sixties, and a few in their early seventies, have given birth. These cases magnify health risks to mother and baby and bring children into the world whose parents may not live to see them reach adulthood.

Today, customers at donor banks can select ova or sperm on the basis of physical characteristics and even IQ. And scientists are devising ways to alter the DNA of human ova, sperm, and embryos to protect against hereditary disorders—techniques that could be used to engineer other desired characteristics. Many worry that these practices are dangerous steps toward “designer babies”—controlling offspring traits by manipulating genetic makeup.

Although reproductive technologies permit many barren adults to become parents, laws are needed to regulate such practices. In Australia, New Zealand, and Europe, in vitro gamete donors and applicants for the procedure must undergo highly regulated screening. Denmark, France, and Italy prohibit in vitro fertilization for women past menopause (Cutas & Smajdor, 2015; Murphy, 2013). Pressure from those working in the field of assisted reproduction may lead to similar policies in the United States.

The ethical problems of surrogate motherhood are so complex that 18 U.S. states and the District of Columbia sharply restrict or ban the practice. Most European nations, along with Australia and Canada, allow only “altruistic” surrogacy, in which the surrogate has no financial gain. More research on how such children grow up, including later-appearing medical conditions and feelings about their origins, is important for weighing the pros and cons of these techniques.

2.2.1 Genetic Counseling

Genetic counseling is a communication process designed to help couples assess their chances of giving birth to a baby with a hereditary disorder and choose the best course of action in view of risks and family goals. Individuals likely to seek counseling are those who have had difficulties bearing children—for example, repeated miscarriages—or who know that genetic problems exist in their families.

In addition, adults who delay childbearing are often candidates because as maternal age rises beyond age 35, the rates of chromosomal abnormalities increase sharply. Older paternal age elevates risk of DNA mutations as well. After age 40, it is associated with increased incidence of several serious psychological disorders. These include autism (see page 23 in Chapter 1); schizophrenia, characterized by hallucinations, delusions, and irrational behavior; and bipolar disorder, marked by alternating periods of elation and depression (Zitzmann, 2013). But because younger parents have children in far higher numbers than older parents, they still bear the majority of babies with genetic defects. Therefore, some experts argue that parental needs, not age, should determine referral for genetic counseling (Berkowitz, Roberts, & Minkoff, 2006).

If prospective parents have a family history of intellectual disability, psychological disorders, physical defects, or inherited diseases, the genetic counselor interviews them and prepares a pedigree, a picture of the family tree in which affected relatives are identified. The pedigree is used to estimate the likelihood that a child will be affected by a disorder. For many disorders traceable to a single gene, molecular genetic testing using a sample of blood, saliva, or body tissue can reveal whether the parent is a carrier of the harmful allele.

Autism, schizophrenia, and bipolar disorder have each been linked to an array of DNA-sequence deviations (called genetic markers) distributed across multiple chromosomes. New genomewide testing methods, which look for these genetic markers, enable genetic counselors to estimate risk for these conditions and other psychological disorders. But estimates are generally low because the genetic markers are found in only a minority of affected people. Also, the genetic markers are not associated with mental illness every time they appear. Their expression—as we will illustrate at the end of this chapter—may depend on environmental conditions. Recently, geneticists have begun to identify rare repeats and deletions of DNA bases that are more consistently related to mental illness (Vissers, Gilissen, & Veltman, 2016). These discoveries may lead to more accurate prediction of the likelihood of passing a psychological disorder from parent to child.

When all the relevant hereditary information is in, genetic counselors help people consider appropriate options. These include taking a chance and conceiving or choosing from among a variety of reproductive technologies (see the Social Issues: Health box starting on page 60).

2.2.2 Prenatal Diagnosis

Several prenatal diagnostic methods—medical procedures that permit detection of developmental problems before birth—are available to couples at risk of bearing a child with abnormalities who decide to conceive (see Table 2.2). Women of advanced maternal age are prime candidates for amniocentesis or chorionic villus sampling. Ultrasound, commonly used during pregnancy to track fetal growth, permits detection of gross structural abnormalities. When ultrasound suggests problems but diagnosis is uncertain, ultrafast fetal magnetic resonance imaging, in which a scanner magnetically records detailed pictures of fetal structures, can be used for greater accuracy (see Figure 2.6). Except for maternal blood analysis, prenatal diagnostic methods should not be used routinely because of injury risks to the developing organism.

Prenatal diagnosis has led to advances in fetal medicine. For example, by inserting a needle into the uterus, doctors can administer drugs to the fetus. Surgery has been performed to repair such problems as heart, lung, and diaphragm malformations, urinary tract obstructions, and neural defects (Nassr et al., 2018). Fetuses with blood disorders have been given blood transfusions. And those with immune deficiencies have received bone marrow transplants that succeeded in creating a normally functioning immune system (Deprest et al., 2010).

Figure 2.6 Ultrafast MRI of a fetus, showing body structures. Ultrafast MRI is increasingly being used as a supplement to ultrasound because it records detailed pictures of body structures, permitting greater diagnostic accuracy. In this colorized MRI of a 26-week-old fetus, the yellow area highlights a brain abnormality.

© SIMON FRASER/SCIENCE SOURCE

These techniques frequently result in complications, the most common being premature labor and miscarriage (Danzer & Johnson, 2014). Yet parents may be willing to try almost any option, even one with only a slim chance of success. Currently, the medical profession is struggling with how to help parents make informed decisions about fetal surgery.

Table 2.2 Prenatal Diagnostic Methods

Method

Description

Amniocentesis

The most widely used technique. A hollow needle is inserted through the abdominal wall to obtain a sample of fluid in the uterus. Cells are examined for genetic defects. Can be performed by the 14th week after conception; 1 to 2 more weeks are required for test results. Small risk of miscarriage.

Chorionic villus sampling

A procedure that can be used if results are desired or needed very early in pregnancy. A thin tube is inserted into the uterus through the vagina, or a hollow needle is inserted through the abdominal wall. A small plug of tissue is removed from the end of one or more chorionic villi, the hairlike projections on the membrane surrounding the developing organism. Cells are examined for genetic defects. Can be performed at 9 weeks after conception; results are available within 24 hours. Entails a slightly greater risk of miscarriage than amniocentesis and is also associated with a small risk of limb deformities.

Fetoscopy

A small tube with a light source at one end is inserted into the uterus to inspect the fetus for defects of the limbs and face. Also allows a sample of fetal blood to be obtained, permitting diagnosis of such disorders as hemophilia and sickle cell anemia, as well as neural defects (see below). Usually performed between 15 and 18 weeks after conception but can be done as early as 5 weeks. Entails some risk of miscarriage.

Maternal blood analysis

By the second month of pregnancy, some of the developing organism’s cells enter the maternal bloodstream. An elevated level of alpha-fetoprotein may indicate kidney disease, abnormal closure of the esophagus, or neural tube defects, such as anencephaly (absence of most of the brain) and spina bifida (bulging of the spinal cord from the spinal column). Isolated cells can be examined for genetic defects.

Ultrasound

High-frequency sound waves are beamed at the uterus; their reflection is translated into a picture on a video screen that reveals the size, shape, and placement of the fetus. By itself, permits assessment of fetal age, detection of multiple pregnancies, and identification of gross physical defects. Also used to guide amniocentesis, chorionic villus sampling, and fetoscopy. When used five or more times, may increase the chances of low birth weight.

Ultrafast magnetic resonance imaging (MRI)

Sometimes used as a supplement to ultrasound, where brain or other abnormalities are detected and MRI can provide greater diagnostic accuracy. Uses a scanner to magnetically record detailed pictures of fetal structures. The ultrafast technique overcomes image blurring due to fetal movements. No evidence of adverse effects.

Preimplantation genetic diagnosis

After in vitro fertilization and duplication of the zygote into a cluster of cells, one or two cells are removed and examined for genetic defects. Only if that sample is normal is the fertilized ovum implanted in the woman’s uterus.

Sources: Akolekar et al., 2015; Griffin et al., 2017; Jokhi & Whitby, 2011; Kollmann et al., 2013; Moore, Persaud, & Torchia, 2016.

Applying What We Know

Steps Prospective Parents Can Take Before Conception to Increase the Chances of a Healthy Baby

Recommendation

Explanation

Arrange for a physical exam.

A physical exam before conception permits getting up to date on vaccinations and detection of diseases and other medical conditions that might reduce fertility, be difficult to treat during pregnancy, or affect the developing organism.

Consider your genetic makeup.

Find out if anyone in your family has had a child with a genetic disease or disability. If so, seek genetic counseling before conception.

Reduce or eliminate toxins under your control.

Because the developing organism is highly sensitive to damaging environmental agents during the early weeks of pregnancy, couples trying to conceive should avoid drugs, alcohol, cigarette smoke, radiation, pollution, chemical substances in the home and workplace, and exposure to infectious diseases. They should also stay away from ionizing radiation, which poses risks for mutations.

Ensure proper nutrition.

A doctor-recommended vitamin–mineral supplement, begun before conception, helps prevent many prenatal problems. It should include folic acid, which reduces the chances of neural tube defects, prematurity, and low birth weight (see Chapter 3, page 00).

Consult your doctor after 12 months of unsuccessful efforts at conception.

Long periods of infertility may be due to undiagnosed spontaneous abortions, which can be caused by genetic defects in either partner. If a physical exam reveals a healthy reproductive system, seek genetic counseling.

Advances in genetic engineering also offer hope for correcting hereditary defects. As part of the Human Genome Project—an ambitious international research program, extending from 1990 to 2003, that identified the sequence of DNA bases in the human genome—thousands of genes have been identified, including those involved in disorders of the heart, blood, eyes, lungs, digestive and nervous systems, and in many forms of cancer (National Institutes of Health, 2018b). As a result, new treatments are being explored.

One such approach is gene therapy—correcting genetic abnormalities by delivering DNA carrying a functional gene to the cells. Testing of gene therapies for treating severe immune system dysfunction, several forms of cancer, and certain blood disorders has been encouraging (Kaufmann et al., 2013). In a recent breakthrough, researchers successfully replaced an abnormal gene with a normal one in the red blood cells of young children with beta thalassemia, a disease in which low levels of hemoglobin cause life-threatening anemia and widespread organ damage (Thompson et al., 2018). In another approach, called proteomics, that shows special promise for treating heart disease and cancer, scientists modify genespecified proteins involved in particular diseases (Lippolis & De Angelis, 2016).

A 9-year-old with cystic fibrosis undergoes a breathing test to assess lung functioning. This recessive disorder causes the lungs, liver, and pancreas to secrete large amounts of thick mucus, leading to breathing and digestive difficulties. Worsening lung disease causes premature death in early adulthood. Today, researchers are testing gene therapies aimed at regenerating the lining of the lungs.

© ANDREW FRANCIS WALLACE/Toronto Star/GETTY IMAGES

Despite some successes, genetic treatments are still some distance away for most single-gene defects and farther off for diseases involving multiple genes that combine in complex ways with each other and the environment. Applying What We Know above summarizes steps that prospective parents can take before conception to protect the genetic health of their child.

2.2.3 Adoption

Adults who are infertile or likely to pass along a genetic disorder, same-sex couples, and single adults who want a family are turning to adoption in increasing numbers. Couples who have children by birth, too, sometimes choose to expand their families through adoption. Because the availability of healthy babies has diminished (fewer young unwed mothers give up their babies than in the past), Americans, and people in other Western nations, often seek to adopt internationally. But despite a dramatic rise in orphaned, abandoned, and voluntarily surrendered children worldwide, intercountry adoption has declined substantially, due to host-country and U.S. adoption policies. Rising numbers of children are being adopted from U.S. foster care (Jones & Placek, 2017). And more families are accepting children who are past infancy or who have known developmental problems.

Adopted children and adolescents—whether or not born in their adoptive parents’ country—tend to have more learning and emotional difficulties than other children, a difference that increases with the child’s age at time of adoption (Askeland et al., 2017; Diamond et al., 2015; van den Dries et al., 2009). Various explanations exist for adoptees’ more problematic childhoods. The biological mother may have been unable to care for the child because of problems believed to be partly genetic, such as alcoholism or severe depression, and may have passed this tendency to her offspring. Or perhaps she experienced stress, poor diet, or inadequate medical care during pregnancy—factors that can affect the child. Furthermore, children adopted after infancy often have a preadoptive history of conflict-ridden family relationships, lack of parental affection, neglect and abuse, or deprived institutional rearing. Finally, adoptive parents and children, who are genetically unrelated, are less alike in intelligence and personality than are biological relatives—differences that may threaten family harmony.

Adoption is one option for adults who want a family but are infertile or have a family history of genetic disorders. This father and his 15-month-old daughter attend a reunion of families who traveled together to adopt babies from China. As she gets older, parental warmth and openness about her adoption will promote this child’s adjustment.

© Shari Lewis/AP Images

Despite these risks, most adopted children fare well, and those with preexisting problems who experience sensitive parenting usually make rapid progress (Arcus & Chambers, 2008; Juffer & van IJzendoorn, 2012). Overall, international adoptees develop much more favorably than birth siblings or institutionalized agemates who remain in their birth country (Christoffersen, 2012). And children with troubled family histories who are adopted at older ages generally improve in feelings of trust and affection for their adoptive parents as they come to feel loved and supported (Veríssimo & Salvaterra, 2006). As we will see in Chapter 5, however, later-adopted children—especially those with multiple early-life adversities—are more likely than their agemates to have persistent cognitive, emotional, and social problems.

By adolescence, adoptees’ lives are often complicated by unresolved curiosity about their roots. As they try to integrate aspects of their birth family and their adoptive family into their emerging identity, teenagers face a challenging process of defining themselves. When parents have been warm, open, and supportive in their communication about adoption, their children typically forge a positive sense of self and display fewer emotional and behavior problems (Brodzinsky, 2011; LeMare & Audet, 2014). Also, as long as parents took steps to help them learn about their birth heritage in childhood, young people adopted into a different ethnic group or culture generally develop identities that are healthy blends of their birth and rearing backgrounds (Barn, 2013; Thomas & Tessler, 2007). At the same time, intercountry adoptees who express a strong host-culture identity also tend to be well adjusted (Boivin & Hassan, 2015). If parents do not know enough about their child’s birth heritage to transmit it, adoptees may explore it later, in adulthood.

Ask Yourself

Connect ■ Why is genetic counseling called a communication process? Who should seek it, and why?

Apply ■ Imagine that you must counsel a couple considering in vitro fertilization using donor ova to overcome infertility. What medical and ethical risks would you raise?

Reflect ■ Suppose you are a carrier of fragile X syndrome and want to have children. Would you choose pregnancy, adoption, or surrogacy? If you became pregnant, would you opt for prenatal diagnosis? Explain your decisions.

2.3 ENVIRONMENTAL CONTEXTS FOR DEVELOPMENT

2.3 Discuss aspects of children’s multi-layered environment that influence their development and well-being.

Just as complex as genetic inheritance is the surrounding environment—a multi-layered set of influences that combine to help or hinder physical and psychological well-being. Jot down a brief description of events and people that have significantly influenced your development. Do the items on your list resemble those of my students, who mostly mention experiences that involve their families? This emphasis is not surprising, since the family is the first and longest-lasting context for development. Other influences that make most students’ top ten are friends, neighbors, school, and community and religious organizations.

Return to Bronfenbrenner’s ecological systems theory, discussed in Chapter 1. It emphasizes that environments extending beyond the microsystem—the immediate settings just mentioned—also powerfully affect development. Indeed, my students rarely mention one important context. Its impact is so pervasive that we seldom stop to think about it in our daily lives. This is the macrosystem, or broad social climate of society—its values and programs that support and protect children’s development. All families need help in rearing children—through affordable housing and health care, safe neighborhoods, good schools, well-equipped recreational facilities, and high-quality child care and other services that permit them to meet both work and family responsibilities. And some families, because of poverty or special tragedies, need considerably more help than others.

In the following sections, we take up these contexts for development. Because they affect every age and aspect of change, we will return to them in later chapters. For now, our discussion emphasizes that environments, as well as heredity, can enhance or create risks for development.

2.3.1 The Family

In power and breadth of influence, no other microsystem context equals the family. The family creates unique bonds among people. Attachments to parents and siblings are usually lifelong and serve as models for relationships in the wider world. Within the family, children learn the language, skills, and social and moral values of their culture. Furthermore, research conducted in over 20 countries across six continents with tens of thousands of children and adults reveals that warm, affectionate family ties, especially with parents, consistently predict physical and psychological health throughout development (Khaleque & Rohner, 2012). In contrast, parental rejection—coldness, hostility, or indifference—is generally associated with developmental problems.

Contemporary researchers view the family as a network of interdependent relationships (Bronfenbrenner & Morris, 2006; Russell, 2014). Recall from ecological systems theory that family members exert bidirectional influences on one another, the behaviors of each affecting those of others. Indeed, the very term system implies that the responses of family members are related. These system influences operate both directly and indirectly.

Direct Influences

The next time you have a chance to observe family members interacting, watch carefully. You are likely to see that kind, patient communication evokes cooperative, harmonious responses, whereas harshness and impatience engender angry, resistive behavior. Each of these reactions, in turn, forges a new link in the interactive chain. In the first instance, a positive message tends to follow; in the second, a negative or avoidant one is likely.

This family is a network of interdependent relationships, in which each person’s behavior influences that of the others. As parents and children play a game, warm, considerate parental communication encourages children’s cooperation, which promotes further parental warmth and caring.

© LAURA DWIGHT PHOTOGRAPHY

These observations fit with a wealth of research on the family system. Studies of families of diverse ethnicities show that when parents are firm but warm, children tend to comply with their requests. And when children cooperate, their parents are likely to be warm and gentle in the future. In contrast, children whose parents discipline harshly and impatiently are likely to refuse and rebel. And because children’s misbehavior is stressful, parents may increase their use of punishment, leading to more unruliness by the child (Lorber & Egeland, 2011; Shaw, Hyde, & Brennan, 2012). In each case, the behavior of one family member helps sustain a form of interaction in the other that either promotes or undermines children’s psychological well-being.

Indirect Influences

The impact of family relationships on development becomes even more complicated when we consider that interaction between any two members is affected by others present in the setting. Recall from Chapter 1 that Bronfenbrenner called these indirect influences the effect of third parties.

Look and Listen

Observe several parent–young child pairs in a supermarket or department store, where parents are likely to place limits on children’s behavior. How does the quality of parent communication seem to influence the child’s response? How does the child’s response affect the parent’s subsequent interaction?

Third parties can serve to enhance or impede development. For example, when a marital relationship is warm and considerate, parents are more likely to engage in coparenting, or coordination of parenting roles, that is mutually supportive and collaborative. Such parents are warmer, praise and stimulate their children more, and nag and scold them less (Morrill et al., 2010). In contrast, parents whose marriage is tense and hostile often coparent ineptly. They interfere with each other’s child-rearing efforts, are less responsive to children’s needs, and are more likely to criticize, express anger, and punish (Palkovitz, Fagan, & Hull, 2013; Stroud et al., 2015).

Children who are chronically exposed to angry, unresolved parental conflict have serious behavior problems resulting from disrupted emotional security (Cummings & Miller-Graff, 2015). These include both internalizing difficulties, such as feeling anxious and fearful and trying to repair their parents’ relationship, and externalizing difficulties, including anger and aggression (Goeke-Morey, Papp, & Cummings, 2013; Stroud et al., 2015).

Adapting to Change

Think back to the chronosystem in ecological systems theory (see page 27 in Chapter 1). The interplay of forces within the family is dynamic and ever-changing as each member adapts to the development of other members.

For example, as children acquire new skills, parents adjust the way they treat their more competent youngsters. Consider the way a parent relates to a young infant compared to a walking, talking toddler. During the first few months, parents spend much time feeding, bathing, and cuddling the baby. Within a year, things change dramatically. The 1-year-old points, shows, names objects, and explores the household cupboards. In response, parents devote more time to talking, playing games, and disciplining. These new ways of interacting, in turn, encourage the child’s expanding motor, cognitive, and social skills.

Parents’ development affects children as well. The rise in parent–child conflict that often occurs in early adolescence is not solely due to teenagers’ striving for independence. This is a time when most parents have reached middle age and—conscious that their children will soon leave home and establish their own lives—are reconsidering their own commitments (Steinberg & Silk, 2002). While the adolescent presses for greater autonomy, the parent presses for more togetherness. This imbalance promotes friction, which parent and teenager gradually resolve by accommodating to changes in each other.

Historical time period also contributes to a dynamic family system. In recent decades, a declining birth rate, a high divorce rate, expansion of women’s roles, increased acceptance of same-sex relationships, and postponement of parenthood have led to a smaller family size and a greater number of single parents, remarried parents, lesbian and gay parents, employed mothers, and dual-earner families. Clearly, families in industrialized nations have become more diverse than ever before. In later chapters, we will take up these family forms, examining how each affects family relationships and children’s development.

Nevertheless, some general patterns in family functioning do exist. In the United States and other industrialized nations, one important source of these consistencies is socioeconomic status.

2.3.2 Socioeconomic Status and Family Functioning

People in industrialized nations are stratified on the basis of what they do at work and how much they earn for doing it—factors that determine their social position and economic well-being. Researchers assess a family’s standing on this continuum through an index called socioeconomic status (SES), which combines three related, but not completely overlapping, variables: (1) years of education and (2) the prestige of one’s job and the skill it requires, both of which measure social status; and (3) income, which measures economic status. As SES rises and falls, families face changing circumstances that profoundly affect children’s development and well-being.

SES is linked to timing of parenthood and to family size. People who work in skilled and semiskilled manual occupations (for example, construction workers, truck drivers, and custodians) tend to marry and have children earlier as well as give birth to more children than people in professional and technical occupations. The two groups also differ in child-rearing values and expectations. When more than 200,000 parents in 90 nations around the world were asked about personal qualities they desire for their children, lower-SES parents more often emphasized obedience, whereas higher-SES parents placed greater weight on independence (Park & Lau, 2016). In other research, low-SES parents tended to stress external characteristics, such as politeness, neatness, and cleanliness. In contrast, higher-SES parents focused on psychological traits, such as curiosity, happiness, self-esteem, self-direction, and cognitive and social maturity (Duncan & Magnuson, 2003; Hoff, Laursen, & Tardif, 2002).

These differences are reflected in family interaction. Parents higher in SES talk to, read to, and otherwise stimulate their infants and preschoolers more and grant them greater freedom to explore. With older children and adolescents, higher-SES parents use more warmth, explanations, and verbal praise; set higher academic and other developmental goals; and allow their children to make more decisions. Commands (“You do that because I told you to”), criticism, and physical punishment occur more often in low-SES households (Bush & Peterson, 2008; Mandara et al., 2009).

Education contributes substantially to these variations. Higher-SES parents’ interest in providing verbal stimulation, nurturing inner traits, and promoting academic achievement is supported by years of schooling, during which they learned to think about abstract, subjective ideas and, thus, to invest in their children’s cognitive and social development (Mistry et al., 2008). At the same time, greater economic security enables parents to devote more time, energy, and material resources to fostering their children’s psychological characteristics (Duncan, Magnuson, & Votruba-Drzal, 2015). In diverse cultures around the world, as the Social Issues: Education box on page 68 makes clear, education of women in particular fosters patterns of thinking and behaving that greatly improve quality of life, for both parents and children.

Because of limited education and low social status, many low-SES parents feel a sense of powerlessness in their relationships beyond the home. At work, for example, they must obey rules made by others in positions of authority. When they get home, they often expect the same unquestioning obedience from their children (Belsky, Schlomer, & Ellis, 2012; Conger & Donnellan, 2007). High levels of stress sparked by economic insecurity contribute to low-SES parents’ reduced provision of stimulating interaction and activities as well as greater use of coercive discipline.

Social Issues: EducationWorldwide Education of Girls: Transforming Current and Future Generations

In 2012, Malala Yousafzai, a Pakistani teenager, rose to international prominence after surviving an assassination attempt by a Taliban gunman for her persuasive activism favoring girls’ right to education. Three years earlier, at age 11, Malala had begun writing a blog for the BBC, using a pseudonym to protect her safety. The blog reported her experiences under Taliban rule, which had at times banned girls in her province from attending school. After the New York Times released a documentary about Malala’s life and courage, she began giving interviews that were broadcast around the world. In retaliation, the Taliban gunned her down on a school bus.

Malala’s recovery from life-threatening gunshot wounds sparked worldwide support for her cause. Among the outcomes were a 2012 United Nations petition called “I am Malala,” advocating school enrollment for all the world’s children, and a UNESCO fund directed at expanding girls’ access to high-quality, safe learning environments, especially in countries affected by conflict and disaster. These initiatives led to Pakistan’s first compulsory education law, which guarantees free education to all children between ages 5 and 16.

From 1950 to 2010, the percentage of children in developing nations attending school increased from a small minority of boys to a majority of all children in most regions. Recently, however, progress has slowed. Today, 63 million (9 percent) of the world’s children of elementary-school age are not in school, a rate that climbs to 61 million (16 percent) at the middle-school level and 263 million (36 percent) at the high-school level (UNESCO, 2018). Although gender differences have declined, more girls than boys remain out of school, especially in the poorest countries. Two-thirds of the world’s 750 million illiterate adults are women.

In research carried out in four nations—Mexico, Nepal, Venezuela, and Zambia—investigators examined the impact of variations in maternal language and literacy skills on family health, mother–child interaction, and young children’s literacy skills (LeVine et al., 2012). Participating mothers’ average levels of schooling ranged from 5 years in Nepal to 8 years in Zambia, with some having attended for as little as 1 year and most having left by age 13.

Findings in each country, and across rural and urban areas, were the same. Educating girls had a powerful impact on the welfare of children and families. The diverse benefits largely accrued in two ways: (1) through enhanced verbal skills—reading, writing, and oral communication; and (2) through the cognitive abilities that literacy promotes. Together, these capacities enable girls, as they become adults and mothers, to navigate health and educational settings more effectively and to teach their children in ways that foster school success.

Child Health

Maternal education in developing countries is the most important contributor to steady, worldwide gains in children’s health over the past several decades (Denno & Paul, 2017). In the four-countries study, the higher mothers’ school attainment and literacy level, the better their comprehension of radio and TV health messages and the more easily they could explain their children’s illness symptoms to health professionals.

Clearly, education gives women the knowledge and communication skills to benefit from public health information. As a result, it strongly predicts preventive health behaviors, including prenatal visits, child immunizations, and healthy diet. Also, because women with more schooling have more life opportunities, they are more likely to take advantage of family planning services, delay childbearing, and have more widely spaced and fewer children (Günes, 2015). All these practices are linked to improvements in child survival and health.

Pakastani girls attend class on the first anniversary of the near-fatal shooting of Malalah Yousafzai, a teenage activist who advocates forcefully for education for girls. Within weeks of the assassination attempt, a shocked Pakistan enacted its first compulsory education legislation. After recovering, Malalah resumed her activism. In 2014, she was awarded the Nobel Peace Prize and, in 2017, was named a United Nations Messenger of Peace.

© A MAJEED/AFP/GETTY IMAGES

Implications

Educating girls is the most effective means of combating the most profound, global threats to children’s development: poverty, child mortality, disease, gender inequality, and economic and social instability in the world’s poorest countries (Tao, 2018). Even the limited educational doses available to women in the four-countries study were influential. But because of cultural beliefs about gender roles, reluctance to give up a daughter’s work at home, or war and social upheaval, parents may resist sending their daughters to school.

An even greater barrier is that many low-income countries continue to charge parents a fee for each child enrolled in school. Consequently, poverty-stricken parents—if they send any children—tend to send only sons. When governments abolish enrollment fees, provide information about the benefits of education for girls, and create employment possibilities for women, the overwhelming majority of parents—including the very poor—choose to send their daughters to school and are willing to make sacrifices to do so.

As early as the second year of life, higher SES is associated with enhanced cognitive and language development and with reduced incidence of behavior problems. And throughout childhood and adolescence, children from higher-SES families, on average, do better in school (Bradley & Corwyn, 2003; Hoff, 2013; Melby et al., 2008; Noble et al., 2015a). As a result, they usually attain higher levels of education, which greatly enhances their opportunities for a prosperous adult life.

2.3.3 Affluence

Despite their advanced education and great material wealth, affluent parents—those in prestigious and high-paying occupations—too often fail to engage in family interaction and parenting that promote favorable development. In several studies, researchers tracked the adjustment of youths growing up in wealthy suburbs. By seventh grade, many showed serious problems that worsened in high school (Luthar & Barkin, 2012; Racz, McMahon, & Luthar, 2011). Their school grades were poor, and they were more likely than youths in general to engage in alcohol and drug use, to commit delinquent acts, and to report high levels of anxiety and depression.

Why are so many affluent youths troubled? Compared to their better-adjusted counterparts, poorly adjusted affluent young people report less emotional closeness, less supervision, and fewer serious consequences for misbehaviors from their parents, who lead professionally and socially demanding lives. As a group, wealthy parents are nearly as physically and emotionally unavailable to their youngsters as parents coping with serious financial strain. At the same time, these parents often make excessive demands for achievement and are critical when their children perform less than perfectly (Luthar, Barkin, & Crossman, 2013). Adolescents whose parents value their accomplishments more than their character are more likely to have academic and emotional problems.

For both affluent and low-SES youths, a simple routine—eating dinner with parents—is associated with a reduction in adjustment difficulties, even after many other aspects of parenting are controlled (see Figure 2.7) (Luthar & Latendresse, 2005). Interventions that make wealthy parents aware of the high costs of a competitive lifestyle, weak involvement in children’s lives, and unrealistically high expectations are badly needed.

Figure 2.7 Relationship of regularly eating dinner with parents to affluent youths’ adjustment problems. Among several hundred affluent sixth graders, those who rarely ate dinner with their parents, compared with those who often did so, were far more likely to display anxiety and depression, delinquency and substance use, and poor school grades, even after many other aspects of parenting were controlled. In this study, frequent family mealtimes also protected low-SES youths from delinquency and substance use and from classroom learning problems. (Based on Luthar & Latendresse, 2005a.)

2.3.4 Poverty

When families slip into poverty, development is seriously threatened. In a TV documentary on childhood poverty, a PBS filmmaker explored the daily lives of several American children, along with the struggles of their families (Frontline, 2012). Asked what being poor is like, 10-year-old Kaylie replied, “We don’t get three meals a day…. Sometimes we have cereal but no milk and have to eat it dry.” Kaylie said she felt hungry much of the time, adding, “I’m afraid if we can’t pay our bills, me and my brother will starve.”

Kaylie lives with her 12-year-old brother Tyler and their mother, who suffers from depression and panic attacks and cannot work. The children sometimes gather discarded tin cans from around their rural neighborhood and sell them for a small amount. When money to pay rent ran out, the family moved from its small house to an extended-stay motel. Before the move, Kaylie and Tyler tearfully gave up their pet dog to a shelter.

With family belongings piled haphazardly around her in the cramped motel room, Kaylie complained, “I have no friends, no places to play. I pass the time by.” Kaylie and Tyler had few books and indoor games; no outdoor play equipment such as bicycles, bats and balls, and roller skates; and no scheduled leisure pursuits. Asked to imagine her future, Kaylie wasn’t hopeful. “I see my future poor, on the streets, in a box, asking for money from everyone, stealing stuff…. I’d like to explore the world, but I’m never going to be able to do that.”

Today, 12.7 percent of the U.S. population—nearly 41 million Americans—live in poverty. Among those hit hardest are parents under age 25 with young children, one-fourth of whom are poor. Poverty is also magnified among ethnic minorities and women. For example, 18 percent of U.S. children younger than age 18—about 12 million—live in families with incomes below the federal poverty level, the income judged necessary for a minimum living standard (about $25,000 for a family of four). Poverty rates climb to 27 percent for Hispanic children, 31 percent for African-American children, and 34 percent for Native-American children. For single mothers with preschool children, the poverty rate is close to 50 percent (Semega, Fontenot, & Kollar, 2017; U.S. Census Bureau, 2017a).

Figure 2.8 Child poverty in 20 of the world’s economically advanced nations. Among the countries listed, the United States has the highest percentage of children and youths under age 18 living in families with incomes below 60 percent of the national median income. (Based on OECD, 2018b.)

As we will see later, government programs with insufficient resources to meet family needs are responsible for these disheartening statistics. The U.S. poverty rate is higher among children than any other age group. And of all Western nations, the United States has the highest percentage of extremely poor children. Eight percent of U.S. children live in deep poverty (at less than half the poverty threshold, the income level judged necessary for a minimum living standard). In contrast, in most economically advanced nations, child poverty rates have remained well below the U.S. poverty rate for several decades (see Figure 2.8), and extreme poverty is rare (UNICEF, 2017c). The earlier poverty begins, the deeper it is, and the longer it lasts, the more devastating are its effects. Children of poverty are more likely than other children to suffer from lifelong poor physical health, persistent deficits in cognitive development and academic achievement, high school dropout, mental illness, and impulsivity, aggression, and antisocial behavior (Duncan, Magnuson, & Votruba-Drzal, 2015; Yoshikawa, Aber, & Beardslee, 2012).

The constant stressors that accompany poverty gradually weaken the family system. Poor families have many daily hassles: loss of welfare and unemployment payments, basic services—phone, TV, electricity, hot water—being shut off because of inability to pay bills, and limited or uncertain access to food, to name just a few. When daily crises arise, family members become depressed, irritable, and distracted; hostile interactions increase; and children’s development suffers (Conger & Donnellan, 2007; Kohen et al., 2008).

Negative outcomes are especially severe in single-parent families and in families who must live in run-down, overcrowded housing and dangerous neighborhoods—conditions that make everyday existence even more difficult while reducing social supports that help people cope with economic hardship (Leventhal, Dupéré, & Shuey, 2015). On average, poverty rates are higher, neighborhood disorganization greater, and community services scarcer in rural communities—like the one where Kaylie, Tyler, and their mother live—than in urban areas (Hicken et al., 2014; Vernon-Feagans & Cox, 2013). These circumstances heighten risks for disrupted family functioning and physical and psychological adjustment difficulties.

Homelessness poses enormous challenges for maintaining positive family relationships and physical and mental health. This mother and her three young children prepare to move out of the motel room they share with her boyfriend and her father.

© John Raoux/AP Images

A related problem has reduced the life chances of substantial numbers of children: More than 3 percent—nearly 2.5 million—experienced homelessness in the most recently reported year (Bassuk et al., 2014). Most homeless families consist of women with children under age 5. Besides health problems (which affect the majority of homeless people), many homeless children suffer from developmental delays and chronic emotional stress due to harsh, insecure daily lives (Kilmer et al., 2012). Homeless children achieve less less well academically than other poverty-stricken children because of poor school attendance, frequent moves from school to school, and physical and emotional health problems (Cutuli et al., 2010; National Coalition for the Homeless, 2012).

Although gaps in overall health and achievement between poverty-stricken children and their economically better-off peers are substantial, a considerable number of children from financially stressed families are resilient, faring well. A host of interventions have emerged to help children and youths surmount the risks of poverty. Some address family functioning and parenting. In a study of one such program, poverty-stricken families with preschool through adolescent children were randomly assigned to a family-strengthening intervention or to a no-intervention control group. The intervention involved 14 hours of intensive parent training devoted to learning about and practicing effective strategies for coping with stress, solving everyday problems, and engaging in positive family communication and parenting. Compared with controls, participating parents reported an improved capacity to manage stressful situations, lessening of economic strain, warmer parent–child interaction, and fewer depressive symptoms—benefits that translated into a reduction in child internalizing and externalizing difficulties (Wadsworth et al., 2013). These positive outcomes were still evident 18 months after the intervention ended.

Other interventions directly target children’s academic, emotional, and social skills in child-care centers, preschools, and elementary and secondary schools. And more programs recognize that because poverty-stricken children often experience multiple adversities, they benefit most from multifaceted efforts that focus on family, parenting, and children’s needs at once (Kagan, 2013a). We will discuss many such interventions later in this text.

2.3.5 Beyond the Family: Neighborhoods and Schools

As the concepts of mesosystem and exosystem in ecological systems theory make clear, connections between family and community are vital for psychological well-being. From our discussion of poverty, perhaps you can see why: In poverty-stricken areas, community life is often disrupted. Families move often, parks and playgrounds are in disarray, and community centers providing organized leisure-time activities do not exist. In poor urban neighborhoods, family violence, child abuse and neglect, child and youth internalizing and externalizing difficulties, and adult criminal behavior are widespread (Chen, Howard, & Brooks-Gunn, 2011; Dunn, Schaefer-McDaniel, & Ramsay, 2010; Ingoldsby et al., 2012; Lang et al., 2008). And in poor rural communities, family isolation and scarcity of supportive services are especially high (Vernon-Feagans & Cox, 2013). In contrast, strong family ties to the surrounding social context—as indicated by frequent contact with relatives and friends and regular church, synagogue, temple, or mosque attendance—reduce stress and enhance adjustment.

Neighborhoods

Neighborhoods offer resources and social ties that play an important part in children’s development. In an experimental study of neighborhood mobility, low-SES families were randomly assigned vouchers to move out of public housing into neighborhoods varying widely in affluence. Compared with their peers who remained in poverty-stricken areas, children and youths who moved into low-poverty neighborhoods and remained there for at least several years showed better physical and mental health and school achievement. Children exposed to advantaged neighborhoods consistently, from early childhood into adolescence, benefit the most academically (Anderson & Leventhal, 2014; Leventhal & Brooks-Gunn, 2003; Leventhal & Dupéré, 2011). The ability of low-income families to integrate into the social life of their new neighborhoods is also key to favorable outcomes.

Neighborhood resources have a greater impact on economically disadvantaged than on well-to-do young people. Higher-SES families can afford to transport their children to lessons and entertainment and, if necessary, to better-quality schools in distant parts of the community. In low-income neighborhoods, in-school and after-school programs that substitute for lack of other resources by providing art, music, sports, and other enrichment activities are associated with improved academic performance and a reduction in emotional and behavior problems in elementary and middle school (Durlak, Weissberg, & Pachan, 2010; Kataoka & Vandell, 2013; Vandell, Reisner, & Pierce, 2007). Neighborhood organizations, such as religious youth groups and special interest clubs, contribute to favorable development in adolescence, including increased self-confidence, school achievement, and educational aspirations (Barnes et al., 2007).

Look and Listen

Ask several parents to list their school-age children’s regular lessons and other enrichment activities. Then inquire about home and neighborhood factors that either encourage or impede their children’s participation.

Yet in dangerous, disorganized neighborhoods, high-quality activities for children and adolescents are scarce. Even when they are available, crime and social disorder limit young people’s access, and parents overwhelmed by financial and other stressors are unlikely to encourage their children to participate. In an investigation of a large sample of elementary school students diverse in SES and neighborhood residence, those living in the least stimulating homes and the most chaotic neighborhoods were least likely to enroll in after-school and community-center enrichment activities (Dearing et al., 2009). Thus, the neediest children and youths were especially likely to miss out on these development-enhancing experiences.

Just how do family–neighborhood ties reduce parenting stress and promote children’s development? One answer lies in their provision of social support, which leads to the following benefits:

Parental self-worth. A neighbor or relative who listens and tries to relieve a parent’s concern enhances her self-esteem. The parent, in turn, is likely to interact in a more sensitive and involved manner with her children.

Parental access to valuable information and services. A friend who suggests where a parent might find a job, housing, and affordable child care and youth activities helps make the multiple roles of spouse, parent, and provider easier to fulfill.

Child-rearing controls and role models. Friends, relatives, and other community members may encourage and demonstrate effective parenting practices and discourage ineffective practices.

Direct assistance with child rearing. As children and adolescents participate in their parents’ social networks and in neighborhood settings, other adults can influence children through warmth, stimulation, and exposure to a wider array of competent models. In this way, family–neighborhood ties can reduce the impact of ineffective parenting (Silk et al., 2004). Nearby adults can also intervene when they see young people skipping school or behaving antisocially.

The Better Beginnings, Better Futures Project of Ontario, Canada, is a government-sponsored initiative aimed at preventing the dire consequences of neighborhood poverty by strengthening community capacity to create development-enhancing environments for children and families. Using neighborhood elementary schools as its base, Better Beginnings programs provided children ages 4 to 8 years with in-class, before- and after-school, and summer enrichment activities. Program staff also visited each child’s parents regularly, informed them about community resources, and encouraged their involvement in the child’s school and neighborhood life. And a communitywide component focused on improving neighborhood life by offering leadership training and adult education programs and organizing special events and celebrations (Peters, 2005; Peters, Petrunka, & Arnold, 2003).

High-school volunteers take part in a program to clean up and revitalize Detroit’s distressed Brightmoor neighborhood. Participation in such neighborhood organizations contributes to favorable development of economically disadvantaged adolescents.

© Jim West/Alamy Stock Photo

Longitudinal follow-ups of Better Beginnings as participants reached grades 3, 6, 9, and 12 revealed wide-ranging benefits compared with children and families living in impoverished neighborhoods without this set of programs (Peters et al., 2010; Worton et al., 2014). Among these were gains in children’s academic performance and social adjustment, a reduction in adolescent delinquency and drug use, and parent-reported improved family functioning, child-rearing practices, and sense of community connection.

Schools

Unlike the informal worlds of family and neighborhood, the school is a formal institution designed to transmit knowledge and skills needed to become productive members of society. Children and youths in the developed world spend much time in school—a total of about 14,000 hours, on average, by high school graduation. And today, because many children younger than age 5 attend “school-like” child-care centers or preschools, the impact of schooling begins earlier and is even more powerful than these figures suggest.

Schools are complex social systems that affect many aspects of development. Schools vary in their physical environments—space, equipment, and materials available for work and play. They also differ in their educational philosophies—whether teachers regard students as passive learners to be molded by adult instruction; as active, curious beings who determine their own learning; or as collaborative partners assisted by adult experts, who guide their mastery of new skills. The social life of schools varies as well—in the degree to which students cooperate and compete; in the extent to which students of different abilities and SES and ethnic backgrounds learn together; and in whether they are safe, humane settings or riddled with peer harassment and violence. We will discuss these aspects of schooling in later chapters.

As with SES and family functioning, schooling and academic achievement contribute substantially to life chances and well-being. Furthermore, these contextual influences are interrelated: Children from homes in low-income and poverty-stricken neighborhoods are more likely to attend underfunded schools and experience poorer quality education. For these reasons, educational interventions aimed at upgrading the educational experiences and school performance of economically disadvantaged children are best begun in the early years (Crosnoe & Benner, 2015). But intervening at later periods to target specific educational problems is also helpful—for example, by promoting academic self-confidence and motivation in middle childhood and providing high-quality vocational education to non-college-bound youths.

Students whose parents are involved in their education—through participating in school organizations, volunteering at school, attending parent–teacher conferences, and reinforcing school-based learning at home—show better academic achievement. And when followed up in early adulthood, their educational attainment is higher (Benner, Boyle, & Sadler, 2016). Higher-SES parents, whose backgrounds and values are similar to those of teachers, are more likely to sustain regular educational involvement. In contrast, low-SES parents often feel uncomfortable about coming to school and approaching teachers on behalf of their children’s learning, and daily stressors reduce the time and energy they have to do so (Calarco, 2014; Grant & Ray, 2010). Teachers and administrators must take extra steps with low-SES and ethnic minority families to build supportive family–school ties.

2.3.6 The Cultural Context

Our discussion in Chapter 1 emphasized that child development can be fully understood only when viewed in its larger cultural context. In the following sections, we expand on this theme by taking up the role of the macrosystem in development. First, we discuss ways that cultural values and practices affect contexts for development. Then we consider how healthy development depends on laws and government programs that shield children from harm and foster their well-being.

Cultural Values and Practices

Cultures shape family interaction and community settings beyond the home—in short, all aspects of daily life. Many of us remain blind to aspects of our own cultural heritage until we see them in relation to the practices of others.

Consider the question, Who should be responsible for rearing young children? How would you answer it? Here are some common responses from my students: “If parents decide to have a baby, then they should be ready to care for it.” “Most people are not happy about others intruding into family life.” These statements reflect a widely held opinion in the United States—that the care and rearing of children, and paying for that care, are the duty of parents, and only parents. This view has a long history—one in which independence, self-reliance, and the privacy of family life emerged as basic American values (Dodge & Haskins, 2015). It is one reason, among others, that the public has been slow to endorse government-supported benefits for all families, such as high-quality child care and paid employment leave for meeting family needs. It has also contributed to the large number of U.S. children who remain poor, even though their parents are employed (Gruendel & Aber, 2007; UNICEF, 2017c).

Although the culture as a whole may value independence and privacy, not all citizens share the same values. Some belong to subcultures—groups of people with beliefs and customs that differ from those of the larger culture. Many ethnic minority groups in the United States have cooperative family structures, illustrated by the active, involved extended-family bonds common in African-American, Asian, Hispanic, and Native-American subcultures. Within these extended families, grandparents play meaningful roles in guiding younger generations; adults who face employment, marital, or parenting difficulties receive assistance and emotional support; and children are better adjusted, academically and socially (Jones & Lindahl, 2011; Washington, Gleeson, & Rulison, 2013). In Hispanic extended families, grandparents are especially likely to share in child rearing—a collaborative parenting arrangement that is consistent with the Hispanic cultural ideal of familism, which places a particularly high priority on close, harmonious family relationships. As the Cultural Influences box on page 75 indicates, familism is associated with multiple positive developmental outcomes.

Our discussion so far reflects two broad sets of values on which cultures and subcultures are commonly compared: collectivism versus individualism (Triandis & Gelfand, 2012). In cultures that emphasize collectivism, people stress group goals over individual goals and value interdependent qualities, such as interpersonal harmony, obligations and responsibility to others, and collaborative endeavors. In cultures that emphasize individualism, people are largely concerned with their own personal needs and value independence—personal exploration, discovery, achievement, and choice in relationships.

Cultural InfluencesFamilism Promotes Competence in Hispanic Children and Youths

Here are some responses from Dominican, Mexican, and Puerto Rican parents who were asked to describe their most basic values:

You have to help each other out, be there for each other, especially when you’re growing up or when you have families. I mean that togetherness is very important.

Your parents are number one but your grandparents are even greater than number one [in reference to extended family members who play a significant role in a child’s life].

When I was growing up, you had to be crazy to talk back to your parents. No one got away with that kind of falta de respeto [lack of respect] (Calzada, 2010).

Each of these statements expresses the Hispanic core cultural value of familism, which elevates the needs of family above any concerns of the individual. Familism requires family members to establish loyal, cohesive relationships with one another; to be respectful, especially toward elders; and to provide one another with emotional and material support. Familism also holds that the quality of relationships forged with family members is central to each person’s self-esteem and identity. Consequently, frequent contact is the norm in Hispanic extended families (Calzada, Tamis-LeMonda, & Yoshikawa, 2013). As they fulfill the obligations of familism in their everyday lives, many Hispanic adults live in close proximity to or share living arrangements with extended kin.

Parents begin to instill familism in young children by insisting that they interact respectfully with adults through polite greetings, not interrupting, and not challenging what they say (Stein et al., 2014). Mothers strongly committed to familism report greater warmth and closeness with their preschoolers. In one study, the combination of high maternal warmth and valuing of familism predicted better classroom emotional adjustment and peer relations among Mexican-American preschoolers (Gamble & Modry-Mandell, 2008). By emphasizing cohesiveness, support, and respect, familism seems to foster both positive parent–child interaction and preschoolers’ favorable social behavior.

From these early parental teachings, school-age children and adolescents internalize the value of familism, which further supports social competence (Bridges et al., 2012). In an investigation of Mexican-American 9- to 13-year-olds, those whose mothers exposed them to social learning opportunities consistent with familism—such as helping with sibling caregiving or meeting elder family members’ needs—expressed stronger familism beliefs (Calderón-Tena, Knight, & Carlo, 2011). Together, these parenting practices and children’s consequent familism beliefs predicted children’s willingness to help and support peers and others outside the family.

As their adolescent children become more autonomous, Hispanic parents engage in many parenting strategies consistent with familism, including closely monitoring teenagers’ activities, insisting that they follow reasonable rules, and expressing warmth and support (Stein et al., 2015; Updegraff et al., 2012). Although striving for autonomy typically leads to a decline in familism beliefs over the teenage years, young people nevertheless continue to demonstrate many behaviors consistent with familism, including spending considerable time with family members and willingly fulfilling family obligations (Updegraff et al., 2005).

Adolescents’ commitment to familism is linked to diverse aspects of maturity and adjustment. These include greater likelihood of viewing parents as legitimate sources of guidance; higher academic motivation and school grades; greater sense of school belonging; fewer deviant peer associations and risky behaviors; less anxiety and fearfulness; and less anger and aggression (Ayón, Marsiglia, & Bermudez-Parsai, 2010; Sánchez, Colón, & Esparza, 2005; Sánchez et al., 2010; Stein & Polo, 2014; Stein et al., 2015). If family responsibilities become excessive, teenagers can react with stress, depression, and worsening school performance. But overall, familism beliefs seem to make young people more aware of others’ needs and the importance of behaving respectfully and responsibly in contexts beyond the home.

Neighborhood familism—the extent to which Hispanic mothers and fathers in the same neighborhood endorse familism—is a stronger predictor of positive adolescent adjustment than family income and neighborhood economic status (Gonzales et al., 2010). Living in a community in which many adults value familism may provide youths with collective supervision and other supports for favorable development.

Although it is the most common basis for comparing cultures, the collectivism–individualism distinction is controversial because both sets of values exist in most cultures. As societies change, due to immigration and contact with other cultural, political, and economic systems, the values of their people diversify, yielding varying mixtures of collectivism and individualism (Taras et al., 2014). Integration of the two sets of values is positive for children’s development because collectivism fosters access to social support, whereas individualism promotes striving for personal goals (Chen, 2015). Both are vital for psychological well-being.

Nevertheless, consistent cross-national differences in collectivism–individualism remain: The United States is more individualistic than most Western countries, which place greater weight on collectivism. These value priorities affect a nation’s approach to protecting the well-being of children and families.

Public Policies and Child Development

When widespread social problems arise, such as poverty, hunger, and disease, nations attempt to solve them through devising public policies—laws and government programs designed to improve current conditions. In the United States, public policies safeguarding children and youths have lagged behind policies in other developed nations. As Table 2.3 reveals, the United States does not rank well on important key measures of children’s health and well-being.

The problems of children and youths extend beyond the indicators in the table. The U.S. Affordable Care Act, signed into law in 2010, extended government-supported health insurance to all children in low-income families. But expanded coverage for low-income adults, including parents, is not mandatory for the states, leaving millions of low-income parents without an affordable coverage option. Largely because uninsured parents lack knowledge of how to enroll their children, 11 percent of children eligible for the federally supported Children’s Health Insurance Program (CHIP)—more than 5 million—do not receive coverage (Kaiser Family Foundation, 2015, 2017). Furthermore, the United States has been slow to move toward national standards and funding for child care. Affordable care is in short supply, and much of it is mediocre to poor in quality (Burchinal, 2018; Burchinal et al., 2015). In families affected by divorce, weak enforcement of child support payments heightens poverty in mother-headed households. And 8 percent of 16- to 24-year-olds who dropped out of high school have not returned to earn a diploma (U.S. Department of Education, 2017b).

Why have attempts to help children and youths been difficult to realize in the United States? Cultural values of self-reliance and privacy have made government hesitant to become involved in family matters. Furthermore, good social programs are expensive, and they must compete for a fair share of a country’s economic resources. Children can easily remain unrecognized in this process because they cannot vote or speak out to protect their own interests (Ripple & Zigler, 2003). They must rely on the goodwill of others to become an important government priority.

Table 2.3 How Does the United States Compare to Other Nations on Indicators of Children’s Health and Well-Being?

Indicator

U.S. Rankª

Some Countries the United States Trails

Childhood poverty (among 20 economically advanced nations with similar standards of living)

20th

Canada, Iceland, Germany, United Kingdom, Norway, Sweden, Spain

Infant deaths in the first year of life (among 39 industrialized nations considered)

39th

Canada, Greece, Hungary, Ireland, Spain

Teenage birth rate (among 20 industrialized nations considered)

20th

Australia, Canada, Czech Republic, Denmark, Hungary, Iceland, Poland, Slovakia

Public expenditure on elementary education as a percentage of gross domestic productb (among 35 industrialized nations considered)

15th

Belgium, France, Iceland, New Zealand, Portugal, Spain, Sweden

Public expenditure on early childhood education as a percentage of gross domestic productb (among 28 industrialized nations considered)

21st

Austria, Chile, Germany, Italy, France, Sweden, Slovenia

Public expenditure on health as a percentage of total health expenditure, public plus private (among 35 industrialized nations considered)

35th

Austria, Australia, Canada, France, Hungary, Iceland, Switzerland, New Zealand

a 1 = highest, or best, rank.

b Gross domestic product is the value of all goods and services produced by a nation during a specified time period. It provides an overall measure of a nation’s wealth.

Sources: OECD, 2017a, 2017b; Sedgh et al., 2015; UNICEF, 2017c; U.S. Census Bureau, 2017a; World Bank, 2018a.

Looking Toward the Future

Public policies aimed at fostering children’s development can be justified on two grounds. The first is that children are the future—the parents, workers, and citizens of tomorrow. Investing in children yields valuable returns to a nation’s quality of life. Second, child-oriented policies can be defended on humanitarian grounds—children’s basic rights as human beings.

In 1989, the United Nations General Assembly, with the assistance of experts from many child-related fields, drew up the Convention on the Rights of the Child, a legal agreement among nations that commits each cooperating country to work toward guaranteeing environments that foster children’s development, protect them from harm, and enhance their community participation and self-determination. Examples of rights include the highest attainable standard of health; an adequate standard of living; free and compulsory education; a happy, understanding, and loving family life; protection from all forms of abuse and neglect; and freedom of thought, conscience, religion, and expression, subject to appropriate parental guidance and national law.

Public policies fostering development are vital both on humanitarian grounds and as an investment in the future. Upward Bound—a federally funded educational enrichment program—helps prepare high school students from low-income families for successful admission to college. Here, an Upward Bound 16-year-old (right) attending a university-sponsored program on marine conservation assists a researcher during a shark-tagging expedition.

© LYNNE SLADKY/AP IMAGES

The United States played a key role in drawing up the Convention, yet it is the only country in the world whose legislature has not ratified it. American individualism has stood in the way (Ruck et al., 2014; Scherrer, 2012). Opponents maintain that the Convention’s provisions would shift the burden of child rearing from family to state.

Although the worrisome state of many children and families persists, efforts are being made to improve their condition. Throughout this book, we will discuss many successful programs that could be expanded. Also, growing awareness of the gap between what we know and what we do to better children’s lives has led experts in child development to join with concerned citizens as advocates for more effective policies. As a result, influential interest groups devoted to the well-being of children have emerged.

In the United States, one of the most vigorous is the Children’s Defense Fund (CDF), www.childrensdefense.org, a nonprofit organization that engages in public education and partners with other organizations, communities, and elected officials to improve policies for children and adolescents. Another energetic advocacy organization is the National Center for Children in Poverty, www.nccp.org, dedicated to advancing the economic security, health, and welfare of U.S. children in low-income families.

Besides strong advocacy, public policies that enhance development depend on research that documents needs and evaluates programs to spark improvements. Today, more researchers are collaborating with community and government agencies to enhance the relevance of their investigations to public policies aimed at ensuring children’s rights and improving their lives (SRCD Equity and Justice Committee, 2018). Investigators are also doing a better job of disseminating their findings in easily understandable, compelling ways, through reports to government officials, websites aimed at increasing public understanding, and collaborations with the media to ensure accurate and effective reporting. In these ways, researchers are helping to create the sense of immediacy about the condition of children and families that is necessary to spur a society into action.

Ask Yourself

Connect ■ How does poverty affect functioning of the family system, placing all domains of development at risk?

Apply ■ Check your local newspaper or one or two national news websites to see how often articles appear on the condition of children and families. Why is it important for researchers to communicate with the public about children’s needs?

Reflect ■ Do you agree with the widespread American sentiment that government should not become involved in family life? Explain.

2.4 UNDERSTANDING THE RELATIONSHIP BETWEEN HEREDITY AND ENVIRONMENT

2.4 Explain the various ways heredity and environment may combine to influence complex traits.

Throughout this chapter, we have discussed a wide variety of genetic and environmental influences, each of which has the power to alter the course of development. Yet children who are born into the same family, and who therefore share both genes and environments, are often quite different in characteristics. We also know that some individuals are affected more than others by their homes, neighborhoods, and communities. In some cases, a child who is given many advantages nevertheless does poorly, while another, though exposed to unfavorable rearing conditions, does well. How do scientists explain the impact of heredity and environment when they seem to operate in such varied ways?

Behavioral genetics is a field devoted to uncovering the contributions of nature and nurture to this diversity in human traits and abilities. Although scientists are making progress in identifying the multiple variations in DNA sequences associated with such complex attributes as intelligence and personality, so far these genetic markers explain only a small amount of variation in human behavior, and a minority of cases of most psychological disorders (Plomin et al., 2016; Zhao & Castellanos, 2016). For the most part, scientists are still limited to investigating the impact of genes on these characteristics indirectly.

Some believe that it is useful and possible to answer the question of how much each factor contributes to differences among people. A growing consensus, however, regards that question as unanswerable. These investigators believe that heredity and environment are inseparable (Lickliter & Honeycutt, 2015; Moore, 2013). The important question, they maintain, is how nature and nurture work together. Let’s consider each position in turn.

2.4.1 The Question, “How Much?”

To infer the role of heredity in complex human characteristics, researchers use special methods, the most common being the heritability estimate. Let’s look closely at the information this procedure yields, along with its limitations.

Heritability

Heritability estimates measure the extent to which individual differences in complex traits in a specific population are due to genetic factors. We will take a brief look at heritability findings on intelligence and personality here, returning to them in greater detail in later chapters. Heritability estimates are obtained from kinship studies, which compare the characteristics of family members. The most common type of kinship study compares identical twins, who share all their genes, with fraternal twins, who, on average, share only half. If people who are genetically more alike are also more similar in intelligence and personality, then the researcher assumes that heredity plays an important role.

Kinship studies of intelligence provide some of the most controversial findings in the field of developmental science. Some experts claim a strong genetic influence, whereas others believe that heredity is barely involved. Currently, most kinship findings support a moderate role for heredity. When many twin studies are examined, correlations between the scores of identical twins are consistently higher than those of fraternal twins. In a summary of more than 10,000 twin pairs of diverse ages, the correlation for intelligence was.85 for identical twins and.60 for fraternal twins (Plomin & Spinath, 2004; Plomin et al., 2016).

Researchers use a complex statistical procedure to compare these correlations, arriving at a heritability estimate ranging from 0 to 1.00. The typical overall heritability estimate for intelligence is about.50 for child and adolescent twin samples in Western industrialized nations, suggesting that differences in genetic makeup explain half the variation in intelligence. However, heritability increases with age, from approximately.20 in infancy, to.40 in childhood, to.55 in adolescence, to.65 in early adulthood (Plomin & Deary, 2015). As we will see later, one explanation is that, compared to children, adolescents and adults exert greater personal control over their intellectual experiences—for example, how much time they spend reading or solving challenging problems. Adopted children’s intelligence test scores are more strongly related to their biological parents’ scores than to those of their adoptive parents, offering further support for the role of heredity (Petrill & Deater-Deckard, 2004).

Heritability research also reveals that genetic factors are important in personality. For frequently studied traits, such as sociability, anxiety, agreeableness, and activity level, heritability estimates obtained on child, adolescent, and young adult twins are moderate, in the.40s and.50s (van Beijsterveldt et al., 2016; Vukasović & Bratko, 2015). Unlike intelligence, however, heritability of personality does not increase with age (Turkheimer, Pettersson, & Horn, 2014).

Celena Kopinski (left) and Sarah Heath (right) were born in China and adopted during their first year into different American families. Both were unaware they were related until, during Sarah’s first year of college, a classmate called her by the wrong name and sent a photo of her to Celena. Two years later, when the adoptees met, DNA testing confirmed that they are identical twins. About getting to know each other, Celena commented, “It’s kind of like looking in the mirror, except there is no mirror!” They discovered striking similarities, including the same taste in fashion. Clearly, heredity contributes to personality, but generalizing from twin evidence to the population is controversial.

© CBS NEWS

Twin studies of schizophrenia, bipolar disorder, and autism generally yield high heritabilities, above.70. Heritabilities for antisocial behavior and major depression are considerably lower, in the.30s and.40s (Ronald & Hoekstra, 2014; Sullivan, Daly, & O’Donovan, 2012). Again, adoption studies are consistent with these results. Biological relatives of adoptees with schizophrenia, bipolar disorder, or autism are more likely than adoptive relatives to share the same disorder (Plomin, DeFries, & Knopik, 2013).

Limitations of Heritability

The accuracy of heritability estimates depends on the extent to which the twin pairs studied reflect genetic and environmental variation in the population. Within a population in which all people have very similar home, school, and community experiences, individual differences in intelligence and personality are assumed to be largely genetic, and heritability estimates should be close to 1.00. Conversely, the more environments vary, the more likely they are to account for individual differences, yielding lower heritability estimates. In twin studies, most twin pairs are reared together under highly similar conditions. Even when separated twins are available for study, social service agencies have often placed them in advantaged homes that are alike in many ways (Charney, 2017; Richardson & Norgate, 2006). Because the environments of most twin pairs are less diverse than those of the general population, heritability estimates are likely to exaggerate the role of heredity.

Heritability estimates can easily be misapplied. For example, high heritabilities have been used to suggest that ethnic differences in intelligence test scores, such as the poorer performance of African-American children compared to European-American children, have a genetic basis (Jensen, 1969, 2001; Rushton, 2012). Yet heritabilities computed on mostly White twin samples do not explain test score differences between ethnic groups. We have already seen that large SES differences are involved. In Chapter 12, we will discuss research indicating that when African-American children are adopted into economically advantaged homes at an early age, their scores are well above average and substantially higher than those of children growing up in impoverished families.

Consistent with these findings, the heritability of children’s intelligence increases as parental education and income increase—that is, as children grow up in conditions that allow them to make the most of their genetic endowment. In impoverished environments, children are prevented from realizing their potential. Consequently, enhancing these children’s experiences through interventions—such as parent education and high-quality preschool or child care—has a greater impact on development (Bronfenbrenner & Morris, 2006; Phillips & Lowenstein, 2011).

2.4.2 The Question, “How?”

Today, most researchers view development as the result of a dynamic interplay between heredity and environment. How do nature and nurture work together? Several concepts shed light on this question.

Gene–Environment Interaction

The first of these ideas is gene–environment interaction, which means that because of their genetic makeup, individuals differ in their responsiveness to qualities of the environment (Rutter, 2011). Gene–environment interaction can apply to any characteristic; it is illustrated for intelligence in Figure 2.9. Notice that when environments vary from extremely unstimulating to highly enriched, Ben’s intelligence increases steadily, Linda’s rises sharply and then falls off, and Ron’s begins to increase only after the environment becomes modestly stimulating.

Gene–environment interaction highlights two important points. First, it shows that because each of us has a unique genetic makeup, we respond differently to the same environment. Notice in Figure 2.9 how a poor environment results in similarly low scores for all three individuals. But when the environment provides a moderate level of simulations, Linda is by far the best-performing child. In a highly enriched environment, Ben does best, followed by Ron, both of whom now outperform Linda. Second, sometimes different gene–environment combinations can make two people look the same! For example, if Linda is reared in a minimally stimulating environment, her score will be about 100—average for people in general. Ben and Ron can also obtain this score, but to do so, they must grow up in fairly enriched circumstances (Gottlieb, Wahlsten, & Lickliter, 2006).

Figure 2.9 Gene–environment interaction, illustrated for intelligence by three children who differ in responsiveness to quality of the environment. As environments vary from extremely unstimulating to highly enriched, Ben’s intelligence test score increases steadily, Linda’s rises sharply and then falls off, and Ron’s begins to increase only after the environment becomes modestly stimulating.

Recently, researchers have made strides in identifying gene–environment interactions in personality development. In Chapter 7 we will see that young children with certain genes that increase their risk of an emotionally reactive temperament respond especially strongly to variations in parenting quality (Bakermans-Kranenburg & van IJzendoorn, 2015; Halldorsdottir & Binder, 2017). When parenting is supportive, they gain control over their emotions and adjust as well or better than other children. But when parenting is harsh and insensitive, they become increasingly irritable, difficult, and poorly adjusted, more so than children not at genetic risk. Notice how such children are genetically constituted to react to both positive and negative parenting with especially high plasticity.

Gene–Environment Correlation

A major problem in trying to separate heredity and environment is that they are often correlated (Rutter, 2011; Scarr & McCartney, 1983). According to the concept of gene–environment correlation, our genes influence the environments to which we are exposed. The way this happens changes with age.

Passive and Evocative Correlation

At younger ages, two types of gene–environment correlation are common. The first is called passive correlation because the child has no control over it. Early on, parents provide environments influenced by their own heredity. For example, parents who are good athletes emphasize outdoor activities and enroll their children in swimming and gymnastics. Besides being exposed to an “athletic environment,” the children may have inherited their parents’ athletic ability. As a result, they are likely to become good athletes for both genetic and environmental reasons.

The second type of gene–environment correlation is evocative. Children evoke responses that are influenced by their heredity, and these responses strengthen their original behavior pattern. For example, an active friendly baby is likely to receive more social stimulation than a passive, quiet infant. And a cooperative, attentive child probably receives more sensitive, patient interactions from parents than an inattentive, distractible child. In support of this idea, the more genetically alike siblings are, the more their parents treat them alike, in both warmth and negativity. Parents’ treatment of identical twins is highly similar, whereas their treatment of fraternal twins and nontwin biological siblings is only moderately so. And little resemblance exists in parents’ warm and negative interactions with unrelated stepsiblings (Reiss, 2003). Likewise, identical-twin pairs—who resemble each other more in sociability than fraternal twins do—tend to be more alike in the degree of friendliness they evoke from new playmates (DiLalla, Bersted, & John, 2015).

Active Correlation

At older ages, active gene–environment correlation becomes common. As children extend their experiences beyond the immediate family and are given the freedom to make more choices, they actively seek environments that fit with their genetic tendencies. The well-coordinated, muscular child spends more time at after-school sports, while the intellectually curious child is a familiar patron at the local library.

This tendency to actively choose environments that complement our heredity is called niche-picking (Scarr & McCartney, 1983). Infants and young children cannot do much niche-picking because adults select environments for them. In contrast, older children and adolescents are increasingly in charge of their own environments.

A mother imparts her ceramic skills to her daughter, who may have inherited her mother’s artistic ability. When heredity and environment are correlated, the influence of one cannot be separated from the influence of the other.

© Albert Shakirov/Alamy Stock Photo

Niche-picking explains why pairs of identical twins reared apart during childhood and later reunited may find, to their surprise, that they have similar hobbies, food preferences, and vocations—a trend that is especially marked when twins’ environmental opportunities are similar. Niche-picking also helps us understand why identical twins become somewhat more alike, and fraternal twins and adopted siblings less alike, in intelligence with age (Bouchard, 2004). And niche-picking sheds light on why identical twin pairs—far more often than same-sex fraternal twin pairs—report similar stressful life events influenced by personal decisions and actions, such as failing a course or getting in trouble for drug-taking (Bemmels, et al., 2008).

The influence of heredity and environment is not constant but changes over time. With age, genetic factors may become more important in influencing the environments we experience and choose for ourselves.

Environmental Influences on Gene Expression

Notice how, in the concepts just considered, heredity is granted priority. In gene–environment interaction, it affects responsiveness to particular environments. Similarly, gene–environment correlation is viewed as driven by genetics, in that children’s genetic makeup causes them to receive, evoke, or seek experiences that actualize their hereditary tendencies (Rutter, 2011).

A growing number of researchers contend that heredity does not dictate children’s experiences or development in a rigid way. For example, in a large Finnish adoption study, children with a genetic tendency for mental illness (based on having a biological mother diagnosed with schizophrenia) but who were being reared by healthy adoptive parents showed little mental illness. In contrast, schizophrenia and other psychological impairments piled up in adoptees whose biological and adoptive parents were both mentally ill (Tienari, Wahlberg, & Wynne, 2006; Tienari et al., 2003).

Furthermore, parents and other caring adults can uncouple unfavorable gene–environment correlations by providing children with positive experiences that modify the expression of heredity, yielding positive outcomes. In an investigation that tracked the development of 5-year-old identical twins, pair members tended to resemble each other in level of aggression. The more aggression children displayed, the more maternal anger and criticism they received (a gene–environment correlation). Nevertheless, some mothers treated their twins differently. When followed up at age 7, twins who had been targets of more maternal negativity engaged in even more aggressive behavior. In contrast, their better-treated counterparts showed a reduction in disruptive acts (Caspi et al., 2004). Good parenting protected them from a spiraling, antisocial course of development.

Accumulating evidence reveals that the relationship between heredity and environment is not a one-way street, from genes to environment to behavior. Rather, like other system influences considered in this and the previous chapter, it is bidirectional: Genes affect people’s behavior and experiences, but their experiences and behavior also affect gene expression. Stimulation both external to the child (home, neighborhood, school, and society) and internal to the child (activity within the cytoplasm of the cell, hormones released into the bloodstream) modifies gene activity.

This view of the relationship between heredity and environment, depicted in Figure 2.10, is called epigenesis, which means development resulting from ongoing, bidirectional exchanges between heredity and all levels of the environment (Cox, 2013; Gottlieb, 1998, 2007). Biologists are clarifying the precise mechanisms through which environment can alter gene expression without changing the DNA sequence—a field of research called epigenetics. The most highly studied mechanism is methylation—a biochemical process triggered by certain experiences, in which a set of chemical compounds (called a methyl group) lands on top of a gene and changes its impact, reducing or silencing its expression. Methylation levels can be measured, and they help explain why identical twins, though precisely the same in DNA sequencing, sometimes display strikingly different phenotypes with age.

Figure 2.10 Epigenesis. Development takes place through ongoing, bidirectional exchanges between heredity and all levels of the environment. Genes affect behavior and experiences. Experiences and behavior also affect gene expression. (Based on Gottlieb, 2007.)

A case study of a pair of identical-twin adults offers an illustration. Researchers reported that they had been highly similar in personality throughout childhood. But after high school, one twin remained close to home, studied law, married, and had children, whereas the other left home, became a journalist, and traveled to war zones around the world, where she repeatedly encountered life-threatening situations. Assessed again in their forties, compared with the “law twin,” the “war twin” engaged in more risky behaviors, including drinking and gambling (Kaminsky et al., 2007). DNA analyses revealed greater methylation of a gene known to affect impulse control in the “war twin” than in the “law twin”—a difference much larger than is typical for identical-twin pairs.

Environmental modification of gene expression can occur at any age, even prenatally. Recall our discussion of genomic imprinting on page 57. It is an epigenetic process occurring within the ovum or sperm, often involving methylation. And one way harmful prenatal environmental factors we will address in Chapter 3 may compromise development is through gene methylation (Markunas et al., 2014). As the Biology and Environment box on page 83 illustrates, severe maternal stress during pregnancy is linked to long-term impairment in children’s capacity to manage stress, with gene methylation likely contributing to unfavorable outcomes. Furthermore, animal evidence indicates that some methylated genes are passed to offspring at conception, thereby affecting development in subsequent generations (Grossniklaus et al., 2013).

We must keep in mind, however, that epigenetic processes also operate positively: Favorable rearing experiences alter gene expression in ways that enhance development! And some negative epigenetic modifications may be reversible through carefully designed interventions (Boyce & Kobor, 2015; van IJzendoorn, Bakermans-Kranenburg, & Ebstein, 2011). The concept of epigenesis reminds us that the genome is constantly in flux, both reflecting and affecting the individual’s ever-changing environment.

The relationship between heredity and environment is bidirectional. This child’s genetic makeup might predispose her to heart disease or obesity. But favorable experiences—such as participation in sports programs that promote regular, vigorous exercise—might alter gene expression in ways that overcome this genetic susceptibility.

KIKE CALVO / Alamy Stock Photo

Biology and EnvironmentThe Tutsi Genocide and Epigenetic Transmission of Maternal Stress to Children

In 1994, in a genocidal rampage against the Tutsi people of Rwanda, nearly 1 million people perished within a three-month period. The horror was so extreme that in surveys of Rwandans during the years following the genocide, an estimated 40 to 60 percent reported symptoms of post-traumatic stress disorder (PTSD) (Neugebauer et al., 2009; Schaal et al., 2011). In PTSD, flashbacks, nightmares, anxiety, irritability, angry outbursts, and difficulty concentrating lead to intense distress, physical symptoms, and loss of interest in relationships and daily life.

Parents with PTSD often have children with PTSD (Brand et al., 2011; Morris, Gabert-Quillen, & Delahanty, 2012). In both children and adults, PTSD is associated with disruptions in the body’s stress response system, reflected in abnormal blood levels of the stress hormone cortisol. In appropriate concentrations, cortisol assists our brains in managing stress effectively. In individuals with PTSD, cortisol levels are either too high or (more often) too low, contributing to persistently disturbed stress regulation.

Figure 2.11 Methylation status of the GR gene in trauma-exposed and non-trauma-exposed Tutsi mothers and their children. Mothers who had been directly exposed to the Rwandan Tutsi genocide, as well as their children, showed elevated methylation of the GR gene, which is centrally involved in functioning of the body’s stress response system. (Based on Perroud et al., 2014.)

Mounting evidence confirms that exposure to extreme adversity increases methylation of a chromosome-5 gene called GR, which plays a central role in stress-hormone regulation. Might this epigenetic process contribute to parent–to–child transmission of PTSD?

To explore this question, researchers identified 50 Tutsi women who had been pregnant during the genocide (Perroud et al., 2014). Half had been directly exposed to the trauma; the other half had been out of the country at the time. Eighteen years later, the mothers and their adolescent children were assessed for PTSD and depression by trained psychologists. Blood samples enabled genetic testing for methylation of the GR gene and assessment of cortisol levels.

Compared with non-exposed mothers, mothers who witnessed the genocidal carnage had substantially higher PTSD and depression scores, and children of the two groups of mothers differed similarly. Also, as Figure 2.11 reveals, exposed mothers and their children displayed stronger GR methylation. And consistent with methylation’s dampening effect on gene expression, trauma-exposed mothers and their children had much lower cortisol levels than their non-exposed counterparts.

These findings are consistent with other evidence, in both animals and humans, indicating that prenatal exposure to the biological consequences of severe maternal stress can induce epigenetic changes, through methylation, that impair functioning of the body’s stress response system (Daskalakis & Yehuda, 2014; Mueller & Bale, 2008). In the Tutsi mothers and children, the effects of genocidal trauma were long-lasting, evident in serious psychological disorders nearly two decades later.

This Rwandan mother gave birth shortly after the Tutsi genocide. Nine years later, she continues to suffer from PTSD caused by first-hand experience of atrocities, including repeated rape and loss of her mother, brother, and two sisters in the massacre. Her daughter’s PTSD and depression might be the result of prenatal exposure to severe maternal stress, which can trigger epigenetic changes that disrupt the body’s stress response system.

JONATHAN TORGOVNIK/GETTY IMAGES

As the researchers noted, more remains to be discovered about exactly how maternal trauma exposure compromised the Tutsi children’s capacity to manage stress. Epigenetic processes, not just prenatally but also at later ages, may have been largely responsible. Alternatively, poor-quality parenting, resulting from maternal anxiety, irritability, anger, and depression, could have been the major influence. More likely, epigenetic changes, inept parenting, and other unfavorable environmental influences combined to place the Tutsi children at high risk for PTSD and depression. In Chapter 3, we will return to the impact of prenatal stress, including evidence showing that its negative impact can be lessened or prevented through social support.

Epigenetics is still an emerging field, and clarifying its mechanisms may prove to be even more complex than efforts to understand DNA sequence variations. But from what we already know, one lesson is clear: Development is best understood as a series of complex exchanges between genes and the child’s multi-layered, surrounding environment. In this way, epigenesis is consistent with the developmental systems perspective, introduced on page 11 in Chapter 1 (Bjorklund & Ellis, 2014). Although people cannot be changed in any way we might desire, environments can modify genetic influences. The success of any attempt to improve development depends on the characteristics we want to change, the genetic makeup of the individual, and the type and timing of our intervention.

Ask Yourself

Connect ■ Explain how each of the following concepts supports the conclusion that genetic influences on human characteristics are not constant but change over time: somatic mutation (page 57), niche-picking (page 81), and epigenesis (page 82).

Apply ■ Bianca’s parents are accomplished musicians. At age 4, Bianca began taking piano lessons. By age 10, she was accompanying the school choir. At age 14, she asked to attend a special music high school. Explain how gene–environment correlation promoted Bianca’s talent.

Reflect ■ What aspects of your own development—for example, interests, hobbies, college major, or vocational choice—are probably due to niche-picking? Explain.

Summary

2.1 Genetic Foundations (p. 51)

2.1a Explain what genes are and how they are transmitted from one generation to the next.

Each individual’s phenotype, or observable characteristics, is a product of both genotype and environment. Chromosomes, rodlike structures within the cell nucleus, contain our hereditary endowment. Along their length are genes, segments of deoxyribonucleic acid (DNA).

Protein-coding genes directly affect our body’s characteristics. Regulator genes modify protein-coding genes’ instructions. Environmental factors also alter gene expression.

Gametes, or sex cells, result from a cell division process called meiosis, which ensures that each individual receives a unique set of genes from each parent. Once sperm and ovum unite, the resulting zygote will then have a full complement of chromosomes.

Genetic sex is determined by whether sperm containing an X-bearing or a Y-bearing chromosome fertilizes the ovum.

Fraternal, or dizygotic, twins result when two ova are released from the mother’s ovaries and each is fertilized. Identical, or monozygotic, twins develop when a zygote divides in two during the early stages of cell duplication.

2.1b Describe various patterns of gene–gene interaction.

Traits controlled by single genes follow dominant–recessive and incomplete-dominance inheritance. Homozygous individuals have two identical alleles, or forms of a gene. Heterozygous individuals, with one dominant and one recessive allele, are carriers of the recessive trait. In incomplete dominance, both alleles are expressed in the phenotype.

X-linked inheritance applies when recessive disorders are carried on the X chromosome and, therefore, are more likely to affect males.

In genomic imprinting, alleles are chemically marked within the ovum or sperm, silencing one pair member and leaving the other to be expressed, regardless of its makeup.

Harmful genes arise from mutation, which can occur spontaneously or be caused by hazardous environmental agents. Germline mutation occurs in the cells that give rise to gametes; somatic mutation can occur in body cells at any time of life.

Traits that vary on a continuum, such as intelligence and personality, result from polygenic inheritance—the effects of many genes.

2.1c Describe major chromosomal abnormalities, and explain how they occur.

Most chromosomal abnormalities result from errors during meiosis. The most common, Down syndrome, leads to intellectual disability, distinctive physical features, and physical defects. Sex chromosome disorders are milder than defects of the autosomes.

2.2 Reproductive Choices (p. 59)

2.2 Discuss counseling, medical procedures, and reproductive options that can assist prospective parents in having healthy children.

Genetic counseling helps couples consider reproductive options when they are at risk for giving birth to children with genetic abnormalities. Prenatal diagnostic methods allow early detection of developmental problems. Advances in fetal medicine and genetic engineering offer hope for treating hereditary disorders.

Reproductive technologies, such as donor insemination, in vitro fertilization, and surrogate motherhood, enable individuals to conceive children who otherwise would not. However, the technologies raise legal and ethical concerns.

Many adults who cannot conceive or who are likely to transmit a genetic disorder choose adoption. Although adopted children tend to have more learning and emotional problems than children in general, most fare well in the long run. Warm, supportive parenting that includes open communication about adoption contributes to favorable development.

2.3 Environmental Contexts for Development (p. 65)

2.3 Discuss aspects of children’s multi-layered environment that influence their development and well-being.

In the family—the first and most enduring context for development—the behaviors of each member affect those of the others in a dynamic, ever-changing system of direct and indirect influences. Warm, gratifying family ties, which foster effective coparenting, help ensure children’s psychological health.

Socioeconomic status (SES) profoundly affects family functioning and children’s development. Higher-SES parents tend to have smaller families, to value independence, and to engage in warm, verbally stimulating interaction with children. Lower-SES parents tend to value obedience and to use more commands, criticism, and physical punishment.

In affluent families, parental physical and emotional unavailability may impair youths’ adjustment. Poverty and homelessness undermine effective parenting and pose serious threats to children’s development.

Children benefit from supportive ties between the family and community, including stable, socially cohesive neighborhoods that provide constructive leisure and enrichment activities. High-quality schooling and parent involvement in children’s education enhance academic achievement, educational attainment, and life chances.

The values and practices of cultures and subcultures affect all aspects of children’s daily life. Extended families, common among many ethnic minorities, help protect family members from the negative effects of poverty and other stressful life conditions. The Hispanic cultural value of familism, which elevates family needs above individual concerns, is associated with multiple positive developmental outcomes.

Cross-national differences in collectivism–individualism powerfully affect public policies aimed at addressing social problems. Largely because of its strongly individualistic values, the United States lags behind other developed nations in policies safeguarding children and youths.

2.4 Understanding the Relationship Between Heredity and Environment (p. 78)

2.4 Explain the various ways heredity and environment may combine to influence complex traits.

Behavioral genetics examines the contributions of nature and nurture to diversity in human traits and abilities. Heritability estimates, derived from kinship studies, attempt to quantify the influence of genetic factors on such complex traits as intelligence and personality. However, the accuracy of this approach has been challenged.

In gene–environment interaction, heredity influences each individual’s responsiveness to qualities of the environment. In gene–environment correlation, children’s genes affect the environments to which they are exposed, at first passively and evocatively. At older ages, children actively choose environments that complement their heredity, a process called niche-picking.

Epigenesis reminds us that development is best understood as a series of complex, bidirectional exchanges between heredity and all levels of the environment. Epigenetic research is uncovering biochemical processes—such as methylation—through which the environment can modify gene expression.

IMPORTANT TERMS AND CONCEPTS

allele (p. 54)

autosomes (p. 53)

behavioral genetics (p. 78)

carrier (p. 55)

chromosomes (p. 51)

coparenting (p. 66)

deoxyribonucleic acid (DNA) (p. 51)

dominant–recessive inheritance (p. 55)

epigenesis (p. 82)

familism (p. 75)

fraternal, or dizygotic, twins (p. 53)

gametes (p. 53)

gene (p. 52)

gene–environment correlation (p. 80)

gene–environment interaction (p. 80)

genetic counseling (p. 60)

genomic imprinting (p. 57)

genotype (p. 51)

heritability estimate (p. 78)

heterozygous (p. 54)

homozygous (p. 54)

identical, or monozygotic, twins (p. 54)

incomplete dominance (p. 55)

kinship studies (p. 78)

meiosis (p. 53)

methylation (p. 82)

mutation (p. 57)

niche-picking (p. 81)

phenotype (p. 51)

polygenic inheritance (p. 58)

prenatal diagnostic methods (p. 62)

protein-coding genes (p. 52)

public policies (p. 76)

regulator genes (p. 52)

sex chromosomes (p. 53)

socioeconomic status (SES) (p. 67)

subculture (p. 74)

X-linked inheritance (p. 56)

zygote (p. 53)

Descriptions of Images and Figures

Back to Figure

At the center of the cell is the nucleus, where the chromosomes are located. These chromosomes were used to create the karyotype, which shows 23 distinct pairs of chromosomes.

Back to Figure

At the top of the diagram are the carrier father and carrier mother, each labeled Np. Below are 4 children, a normal son labeled NN, a carrier daughter labeled Np, another carrier daughter labeled Np, and a PKU son labeled pp.

Back to Figure

At the top of the diagram are a normal father labeled XY and a carrier mother labeled XX. One X is circled and labeled as follows: this X chromosome has one abnormal recessive allele. Below are 4 children, a normal female labeled XX, a normal male labeled XY, a carrier female labeled XX, with 1 X circled, and an affected male, labeled XY, with the X circled.

Back to Figure

The x-axis is labeled maternal age in years, and the y-axis is labeled incidence per 1,000 births. The data for Down syndrome specifically is as follows. Age 20, 1 in 1,770. Age 35: 1 in 400. Age 40: 1 in 100. Age 45: 1 in 30. Age 50: 1 in 11. The data for all chromosomal abnormalities is as follows. Age 20: 1 in 1,525. Age 35: 1 in 200. Age 40: 1 in 65. Age 45: 1 in 20. Age 50: 1 in 7.

Back to Figure

The data shows that children who do not eat dinner with their parents have a significantly higher risk of developing anxiety and depression, delinquency and substance abuse, and poor school grades than children who do eat dinner with their parents.

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The data is as follows.

• United States: 20

• Lithuania: 19

• Greece: 18.5

• Canada: 16.5

• Portugal: 16

• Slovakia: 15

• Poland: 13

• Latvia: 12

• Estonia: 12

• France: 11.5

• Belgium: 11

• United Kingdom: 11

• Czech Republic: 10.5

• Netherlands: 10.5

• Austria: 9.5

• Sweden: 9

• Norway: 7.5

• South Korea: 7.5

• Slovenia: 7

• Finland: 4

All values are estimated.

Back to Figure

The x-axis is labeled quality of environment with an arrow pointing from left to right, the left labeled extremely unstimulating and the right labeled highly enriched. The y axis is labeled intelligence scores and ranges from 0 to 150. The graph shows 3 curves. The first is labeled Ben and rises linearly from about 50 to 130 from left to right. The second is labeled Linda and it starts at about 50, rises to about 140, and then falls to about 110. The last curve is labeled Ron and it starts at about 60, falls to about 30, and then rises almost linearly to about 125.

Back to Figure

Three rings surround a person. The rings are labeled gene expression, behavior, and environment, which consists of home, neighborhood, school, and community. Arrows labeled bidirectional changes point between the rings. An arrow below the rings points in one direction and is labeled individual development.

Back to Figure

The mean extent of GR methylation, by person, is as follows.

• Trauma-exposed mothers: 6.5

• Children of trauma-exposed mothers: 9.5

• Non-exposed mothers: 5.

• Children of non-exposed mothers: 5.5.

All values are estimated.