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5Physical Development

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Learning Objectives

After completing this module, you should be able to:

ሁ Describe changes in body and brain structure from birth through adolescence. ሁ Detail the process of nerve function and how neurons transmit signals. ሁ Provide behavioral examples that demonstrate how the brain is organized. ሁ Outline major milestones in motor development. ሁ Clarify important issues related to toilet training. ሁ Identify warning signs of various physical disabilities that may first appear in early childhood. ሁ Describe physical changes that take place during puberty, including historical and cultural trends,

and the differential impact on males and females.

Section 5.1General Patterns of Growth

Prologue Among infants and young children, tremendous changes occur in every domain of develop- ment. However, none are more apparent than the physical changes. When new parents talk about their baby’s growth, the first thing that usually comes to mind is height, weight, and motor activity. Imaging devices now allow us to track coinciding changes in brain tissue. We can conclusively differentiate between a male brain and a female brain—even at birth. Though we are far from making predictions about physical development based on brain scans, we can predict some effects of deprivation. For instance, malnutrition can have far-reaching conse- quences, extending into physical, cognitive, and even psychosocial domains.

Quite unlike other animal species, human infants are virtually helpless at birth. Babies can eat only if a nipple is provided; they cannot move objects out of the way or closer; and for the most part they cannot manipulate the physical structure of the environment. Initially they do not even have the muscle strength needed to hold up their heads. It is only with adult assistance that infants can survive and eventually optimize growth. Technology and scien- tific advancement have allowed us to better understand how we transition from completely dependent beings into adolescents who are perfectly capable of walking away from their par- ents. This module focuses on those physical developments.

5.1 General Patterns of Growth Though parents do not often notice, the heads of infants are disproportionately large com- pared to the rest of their bodies. On their way to adult proportions, the torso and limbs grow faster than the head. This pattern of growth is an example of directionality, one of the gen-

eral principles of human growth. In this case, the direction is cephalocaudal, literally meaning “head to tail.” At birth not only is the head more developed physically than the rest of the body, but also vision and hearing precede growth of the limbs. That is, babies begin to focus their eyes on what they hear well before they begin walking or perform coordinated hand movements.

Physical growth also occurs in a proximodistal pattern— from the inside out. In the prenatal environment, this prin- ciple is displayed as the spinal cord develops before fingers and toes. The pattern continues after birth, as infants learn to move their torsos before their extremities. Babies learn to use their arms to maintain balance before they use their hands and fingers to reach for an object.

Another general principle of physical growth is indepen- dence of systems. This principle suggests that different body systems grow and mature independently. As seen in Figure 5.1, the nervous system matures quite rapidly begin- ning in childhood, whereas the pattern of growth of overall stature (body size) is a bit more even. And neither the tim- ing nor the rate of sexual maturation mirrors that of either the nervous system or stature.

David De Lossy/Photodisc/Thinkstock ሁ Physical development

depends on maturation but still involves interchange with the environment.

Section 5.2Neuropsychology and Brain Development

Figure 5.1: Independence of systems ሁ This graph illustrates that different body systems grow and mature independently.

S iz

e i n

t e rm

s o

f p

e rc

e n

ta g

e o

f to

ta l

g ro

w th

Age in years

Birth 2 4 6 8 10 12 14 16 18 20

100

120

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160

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200

Lymph tissue Brain and head

General growth curveGenitals

Source: Tanner, J. M. (1962) Growth At Adolescence, 2nd ed., Oxford: Blackwell Scientific Publications.

S E C T I O N R E V I E W Provide examples that demonstrate the three general patterns of growth.

5.2 Neuropsychology and Brain Development As the cephalocaudal principle implies, the brain is closer to its adult size than is any other physical structure in the newborn human. Embryonic cells have been transformed into a sophisticated machine with all kinds of specialized processes. The brain integrates informa- tion from the environment and from the body’s multiple systems. Children learn to walk, run, and hop, leading to more complex physical feats like executing studied gymnastics moves, diving into a pool, or high jumping. These changes necessarily begin with the brain and ner- vous system. In this section, we explore these developments as they relate to early physical growth. We look first at the brain from a cellular level, and then explore how the different parts of the brain communicate with each other and the rest of the body.

Neurons and Synaptic Development There are at least 100 billion neurons, or nerve cells, in the human brain. The neuron is the basic element of the nervous system, as displayed in Figure 5.2. Unlike other cells, neurons communicate with each other in an elaborate relay system. Information is first transmitted by

Section 5.2Neuropsychology and Brain Development

dendrites, structures that receive incoming signals. The message then travels to the soma (cell body). If the signal is to be continued, it travels via the axon. The transmission may be sped up by a myelin sheath, which eventually covers most of the long, threadlike axons.

The neuron transmits the impulse to the next neuron (or gland or muscle fiber) at bulblike structures called terminal but- tons. This transmission is achieved with- out the neurons actually touching each other. Instead, they form a synapse, or gap between the sending and receiving neurons. Every terminal button contains vesicles that release chemicals called neurotransmit- ters into the synapse. Depending on a num- ber of factors, especially the concentration of the specific neurotransmitter, the receiv- ing neuron will either carry the message forward or not. That is why sometimes peo- ple can perceive a faint sound or a distant light while at other times they cannot. The

chemical messengers have either reached a particular threshold to transmit the sensory mes- sage or not.

Figure 5.2: The neuron ሁ The neuron is the basic element of the nervous system. Information is first received by the

dendrites. The message travels to the cell body (soma). If the message is to be continued, it travels to the axon, where transmission may be sped up by the myelin sheath, which covers many axons. At the terminals, neurotransmitters are released into the synapse between the sending and receiving neurons.

Dendrite

Nucleus

Myelin sheath

Terminals

Axon

Courtesy of Ron Mossler ሁ Every potential visual, auditory, and tactile

stimulus sparks production of synaptic growth.

Section 5.2Neuropsychology and Brain Development

It was previously thought that we do not manufacture neurons after we are born. How- ever, recent research has confirmed that some sensory neurons continue to regenerate throughout the lifespan, and there are even indications of the growth of some neurons related to cognition. For instance, evidence indicates that neural growth can be promoted in the hippocampus, possibly slowing or reversing the effects of memory loss associated with dementia (Frielingsdorf, Simpson, Thala, & Pizzo, 2007; Ho, Hooker, Sahay, Holt, & Roffman, 2013).

Timing Although the infant brain is proportionately closer to adult size than are other body parts, it weighs only about 13 ounces (370 grams), whereas an adult brain weighs a bit more than 3 pounds (1,400 grams). The brain grows faster by weight than any other body part. By the time children are 2 years old, the brain is about 75% of the size and weight of an adult brain. Put another way, it is quite apparent that evolution has provided the brain a “head start,” relative to the rest of the body, in order to direct development.

Though the quantity of neurons remains relatively constant after birth, the number of postna- tal synaptic connections multiplies tremendously. Therefore, the rapid increase in mass is due to the axons and dendrites that grow to form new synapses in response to stimuli. As a new object is seen, a new sound is heard, or a new movement is made, neurons branch and extend their reach to other neurons and form new synapses. By the time a child is 2 years old, some cells may have up to 10,000 connections (Sadava, Hills, Heller, & Berenbaum, 2009). In total, those 100 billion neurons establish trillions of synapses forming a complex yet integrated communication network. When brain development peaks, as many as 250,000 synapses are created every minute.

For every potential stimulus in a person’s environment, there is massive overproduction of synapses during infancy. As new synapses grow, continued stimulation of those connections is key to their survival, maintaining a principle of “use it or lose it.” This physical develop- ment serves as a biological foundation for learning. But as discussed earlier, with regard to sensitive periods and independence of systems, all development does not happen at the same time or at the same rate. The same is true for brain development, as shown in Figure 5.3. Synapses in the visual cortex that are responsible for sight reach peak production between the 4th and 8th postnatal months; synapses in the more sophisticated reasoning centers of the prefrontal cortex do not peak until the 15th month. Notice also that growth in language areas peaks just before infants begin to speak. Therefore, the rate and timing of synapse and dendrite formation are important to understanding development (Tierney & Nelson, 2009; Twardosz, 2012).

Section 5.2Neuropsychology and Brain Development

Figure 5.3: Timing of synapse and dendrite formation ሁ The rate and timing of synapse and dendrite formation vary by age and are important to

understanding development. Notice, for example, that growth in language areas peaks just before infants begin to speak.

Age in yearsAge in months 0 1 2 3-3 -2 -1 4 5 6 7 8 9 10 11 122 3 4 5 6 7 8 9 10 15 1613 1411 12 1

Visual/auditory cortex (seeing and hearing)

Prefrontal cortex (higher cognitive functions)

Angular gyrus/Broca’s area (language areas/speech production)

R e la

ti v e g

ro w

Source: From R. A. Thompson and C. A. Nelson, “Developmental science and the media: Early brain development,” American Psychologist, 56(1): 5–15. Copyright © 2001. Reprinted by permission of the American Psychological Association.

The timing of brain development is important to understanding its processes. When peak development for a particular process occurs at a later age, the brain remains plastic (more adaptable) for a longer time. That is, if a part of the brain is damaged before it has begun its major synaptic growth, other cells can take the place of those that are damaged. This ability to adapt due to experience (whether due to damage or ordinary behavior) is called neuroplasticity.

Pruning To facilitate neuroplasticity, the brain goes through a process of overproduction of synapses (as shown in Figure 5.4) before engaging in a process of reduction. Although synaptic devel- opment unfolds by genetic programming (maturation), experience dictates which synapses receive the most stimulation and are likely to remain. Conversely, synapses that are not stimu- lated to a particular threshold will go through a natural reduction process called synaptic pruning. Neurons that are less used—and therefore less necessary—are eliminated. This favoritism allows neurons that receive the most stimulation—and thus are interpreted as the most important—to be given space to grow more elaborate connections.

Section 5.2Neuropsychology and Brain Development

Figure 5.4: Neuron growth and pruning ሁ According to scientists, the brain overproduces synapses during early childhood and then goes

through a pruning process later. Neurons that receive the most stimulation are favored over those that receive less stimulation.

Though paradoxical, in this way development of the nervous system actually profits most effectively from the purging of cells. In a manner that is similar to synapse production, timing of pruning also varies by different brain areas. In some instances, pruning of specific areas of the brain is not complete until adolescence or beyond (Selemon, 2013). This process of overproduction and pruning continues while the mind remains adaptive to unique individual experiences.

Myelination In addition to the growth of synapses, the axons of neurons get coated with myelin (refer back to Figure 5.2), which represents the last stage of sophistication of brain development. Myelination, or the process of coating neurons with myelin, is responsible for speeding up the transmission of impulses. This is an important activity, as faster neural processing is neces- sary to move faster physically and to think in more complex ways. Like other aspects of brain development, myelination occurs in a manner predetermined by maturational processes and follows the same patterns described previously with regard to synaptic growth and pruning (Staudt et al., 1993; Tierney & Nelson, 2009).

The myelination of sensory and motor neurons that is essential to early physical development is mostly complete by 40 months, whereas the neurons that are responsible for higher brain functions like reasoning and complex decision making are not myelinated until early adult- hood. Compared to infants with richer experiences, those raised in more limited environ- ments indicative of low socioeconomic status show overall brain differences in both structure and weight (Lawson, Duda, Avants, Wu, & Farah, 2013). That is, when experiences are limited, it makes sense that brain growth is similarly restricted. Not surprisingly, poor nutrition leads to less myelin development as well as a general reduction in brain size, though early treat- ment can often reverse these negative effects (Atalabi, Lagunju, Tongo, & Akinyinka, 2010; El-Sherif, Babrs, & Ismail, 2012; Hazin, Alves, & Rodrigues Falbo, 2007).

Section 5.3Increased Complexity in Neural Organization

S E C T I O N R E V I E W Diagram the transmission of a signal from one neuron to another.

5.3 Increased Complexity in Neural Organization Although maturation processes dictate the course of peak brain development, neurons con- tinue to migrate and form new synapses when children learn how to throw a ball, experience what it is like to get one’s feelings hurt, and acquire the skill needed to graph a geometric equation. Developing brain processes become more apparent when we see the effects of lat- eralization and sex and gender differences. We explore these processes in this section.

Brain Development in Later Childhood The sophistication of brain growth throughout childhood is evident in the growth patterns illustrated in Figure 5.5. These images show extensive mapping of cortical development among individuals between 5 and 20 years of age. Brain scans were obtained every 2 years, and a dynamic map of development was constructed (Gogtay et al., 2004).

Figure 5.5: Brain development through childhood and adolescence ሁ In an extensive project to map brain development, scientists found that axons (white matter)

continued to replace cell bodies (gray matter) well into adolescence.

Source: Image courtesy of Paul Thompson (USC) and the NIMH.

Section 5.3Increased Complexity in Neural Organization

As you can see from Figure 5.5, well into adolescence axons continue to grow and expand connections, supplanting cell bodies in the process. Basic sensory and motor functions mature first, coinciding with the basic learning outcomes of infancy. Speech and language areas come next; areas in the frontal lobe (one of four major brain divisions) that are related to judgment and the inhibition of impulses are last to develop. Because these centers are not mature until after adolescence, some researchers have speculated that immature fron- tal lobe development is linked to the risky behaviors that are indicative of adolescence. This possibility also raises questions about public policy and whether adolescents should be considered more like children or more like adults with regard to forensic examinations, driving, and other adultlike responsibilities. (See especially Bonnie & Scott, 2013, Stein- berg, 2013, and Steinberg & Scott, 2003). For instance, if judgment among teens is develop- mentally compromised, then there are implications for holding them completely account- able for crimes.

Adolescence also marks a second wave of overproduction of synapses and neural pruning (Hedman, van Haren, Schnack, Kahn, & Hulshoff Pol, 2012). The architecture of the prefron- tal cortex begins to change rapidly during this time and may give rise to specific behaviors that are associated with adolescence (Blakemore & Choudhury, 2006). These behavioral changes include improved cognitive control over activities like plan- ning, attention, and other goal-directed behavior. On the other hand, teenagers perform relatively poorly on tests of emotional control. They are more reactive rather than reflective. This behavior is consistent with their brains performing more functions in the primitive parts of the brain. It is during later adolescence that emotional processing shifts to the frontal lobes, con- tributing to improved reasoning, judgment, and control over hypothesized outcomes (Zimmer, 2011).

Lateralization The brain shows further sophistication in its contralateral (“other side”) organization pat- tern. That is, input from the right side of the body is transmitted to the left half, or hemi- sphere, of the brain; input from the left side of the body is received in the right hemisphere, as depicted in Figure 5.6. For nearly everyone, each hemisphere specializes in specific functions. For example, most people have language centers located in the left half of the brain. The left hemisphere processes tasks like thinking, reading, and speaking. Right hemisphere special- izations usually include emotional expression, music, and visual-spatial relationships used in geometry, art, and finding directions. This specialization is called lateralization.

Critical Thinking Should the knowledge that the reasoning cen- ters of adolescents are not fully mature have an impact on how they are treated when they commit crimes? For further information and dis- cussion, see Aronson (2007), Beckman (2004), Bonnie and Scott (2013), Steinberg (2013), and the case against Christopher Simmons (International Justice Project, 2005).

Section 5.3Increased Complexity in Neural Organization

Figure 5.6: Brain lateralization ሁ Even though humans process a great deal of sensory information in a contralateral fashion, other

functions, like language and emotion, are lateralized.

Left hand Right hand

Left hemisphere Right hemisphere

Lateralization (language and emotion)

Contralateral control (sensory information)

Lateralization is also demonstrated when we show a preference for using either the right or the left hand, called handedness. Across cultures and continents, the proportion of right- handedness remains at about 90% (Vuoksimaa, Koskenvuo, Rose, & Kaprio, 2009). Though some children exhibit handedness as soon as 12 months old, except for the 3% of children who are ambidextrous (showing equal preference for use of either hand), dominance usu- ally becomes well established by kindergarten (Hinojosa, Sheu, & Michel, 2003).

Ninety-five percent of right-handed people show typical lateralization patterns; they have language centers located in the left hemisphere of the brain. Among left-handers, though, only about 75% are dominant for language in the left hemisphere. It has been suggested that differences in typical patterns of lateralization may lead to greater engagement of both brain hemispheres and result in cognitive flexibility (Beratis, Rabavilas, Kyprianou, Papadimitriou, & Papageorgiou, 2013; Szaflarski et al., 2002). Left-handedness is often found to be more common among those who have jobs related to visual-spatial (right hemisphere) processing, and other evidence indicates that more left-handed children have language-based (left hemi- sphere) learning problems. As yet, evidence for any consistent overall patterns in intelligence or academics as they relate to handedness appears to be lacking.

The process of lateralization is apparent even at birth. Verbal stimuli are usually more respon- sive in the left hemisphere, whereas emotion is associated with more right-brain activity (Dubois et al., 2009; Montirosso, Cozzi, Tronick, & Borgatti, 2012). Even so, both hemispheres of the brain are usually engaged. They work in tandem to understand experiences and to respond. Consequently, there is no such thing as a “right-brain” or “left-brain” person. The interpretation of emotions that occurs in the left hemisphere needs language areas in the right hemisphere to understand the subtleties of how to act; verbal conversations include

Section 5.3Increased Complexity in Neural Organization

a fair amount of imagery and other right-brain functions. Therefore, the hemispheres of the brain should be thought of as interdependent.

Neuroplasticity remains important in lateralization as well. When young children suffer dam- age to one hemisphere of the brain, depending on the age at which the trauma occurs, the other side is often able to compensate. Among others, we can reorganize large areas of the brain related to language development, memory, emotions, and vision (Cramer et al., 2011).

Sex and Gender Differences in Brain Development A controversial topic involves findings that there are distinct patterns and organization of growth in the brains of males and females. In addition, girls consistently perform better at left-brain-dominated language tasks and boys consistently score better on tests of right- brain-dominated spatial perception and mathematical reasoning (Guiso, Monte, Sapienza, & Zingales, 2008). In their controversial book Brain Sex, Moir and Jessel (1992) make a strong case that measureable differences in the brains of males and females are already apparent soon after birth, before the environment has a major impact. Other researchers have also concluded that sex differences in brain organization and cognition have their origins before birth (e.g., Achiron, Lipitz, Hering-Hanit, & Achiron, 2001). Studies show that newborn male and female infant brains are quite different physically. Specific structures like the corpus callosum (the bundle of nerves that connects the two brain hemispheres) and, at the cellular level, the length and function of cer- tain chromosomes are indeed distinct (Hammer, Men- dez, Cox, Woerner, & Wall, 2008). Relative size differ- ences also exist in structures related to memory, vision, and language processing (e.g., see Cahill, 2005).

In a widely publicized new brain study, researchers found striking physical evidence of differences in how male and female brains are organized (Ingalhalikar et al., 2014). This information appears to confirm behavioral differences (such as verbal and math ability) that are often only observed. Brain imaging of 521 females and 428 males aged 8 to 22 years showed that male neural networks formed superior connections from front to back in each of the brain hemispheres (see Figure 5.7).

According to the researchers, there is some indication that males have greater poten- tial to connect perception with coordinated action, like learning the single task of riding a bicycle. In contrast, female brains have more neural communication between the two hemi- spheres, coinciding with a stronger connection between analyses and intuition. This pattern suggests that females are better equipped for

Critical Thinking When there are innate differences between children, how should teachers modify education planning to acknowledge individual or group differences?

Stockbyte/Thinkstock ሁ It appears that there is at least some

biological basis for sex differences in abilities..

Section 5.3Increased Complexity in Neural Organization

multitasking and working toward solutions that focus on group outcomes. The clear differ- ences provide additional evidence that males and females may be prewired to excel at differ- ent tasks.

Figure 5.7: Neural connectivity ሁ These “connectome” maps show relatively more interhemispheric connections among females

(orange) and relatively more intrahemispheric connections in male brains (blue).

f05.07_PSY104.ai

Source: Ingalhalikar, M., Smith, A., Parker, D., Satterthwaite, T. D., Elliott, M. A., Ruparel, K., & Verma, R. (2014). Sex differences in the structural connectome of the human brain. Proceedings of the National Academy of Sciences of the United States of America.

Section 5.4Maturation

It should be emphasized that there are many more differences within groups than between groups when it comes to sex differences. That is, there is much more variation among males as a group and among females as a group than between males and females. This means that there are many females who excel at math, for example, and many males who excel at verbal skills. Further, there is no convincing evidence that either males or females are cognitively advantaged overall relative to the other. Moreover, academic performance gaps in language and math vary widely between different countries, suggesting a strong sociocultural effect (Guiso et al., 2008).

So, even though there is strong evidence for distinctions between male and female brains, at both the cellular and structural levels, those distinctions may not translate into marked dif- ferences in ability in the absence of a social bias. Importantly, the small differences in perfor- mance as measured by standardized tests do not justify special academic tracks based on sex. Given equal abilities, advantageous home and school learning environments remain the most important determinants in academic outcome. These issues are explored further in Module 14. In the next section, we see how maturation of the brain in general coincides with physical and behavioral changes.

S E C T I O N R E V I E W Explain how various behaviors can predict growth in brain development.

5.4 Maturation Even though our individual genetic codes prescribe when and how our postnatal bodies grow, physical development begins in the womb. Fetuses are affected by nutrition and environ- mental factors like smoking and some illnesses. As discussed in Module 4, newborns exhibit innate physical reflexes and are born with many of their senses already functioning at a high level. As reflexes become voluntary, they provide a foundation for how brain development translates into physical behaviors. Here we expand on that notion and examine the progres- sion of physical growth milestones that occur during childhood.

Height and Weight Height is perhaps the most obvious feature of physical maturation. Whether a child is short, tall, or average, doctors measure patterns of development by consistency of growth. The chart in Figure 5.8 is typical of those used by pediatricians to gauge change in weight. In this case, it does not matter much which path children follow; it is more important to see that they are following a consistent pattern and that their weight is not fluctuating excessively.

Section 5.4Maturation

Figure 5.8: CDC weight-for-age percentiles, birth to 36 months ሁ This standard growth chart shows weight-for-age percentiles for children up to 36 months old.

lb kg

Age (months)

Weight-for-age percentiles: Girls, birth to 36 months

30 6 9 12 15 18 21 24 27 30 33 36

97th 95th

90th

75th

25th 10th 5th

50th

3rd

lb kg

30 6 9 12 15 18 21 24 27 30 33 36

97th

95th

90th

75th

25th

10th 5th

50th

3rd

Age (months)

Weight-for-age percentiles: Boys, birth to 36 months

Source: Adapted from Kuczmarski, R. J., Ogden, C. L, Guo, S. S., et al. 2000 CDC growth charts for the United States: Methods and development. National Center for Health Statistics. Vital Health Stat, 11(246). 2002.

Infants grow in length by about 50%, on average, in the first year, from about 20 inches (51 centimeters [cm]) to about 30 inches (76 cm). During the second year, they add another 5 inches (13 cm). Until adolescence, the annual growth in height decreases gradually, as shown in Figures 5.9 and 5.10. Because girls begin a period of accelerated growth during adoles- cence earlier than boys, they grow, on average, only about ½ inch (1.4 cm) after the age of 14 years, whereas boys grow another 3⅓ inches (8.5 cm).

Section 5.4Maturation

Figure 5.9: Average annual growth rate of girls and boys ሁ Growth rates for boys and girls show similar patterns, with girls beginning the adolescent growth

spurt, on average, about 2 years earlier than boys.

Age in years 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 2011 12 13

C e n

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1.5

0.5

2.5

3.5

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GirlsBoys

Source: Adapted from Ogden, Fryar, Carroll & Flegal , 2004. Advance Data No. 347. National Center for Health Statistics. October 27, 2004.

Figure 5.10: Average height of U.S. boys and girls ሁ On average, girls are taller than boys during early adolescence. After age 14, though, girls grow, on

average, only a little more than ½ inch (1.4 cm), whereas boys grow another 3⅓ inches (8.5 cm).

Age in years 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 2011 12 13

GirlsBoys

C e n

ti m

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100

110

120

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170

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Source: Adapted from Ogden, Fryar, Carroll & Flegal , 2004. Advance Data No. 347. National Center for Health Statistics. October 27, 2004.

Section 5.4Maturation

Recall that the cephalocaudal principle dictates that growth proceeds from the head down. The head represents about 25% of the body length at birth. Notice from Figure 5.11 that the head-to-body proportion decreases with age. During the first 2 years, the torso and limbs begin to catch up. By adulthood, the head makes up less than one-seventh of an individual’s height, or about half of the body proportion it held at infancy.

Figure 5.11: Change in body proportion, by age ሁ One representation of the cephalocaudal principle is the change in body proportion by age. The

proportion of head-to-body size decreases by about half from infancy to adulthood, and secondary sex characteristics develop through the teenage years until adulthood.

Newborn 2 years 5 years 15 years Adult

A popular theory to explain the disproportionate size of the brain is based in evolution. Natu- ral selection promoted a large and more sophisticated brain. However, an upright gait was also an advantage. A vertical posture changed the position of the pelvis and made for a narrower birth canal that limited brain size. Therefore, in order to have a large, sophisticated brain, it would need to continue growing after exiting the relatively small birth canal. So instead of a brain that is mostly developed in the womb to allow walking immediately after birth, like many other mammals, humans have relatively undeveloped brains at birth that continue to need plenty of attention.

Maximum Height It has been suggested that because of modern advantages in nutrition, it is now possible to gain optimum genetic height, which is a function of both genetic and environmental variables (Silventoinen, 2003; Steckel, 2002). It is estimated that, in modern Western societies, about 20% of final body height is due to environmental variation, including nutrition and physical stimulation; in settings with fewer resources, environmental variation is responsible for even

Section 5.4Maturation

more than 20% of final height. For instance, in undeveloped countries and among some fami- lies in the United States, food variety is limited. As a result, children may lack some vitamins and minerals that are essential for growth. Therefore, heritability of height (the proportion due to genetics) increases as a function of advantages in health, nutrition, and medical science.

Because short stature varies inversely with both education and social position, height can often be used as a contemporary (and historical) indicator of the health and welfare of chil- dren. For instance, in the United States the average person is nearly 3 inches (7.6 cm) taller today than when the country was founded in 1776. And during the 20th century, average body height increased throughout the industrialized world. These figures indicate that tech- nological development leading to a higher standard of living, including the ability to transport foods and services, results in healthier children.

Motor Development As babies grow, parents anxiously look for their children to roll over, stand, and walk. Later, pediatricians will ask about catching a ball, using eating utensils, and manipulating a pencil. These normative milestones are important in the study and understanding of motor develop- ment (body movement). Physical movements are categorized as either gross motor skills or fine motor skills. Gross motor skills involve large movements of the head, torso, arms, and legs, whereas fine motor skills involve more precise dexterity of the hands and fingers, usually coordinat- ing with vision. Like physical growth in general, motor behavior varies as a function of experience, like oppor- tunities to play with blocks or participate in school sports. Table 5.1 offers a sample of milestones we might typically look for.

Table 5.1: Milestones in motor development Age

Behavior

Fine motor (f ) or gross motor (g)

0–6 months Exhibits reflexes —

Holds head up g

Rolls over g

Will reach and grasp f

Physically pursues objects f + g

Can sit without support g

Stands while holding on to a parent’s hand g

Pulls self to standing position g

Critical Thinking How do changes in motor skills affect the way infants interact with their environment?

(continued)

Section 5.4Maturation

Age

Behavior

Fine motor (f ) or gross motor (g)

6–12 months Has the skill to crawl (but may not) g

Walks with support g

Stands alone g

Cruises (walks while holding on to furniture) f + g

Grasps with thumb and forefinger (pincer grip) f

12–18 months Walks without support g

Throws objects f + g

Ascends steps with help g

18–24 months Climbs f + g

Turns on faucet to get water f

Dresses self with help f + g

Drinks from a cup f

Jumps g

2–3 years Dresses self (without buttons) f + g

Ascends steps unaided, alternating feet g

Hops irregularly g

Pours liquid from one container to another f

3–4 years Can run, jump, and ride a tricycle g

Throws and catches a ball f + g

Jumps 12 inches from a climber to the ground g

Puts together simple puzzles f

Strings beads f

Cuts and pastes f

Draws shapes and symbols holding pencil or crayon between thumb and first two fingers

4–5 years Hops with purpose g

Ties shoes f

Prints recognizable letters and numbers f

Walks across a balance beam g

(continued)

Section 5.4Maturation

Age

Behavior

Fine motor (f ) or gross motor (g)

5–6 years Hand dominance usually apparent —

Skips g

Descends stairs unaided g

Skips rope g

Connects zippers, buttons, and snaps f

Traces accurately f

Copies shapes f

Uses school supplies appropriately f

7 Physical movement resembles adult movement —

Uses tools f

Can anticipate trajectory of rolling balls —

Physical Norms and Cultural Variation The historical research by Gesell (1928) and others provides a basis for standardizing average gross motor behavior. Though it has fallen out of favor, the Gesell Developmental Schedules are still used to assess normal developmental milestones. The Bayley Scales is another commonly used instrument to assess physical milestones (Bayley, 1969). It provides a more comprehen- sive battery of instruments and tests a variety of motor and mental skills for children up to 42 months. The general idea of the Gesell Developmental Schedules and other schedules is that development is maturational and does not change very much within a healthy population.

More recently it has been suggested that there is more diversity than was once thought in the acquisition of motor skills, providing substance for the nature-nurture debate. Karasik, Adol- pha, Tamis-LeMonda, and Bornstein (2010) argue that Gesell, Bayley, and other traditional developmental scales are based on Western-educated populations. They highlight a number of cultures in which the environment seems to play a larger role in development. For example, some cultures specifically target infant muscles that are later necessary for walking. These muscles are massaged and stretched, and infants are engaged in various motor exercises, in an effort to get the children walking sooner. This treatment would be an advantage within environments where there are few safe places for children to crawl. Other behaviors specifi- cally encouraged within a culture can also be accelerated. However, in the United States, there would be no particular advantage to accelerating most motor movements, demonstrating the importance of cultural context (Adolph, Karasik, & Tamis-LeMonda, 2010). Conversely, there can be a reverse effect. On average, Western-educated families that follow the back-to-sleep guidelines have babies that scoot forward and crawl later than those who are put to bed in a prone (face downward) position (Davis, Moon, Sachs, & Ottolini, 1998; Pin, Eldridge, & Galea, 2007). These infants are probably delayed in learning to propel themselves in a prone posi- tion because they are placed in the supine (face upward) more often. On the other hand, per- haps the added stimulation of looking up is a cognitive advantage.

Section 5.4Maturation

Contemporary environmental variations can affect other kinds of movement. Even the seem- ingly benign use of diapers has been shown to contribute to differences in motor develop- ment. (In many poorer countries where diapering is a luxury, until children are toilet trained it is typical for them to remain naked during the day.) In a newer study, researchers asked if the relatively new cultural invention of various diapering practices had an effect on walking behavior (Cole, Lingeman, & Adolph, 2012). Infants who had been accustomed to walking in disposable diapers were documented walking in one of three conditions: naked, in a cloth dia- per, and in a disposable diaper. The resultant footprint paths for the three conditions in Figure 5.12 were noticeably different, with the naked condition providing the most mature pattern. This study shows that cross-cultural research that compares locomotion skills may be less reliable if diapering practices are not taken into account. Furthermore, it is not clear whether the contextual differences of diapering lead to significant changes in later development, such as athletic skills or hip injuries among the elderly.

Figure 5.12: Environmental context on walking behavior ሁ Footprint paths of a single child in three conditions show that diapers change walking behavior.

When children are naked, they demonstrate the most mature gait.

f05.12_PSY104.ai

Dynamic base

Step length

Step width

Gait ParametersClothDisposableNaked

(a) (b)

Source: Adapted from Go naked: Diapers affect infant walking, by Whitney G. Cole, Jesse M. Lingeman and Karen E. Adolph. Developmental Science, Volume 15, Issue 6, pages 783–790, November 2012. John Wiley & Sons. © 2012 Blackwell Publishing Ltd.

For the most part, accelerating physical milestones like walking is probably unnecessary in most developed nations. Parents may want to show off that their not-yet-one-year-old is walking, but the fact is that nearly all 16 month olds walk anyway. The child who was pushed to walk early may simply begin walking at 12 months instead of 12 months and 2 weeks. So while Karasik et al. (2010) explain that “the field suffers from long-standing assumptions of universality based on norms established with [Western] populations” (p. 95), a strong case

Section 5.4Maturation

has yet to be made against the continued use of those norms. Whether milestones are repre- sentative of, and appropriate for, non-Western-educated populations appears to be an impor- tant question for further research.

Gross Motor Skills The first signs of gross motor skills related to locomotion occur when children develop the muscle control to roll over at between 2 and 3 months of age (refer to Table 5.1). The average infant obtains the skills needed to scoot and then crawl at between 6 and 10 months. Interest- ingly, although infancy is often associated with a crawling baby, it is not unusual for infants to skip the crawling stage and move right into cruising (walking while holding on to furniture) and then walking.

As a child’s body begins to stretch and resemble more of an adult body type, movements become more adultlike as well. For the most part, school-age children can perform the same movements as adults, though without the same skill or strength. For example, younger chil- dren do not yet fully comprehend visual-spatial movement such as the trajectory of a roll- ing ball in soccer, a bouncing ball in basketball, or a pitched ball in baseball, but they can still engage physically in those activities. Because movement is slower and reaction time is thrown off, accommodations like a batting tee (“T-Ball”) are made for younger elementary- school-age children.

By late elementary school (10 or 12 years of age), children can throw a ball, run smoothly, hop, jump with purpose, and kick with great agility and skill. They show outstanding coordi- nation dribbling a soccer ball or a basketball. They have great body control on a skateboard or rollerblades. Though they still lag adults in strength and speed, 12 year olds show adultlike hand-eye coordination in most physical activities, quite unlike the 6- and 7-year-old bodies they left behind. The advancement of physical skills also depends on brain maturation, since more cognitive sophistication is required to coordinate advanced movements.

S E C T I O N R E V I E W How would you design a research study that investigates the relationship between early motor activity and later athletic ability?

Fine Motor Skills Following the proximal-distal pattern, infants begin to integrate gross motor abilities with smaller hand movements, or fine motor skills, at around 4 months of age. A few months later they will be able to hold a bottle, but immature brain development will at first cause them to have difficulty guiding it to their mouths. Toward the end of the first year, they will transition from using the whole-hand palmar grasp to picking up cereal and other small objects between the thumb and forefinger using what is called the pincer grip. Infants will also begin to bang two toys together and can use eating utensils and cups. These activities coincide with greater mobility. Infants delight in scanning for objects, moving toward them, and picking them up with their more advanced hold. They are also becoming less dependent on others for stimulation. For this reason, it becomes exceedingly important for parents and caregivers to

Section 5.4Maturation

be on the alert for dangerous objects that these newly mobile infants can reach.

Fine motor ability in the second year results in added coordination between eye and hand movements. Children learn to get water from a faucet and put together and take apart simple toys. Preschoolers can manipulate pencils and crayons and can color within boundaries. They can also use safety scissors to cut out objects from paper. Before they reach elementary school, most children are able to acquire the skills needed to accurately use a touch screen, computer keyboard, and mouse.

If children are exposed to the fine motor activity necessary for playing musical

instruments like the piano and violin, most 5 year olds can begin to play. With some prac- tice, the average kindergartener can tie shoes and manipulate zippers, snaps, and buttons. Advancement in fine motor ability will be demonstrated throughout elementary school as children become more skilled at writing, drawing, playing music, knitting, keyboarding, constructing puzzles, and performing more exact movements with a computer. Like gross motor skills, by the end of elementary school, fine motor skills will be comparable to those of an adult.

F O C U S O N B E H A V I O R : K e e p i n g Y o u n g C h i l d r e n S a f e One way for adults to know how safe an environment is for babies, is to crawl in their envi- ronment. Every hanging cord, paper clip, dead bug, dropped food particle, or piece of trash is a potential hazard. Babies are not “testing” parents and caregivers when they put small objects in their mouths or destroy items not meant to be touched. They are simply explor- ing new textures, sounds, and other stimuli.

It is not enough to check a home once. Adults need to be vigilant because objects like an errant plastic bag or paper clip can be a curiosity. Children will continue to try to crawl through pet doors and into cabinets, even if they have been told not to. Parents need to periodically rattle locks and handles, inspect loose screws and nails, and look for standing water or any toxic substances that may have been left out.

Sex Differences in Motor Development There is no doubt that sex differences in brain development affect motor behaviors and skills. Evidence overwhelmingly suggests that physical disparities exist between boys and girls due to physiological and maturational differences (e.g., Eaton & Yu, 1989; Pellegrini & Smith, 1998). It should come as no surprise that boys generally outperform girls in gross motor skills that require speed or strength. Beginning at about 3 years old, the average boy jumps

Image Source/Photodisc/Getty Images ሁ If given the opportunity, elementary-school-

age children begin to exhibit excellent fine motor control.

Section 5.5Bowel and Bladder Control

higher and runs faster than the average girl. These differences are generally due to variability in muscle strength. Even from birth, boys are more active than girls. In contrast, girls perform better at balancing skills like walking on a beam, balancing on one foot, and playing hopscotch.

Perspectives on evolution and neurobiology reveal that the greater activity level of male infants accelerates brain growth of the motor neurons needed for strength and speed. But beginning at an early age the average boy is also conditioned to be more active than the aver- age girl. Adults treat girls more delicately and use softer language within 24 hours of birth (Beal, 1994). Compared to their interactions with boys, mothers cuddle girls more, and they are more emotionally expressive, smile and talk more, and are more responsive to the needs of girls. Boys are given more latitude, whereas girls tend to be more restricted. In this way, boys may learn to be more independent, which translates to greater activity. Regardless of the reasons, boys get more practice using their motor skills, perhaps laying the groundwork for increased strength later.

S E C T I O N R E V I E W Give a brief outline of the changes in size and physical ability that take place during child- hood. Compare children at different periods of development.

5.5 Bowel and Bladder Control Parents do not often think of bowel and bladder control as part of muscular development, yet there are few developmental issues of early childhood that garner more attention than potty training. Achieving muscular control is thought to be mostly a maturational issue that, like other behaviors, varies by individual. Some children are ready to begin the process at 18 months and some are not ready until 30 months or later (Connell-Carrick, 2006). However, if toilet training were mostly maturational, then there would be consistent trends across generations.

Instead, since the 1950s, in the United States and throughout Europe there has been a trend toward later completion of toilet training (Bakker & Wyndaele, 2000; Blum, Taubman, & Nemeth, 2004). Surveys suggest that in 1970 over 95% of U.S. parents expected their children to begin toilet training before 24 months; by 1985, the figure had dropped to 73%; by 1996, only 65% of 30-month-old children had started toilet training (Schum et al., 2001; Seim, 1989; Stehbens & Silber, 1971). Reasons for the change include an increase in daycare facilities that will accept children before they are toilet trained and parents who have not committed to the process (often due to time constraints related to work schedules). Throughout the world, the age at which toilet training begins is deter- mined partly by family income as it relates to the cost of disposable diapers (de Miranda & Machado, 2011; Horn, Brenner, Rao, & Cheng, 2006).

Critical Thinking Many parents and early childhood educators use bowel and bladder control as a marker for the appropriate time to start preschool. Is it appro- priate to use this developmental milestone to determine daycare and preschool placement? Why or why not?

Section 5.5Bowel and Bladder Control

Training Methods A comprehensive review of the literature revealed little research comparing the efficacy of different toilet train- ing methods (Vermandel, Van Kampen, Van Gorp, & Wyn- daele, 2008). Though age trends have changed, without clear clinical guidelines it is difficult to guide parents and professionals about the best methods of training. The American Academy of Pediatrics (2011) does not advo- cate for any specific method, perhaps because evidence for best practices does not exist. Their current guidelines state that adults should not think that there is a set age at which toilet training necessarily begins. Other organi- zations and professionals have similar recommendations (e.g., de Miranda & Machado, 2011).

F O C U S O N B E H A V I O R : T o i l e t T r a i n i n g Young children show toilet-training readiness when they stay dry for at least 2 hours dur- ing the day, have regular and predictable bowel movements, use words or body language that demonstrate a need to urinate or defecate, and have a desire to use the toilet or to wear underwear.

A common strategy for toilet training is T. Berry Brazelton’s child-centered approach (Brazelton, 1962; Brazelton et al., 1999). In this method, a potty chair is a conspicuous part of the child’s environment. Children are left to their own schedule to get used to the potty chair but at the same time are reinforced by shaping. Children are first rewarded with a story or treat for sitting on the chair clothed. They are then given reinforcement each time they get closer to the goal of evacuating in the toilet. It is a variable process wherein suc- cess depends on the child’s toileting behavior, the schedule of the rewards, and the rigor of the supervising adults.

The intensive-training approach described by Azrin and Foxx (1971) is perhaps the only method that truly breaks down toilet training into a systematic process. The method was originally developed for use with children who had developmental disabilities, but it has been applied successfully to other populations and settings as well. Perhaps because of the detailed descriptions Azrin and Foxx present, their book Toilet Training in Less Than a Day remains a popular seller 40 years after its original publication (Azrin & Foxx, 1974, p. 208).

S E C T I O N R E V I E W How is bowel and bladder control related to physical development? Describe the genera- tional changes in the initiation of this milestone.

Ryan McVay/Stockbyte/Thinkstock ሁ There are a number of different

methods for toilet training.

Section 5.6Physical Disabilities

5.6 Physical Disabilities Sometimes motor development does not follow a typical trajectory. This occurs for approx- imately 13 out of 10,000 school-age children, who have serious difficulty with movement (National Center for Education Statistics, 2013b). Having a physical handicap makes navi- gating the school environment more difficult. In older “mainstream” schools, there are not always accommodations for students with physical disabilities. Elevators are rare. Many schools still have doorknobs instead of levers, and accommodations for the sight or hearing impaired are not always present. Consequently, though specialized schools exist, educational opportunities for those with cerebral palsy, juvenile rheumatoid arthritis, spina bifida, and injuries affecting the brain and spinal cord are more limited.

Vision Most visual problems are identified early, but some go undetected until warning signs become more obvious. Parents and edu- cators should take note of children who squint and make facial gestures when read- ing, hold materials too close to the face, and miss simple visual cues. It is difficult to know how many children suffer from visual impair- ment because there are so many ways to define it. There are legal definitions for blindness (vision of less than 20/200 after using corrective lenses) and partial sightedness (visual acuity between 20/70 and 20/200 after correction), but those definitions refer only to dis- tance vision. Other children have difficulty with near vision that severely affects reading, writing, and learning.

Critical and Sensitive Periods in Visual Development The visual system also provides an excellent example of a critical period of development. David Hubel and Torsten Wiesel (1970) won the Nobel Prize in Physiology or Medicine for their work with the visual system of cats, whose eyes are similar to those of humans. They found a number of specialized brain cells that react only to specific kinds of stimulation. For instance, when kittens were deprived of color or certain types of movement, they suffered irreversible damage to the visual system related to those deficits.

Their research also helped us to understand visual problems and how they might be treated. For instance, each human eye normally sends signals to the brain from a slightly different loca- tion (binocular disparity), allowing us to synthesize input and coordinate depth perception. When children have strabismus (are “cross-eyed”), binocular disparity and depth perception suffer because eye movements are not coordinated; one eye is usually dominant while input from the other is neglected. When the brain is deprived of one eye’s input during infancy and early childhood, the dominant eye performs most of the visual tasks. As a result, the part of the brain that is supposed to receive signals from the weaker eye does not develop properly, which leads to permanent deficits in depth perception. If strabismus, which occurs in 2–5% of normal births, is not corrected through surgery or eye exercises during early childhood,

Fotosearch/SuperStock ሁ Some schools are not able to accommodate

students with special needs.

Section 5.6Physical Disabilities

individuals will never gain proper depth perception (Vaegan & Taylor, 1979). Like other criti- cal periods of development, specific kinds of deprivation therefore predict specific kinds of deficits.

Hearing Like visual impairments, auditory impairments vary along a continuum. Some children have difficulty hearing only certain frequencies (higher and lower sounds). For them, amplifica- tion of all frequencies would be uncomfortable, so specialized instructional devices are often needed. The critical period for hearing is from birth to 3 years, so children who have severe hearing loss before the age of 3 have difficulty producing oral language. But even those who experience hearing loss after the age of 3 may experience speech impairments. Early audi- tory impairment is also associated with difficulties in abstract thought, including solving math problems and understanding concepts (Marschark, 2003a, 2003b). It is theorized that these cognitive deficits are due to the ways in which those with hearing impairments pro- cess language, but clear evidence about the causal factors behind differences in cognition has remained elusive.

F O C U S O N B E H A V I O R : I n d u c e d H e a r i n g L o s s How long does it take for loud music or other noise to cause permanent damage to the auditory system? Noise-induced hearing loss (NIHL) occurs with frequent exposure to loud sounds such as from car and personal music players. By the time North American teenagers graduate from high school, 15–20% of them will have measurable hearing loss (Harrison, 2008). And it does not take years to occur. A simple blast of a firecracker or exposure to loud music over just several months can cause permanent damage (Harrison, 2008; Segal, Eviatar, Lapinsky, Shlamkovitch, & Kessler, 2003).

When I used to pick up my children at their elementary schools, I often observed (and heard!) older siblings and parents of young children in cars where the music was blasting, something I have certainly done in the past. But I would never expose young children to such high-volume noise, since they do not have a choice in the matter. It is one thing when adults choose to expose themselves to potentially harmful effects of loud music; children need to be protected.

Speech Nearly 1.4 million children receive special education services for speech or language prob- lems (National Center for Education Statistics, 2013b). As discussed earlier, auditory impair- ments often predict speech difficulties. The causes of other kinds of speech impairments are harder to identify. The most common types are articulation disorders such as stuttering or difficulty pronouncing /s/ (lisping) and /r/. Though predictable patterns are rare, develop- mental risk factors like fetal alcohol spectrum disorders and cerebral palsy usually indicate that special attention should be paid to any emerging speech difficulties.

As a general rule, adults should be able to understand about 50% of what 2 year olds say, about 75% of what 3 year olds say, and nearly 100% of what 4 year olds say. That does not mean that all 4 year olds articulate perfectly, only that they should be understood. Between

Section 5.7Puberty

the ages of 2 and 5, it is also normal for some children to stutter. When this occurs, it is theo- rized that the activities in the language production centers and the articulation centers of the brain are not completely coordinated. In a sense, young children cannot speak as fast as their brains are thinking about words. This type of stuttering usually disappears spontane- ously within a few months. On the other hand, despite extensive research over the past 100 years, the cause of persistent stuttering remains unclear (Drayna & Kang, 2011; Prasse & Kikano, 2008).

It is recommended that adults respond to both types of stuttering the same way—with patience. They should let children finish what they want to say without drawing attention to their speech patterns. Speech problems of all kinds can cause stress and embarrassment, may affect psychosocial development, and can cause children to be less involved in classroom activities. If stuttering persists, or a speech problem of any kind is suspected, early evaluation or intervention with a speech pathologist is the key to effective treatment.

S E C T I O N R E V I E W Explain how various physical disabilities impact development. Speculate about the poten- tial effects on cognitive and psychosocial outcomes.

5.7 Puberty Physical limitations may also affect social opportunities and status, especially as children transition into adolescence. The time of potentially intense psychosocial change associated with the teenage years did not really exist before the Industrial Revolution. One was either a child or an adult. In the 18th and 19th centuries, if youth were fortunate enough to attend secondary school, they could extend childhood until high school graduation. Otherwise, as soon as children were able to help the family by working an adult job, that’s what they would do. The definition of adulthood was therefore largely dependent on physical stat- ure. Boys who looked more like men became “adults” faster than those who were physically less mature.

Beginning in the 20th century, laws protecting children from being physically or sexually exploited were passed, education became mandatory, and child labor was restricted. In short, children were accorded a respectful place in society, including during the time of physical maturation called puberty. This section explores this period of relatively rapid change.

Adolescent Growth Spurt The adolescent body undergoes a number of physical changes that mark the transition into adulthood. Part of the tremendous change is the sudden growth in height and weight. This development is often referred to as the adolescent growth spurt and can add 5 inches (12.7 cm) or more in a single year. Girls begin the spurt at about age 10 and boys at about age 12 (refer back to Figures 5.9 and 5.10). Because girls begin the growth spurt about 2 years ear- lier than boys, 12-year-old girls are taller, on average, than their male counterparts.

Section 5.7Puberty

Sexual Maturation We can know from brain scans whether or not an infant is male or female, but only genitalia differentiates males from females at birth. The internal and external physical characteristics that distinguish the sexes at birth are called primary sex characteristics. For females they include the clitoris, vagina, uterus, fallopian tubes, and ovaries. Male primary sex characteris- tics include the scrotum, testes, penis, and the glands that contribute to semen.

During adolescence, physical changes take place that further differentiate males from females. These visible changes are called secondary sex characteristics and are part of the complex biological process that serves to prepare us for reproduction. Studying the onset and tempo of these changes is challenging. As Table 5.2 shows, secondary sexual character- istics typically occur in a prescribed order, but there are considerable variations in timing. Worldwide, pubertal changes vary with race, ethnicity, income, environmental conditions, location, and nutrition (Eveleth & Tanner, 1990; Herman-Giddens et al., 1997; Parent et al., 2003).

Table 5.2: Average age and normal range of selected pubertal changes in North American boys and girls

Girls

Average

Normal range

Boys

Average

Normal range

Breast budding 10 8–13 Testes begin to enlarge 11.5 9.5–13.5

Growth spurt begins 10 8–13 Growth spurt begins 12 10.5–16

Pubic hair growth begins 11 8–14 Pubic hair growth begins 12 10.5–14.5

Peak of growth 11.7 10–13.5 Peak of growth 14 12.5–15.5

Menarche 12.5 10.5–14 Spermarche Facial hair growth begins Voice deepens

13.5 14 14

12–16 12.5–15.5 12.5–15.5

Mature pubic hair growth 14.5 14–15 Mature pubic hair growth 15.5 14–17

Adult stature reached 13 10–16 Adult stature reached 15.5 13.5–17.5

Source: Based on Herman-Giddens, et. al. (1997 & 2001)

Puberty in Girls In girls, a dramatic increase in the production of the hormone estrogen induces secondary sex characteristics that include breast development, widening of the hips (perhaps to gain a wider birth canal, an evolutionary advantage), and increased fat deposits around the hip area. Additionally, because of the increase in fat, skin becomes softer. Breast budding is the first visual sign of burgeoning sexual development and is a sign that menarche, or the first menstrual period, will follow in 2–3 years. Menarche is an indication that ova are becom- ing mature enough to be fertilized. For many girls, it is the first sign of adulthood. Most girls complete pubertal changes in 3–4 years, but the normal range is anywhere from 2 to 6 years (Archibald, Braber, & Brooks-Gunn, 2006).

Section 5.7Puberty

There are differences between groups as well. One large study of over 17,000 girls found significant racial differ- ences in puberty (Herman-Giddens et al., 1997). While only 15% of white girls begin puberty by age 8 (defined as breast budding or pubic hair growth), nearly half of black girls do. For white girls, the average age of men- arche is about 13 years old; the onset of menarche for black girls is approximately 8 months earlier, as shown in Figures 5.13 and 5.14. However, because young black girls are, on average, heavier than white girls, weight may be a more important factor than race. Additional studies that control for weight and economic and health factors would offer additional guidance as to what is considered normal versus early maturation. Indeed, girls in the United States begin puberty about a year ear- lier than many of their European counterparts, and are also heavier (Kaplowitz, 2006; McDowell, Brody, & Hughes, 2007). Therefore, it is likely that epigenesis is at least partially responsible for onset and timing of puberty, but little research has been performed in this area (Mishra, Cooper, Tom, & Kuh, 2009).

Figure 5.13: Prevalence of menses, by age and race

P e rc

e n

ta g

White African American

Age in years 8 9 10 11 12

Source: From M. E. Herman-Giddens, et al., (1997). Secondary sexual characteristics and menses in young girls seen in office practice: A study from the pediatric research in office settings network. Pediatrics, 99, 505–512. Copyright © 1997 by the American Academy of Pediatrics. Reprinted by permission.

Elena Elisseeva/iStock/Thinkstock ሁ There are wide variations in the

timing and course of puberty.

Section 5.7Puberty

Figure of 5.14: Prevalence of pubic hair or budding in girls, by age and race

P e rc

e n

ta g

100

White African American

Age in years 3 4 5 6 7 8 9 10 11 12

Puberty in Boys In boys, the hormone testosterone causes a deepening of the voice coinciding with an enlargement of the Adam’s apple (larynx), increased muscle mass, facial and body hair, and broadening of the shoulders. Boys’ skin becomes coarser due to fewer fat deposits. Though the first visual sign of puberty is an enlargement of the testes, it is of course not as notice- able (or as well known) as the first indication in girls. Later, pubic hair growth begins, and the penis enlarges and elongates; there is concurrent maturation of the prostate gland and seminal vesicles. Fluid from those glands, together with sperm from the testes, comprises the semen and provides a signal that the male is ready to reproduce. Spermarche, or first ejaculation of semen, occurs about 2 years after secondary sex characteristics begin to appear (Dorn, Dahl, Woodward, & Biro, 2006).

Section 5.7Puberty

Unlike studies with girls, there is a lack of standardization for boys that would allow for easy comparison of trends or prevalence. In one cross-cultural study that collected data from 2,495 youth (summarized in Figures 5.15 and 5.16), black boys began and ended puberty earlier than either white or Mexican American boys (Herman-Giddens, Wang, & Koch, 2001). However, when height and weight were controlled, differences between blacks and Mexican Americans disappeared. That is, height and weight appeared to be stronger predictors than age for onset of puberty.

Figure 5.15: Prevalence of pubic hair in boys, by race and age

P e rc

e n

ta g

100

White

Mexican American

African American

Age in years 8 9 10 11 12 13 14 15+

Section 5.7Puberty

Figure 5.16: Prevalence of genital development of boys, by race and age

P e rc

e n

ta g

100

White

Mexican American

African American

Age in years 8 9 10 11 12 13 14 15+

Secular Trends in Puberty As discussed earlier, height has increased gradually over the centuries. People in industrial- ized nations are now closer to their optimal genetic height than they have ever been. This change over time, or secular trend, occurs in the timing of puberty, too. As Figure 5.17 shows, the average age of menarche has declined from nearly 17 years old in 1840 to around 13 years old today. That decline includes a drop of one year in just 25 years beginning in the late 1960s (Herman-Giddens et al., 1997; Marshall & Tanner, 1969).

Section 5.7Puberty

Figure 5.17: Secular changes in menarche ሁ Between 1840 to 1970, the average age of first period decreased by about 4 years.

A g

e o

f m

e n

a rc

h e i

n y

e a rs

Year of menarche

13.5

14.0

14.5

15.0

15.5

16.0

16.5

17.0

17.5

18 30

18 40

18 50

18 60

18 70

18 80

18 90

19 00

19 10

19 20

19 30

19 40

19 50

19 60

Norway Finland Sweden

Germany

Gt. Britain

Denmark U.S.A

Source: Adapted by permission from Macmillan Publishers, Ltd. “Trend towards Earlier Menarche in London, Oslo, Copenhagen, the Netherlands and Hungary,” by J. M. Tanner, Nature, 243, 95–96 (11 May 1973), Fig 1, p. 96.

Popular media, including many nonscientific websites, have suggested that earlier onset of puberty is caused by hormones fed to poultry, cows, and other livestock. These hormones could theoretically get into human bodies, but they are not digestible (Kaplowitz, 2006). There is simply no scientific evidence that bovine growth hormone fed to cows to increase milk production causes early puberty (Kaplowitz, 2006; Partsch & Sippell, 2001).

Instead, the initial decline in average age of menarche coincides with the beginning of the Industrial Revolution, when improved standards of living and advanced technologies began to change the health of people living in developed countries (Kaplowitz, 2006). There are indeed analogous data today that support this idea. In some underdeveloped countries, where malnu- trition is common and health is relatively poor, menarche is delayed until about age 16; within developing countries, girls from more advantaged homes reach menarche 6–18 months ear- lier than those who are economically disadvantaged (Parent et al., 2003). These gaps are even more noticeable with regard to global health and safety, a topic that is explored next.

S E C T I O N R E V I E W Describe the primary and secondary sexual characteristics among both boys and girls.

Summary and Resources

Wrapping Up and Moving On How the brain is nurtured with both nutritional and experiential stimulation has a profound effect on development. Maturation prescribes when development of normative behavior occurs, but the environment provides the context in which specific skills are obtained. In developed countries, children usually come close to their genetic potential for height and other physical milestones, but much of the world still lacks resources to achieve optimal development. Cultural and contextual differences also exist with motor behaviors (includ- ing bowel control), among children with disabilities, and in the onset and timing of puberty. Physical development is also affected by individual choices and societal conditions related to health and safety. These topics are explored next, as we investigate how children remain healthy so that they can fulfill their developmental potential.

Summary and Resources • The physically dependent child grows quickly into one that strives for autonomy.

During the second year, children learn that they are mobile and can independently explore the environment. Physical growth remains fairly consistent until puberty, when it accelerates rapidly.

• Brain development follows a prescribed maturational pattern. For the brain to respond optimally to stimulation, synapses multiply at a tremendous rate and then go through a pruning process.

• Synaptic growth is stimulated through both maturation and experience. • Specific cognitive functions develop in different parts of the brain, many of which are

found in only one hemisphere. • Substantial evidence shows that male and female brains are different, but that find-

ing must be tempered by the possibility of environmental influences. • Maturation as dictated by genetics predicts the order of motor sophistication,

though some cultural variability may exist. • Some uncertainties exist in the science of bowel and bladder control. Not only are

trends inconsistent, but there is also little discussion about best practices. • Children with special needs are often disadvantaged in multiple areas. Without

proper accommodations and interventions, negative outcomes may be worsened. • Puberty is a time of great physical change. The adolescent growth spurt and the

development of secondary sex characteristics physically transform children into adults. Though scientists have observed a number of secular trends, explanations for how and why they have occurred are not completely clear.

Key Terms adolescent growth spurt A period of rapid growth in height and weight that occurs dur- ing puberty.

ambidextrous Showing equal preference for using the right and left hands.

axon A projection of a nerve cell; it sends signals indicating stimulation.

blindness Commonly defined as vision of less than 20/200 after using corrective lenses.

cephalocaudal A description of develop- mental growth that follows a head-to-toe pattern.

Summary and Resources

contralateral The organizational relation- ship between the brain and the body. Input from one side of the body is transmitted to the opposite side of the brain.

dendrite A projection of a nerve cell; it receives signals from sending neurons.

directionality The general principles of growth, including cephalocaudal and proxi- modistal patterns.

estrogen The hormone that promotes reproduction and guides puberty in females.

fine motor skills Movements using preci- sion, usually with the hands and fingers. Contrast with gross motor skills.

gross motor skills Large body movements using the head, torso, arms, and legs. Con- trast with fine motor skills.

independence of systems A principle of growth that suggests different body systems mature independently.

lateralization The specialized organization of the left and right brain hemispheres.

menarche First menstrual period.

myelin sheath A fatty, insulating substance that coats axons in order to speed transmis- sion of neural signals.

neuron A nerve cell that transmits impulses in the nervous system.

neuroplasticity The ability of the brain to physically adapt as the result of experience.

neurotransmitters Specialized chemicals used to relay signals from one neuron to another.

palmar grasp A whole-hand grasp using the palm and all five fingers.

partial sightedness Commonly defined as visual acuity between 20/70 and 20/200 after correction.

pincer grip A grip using the thumb and forefinger for fine motor manipulation of objects like cereal and pencils.

primary sex characteristics Physical char- acteristics present at birth that distinguish males from females.

proximodistal A description of develop- ment that identifies growth as a pattern that begins from the inside of the body and proceeds outward toward the extremities.

puberty A developmental period and pro- cess when rapid physical changes lead to the formation of secondary sex characteristics and the adolescent growth spurt. Puberty culminates in an adult body.

secondary sex characteristics Visual signs beginning at puberty that distin- guish the sexes and prepare the body for reproduction.

secular trend A relatively long-term pat- tern or variation.

soma The body of a neuron.

spermarche A boy’s first ejaculation of semen.

synapse The space between two neurons into which neurotransmitters are released.

synaptic pruning The process of reducing the number of underused or weak synapses, which thereby strengthens others.

terminal buttons Bulblike structures at the end of axons. Vesicles in terminal buttons contain neurotransmitters.

testosterone The hormone that promotes reproduction and guides puberty in males.

Summary and Resources

Web Resources See links below for additional information on topics discussed in the chapter.

The Brain

http://www.pbs.org/wnet/brain/index.html

Cerebral Palsy

http://vsearch.nlm.nih.gov/vivisimo/cgi-bin/query-meta?v:project= medlineplus&query=cerebral+palsy&x=15&y=13

Corpus Callosum

http://www.buzzle.com/articles/corpus-callosum-function.html

Dementia

http://www.nlm.nih.gov/medlineplus/dementia.html

Frontal Lobe

http://www.ninds.nih.gov/disorders/brain_basics/know_your_brain.htm#fore

Gesell Developmental Schedules

http://www.gesellinstitute.org.php53-10.dfw1-2.websitetestlink.com/wp-content/ uploads/2013/09/GesellSchedules.pdf

Hippocampus

http://biology.about.com/od/anatomy/p/hippocampus.htm

Immature Frontal Lobe

http://www.nimh.nih.gov/health/publications/the-teen-brain-still-under- construction/index.shtml

Juvenile Rheumatoid Arthritis

http://www.nlm.nih.gov/medlineplus/juvenilerheumatoidarthritis.html

Natural Selection

http://evolution.berkeley.edu/evolibrary/article/evo_25

Nobel Prize in Physiology or Medicine

http://nobelprize.org/nobel_prizes/medicine/

Noise-Induced Hearing Loss

http://www.nidcd.nih.gov/health/hearing/noise.aspx

Shaping

http://www.behavioradvisor.com/Shaping.html

Summary and Resources

Stuttering

http://www.aafp.org/afp/2008/0501/p1271.html

Toilet Training in Less Than a Day

http://www.amazon.com/Toilet-Training-Less-Than-Day/dp/0671693808/ref=sr_1_1?s =books&ie=UTF8&qid=1287122520&sr=1-1

Visual Cortex

http://thebrain.mcgill.ca/f lash/d/d_02/d_02_cr/d_02_cr_vis/d_02_cr_vis.html#2