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Psychology Twelfth Edition

Chapter 3 Genes, Evolution,

and Environment

Unlocking the Secrets of Genes

• LO 3.1.A Explain how genes, chromosomes, DNA,

and genomes all relate to one another.

• LO 3.1.B Explain why the study of epigenetics

offers an important avenue for understanding the

genetic components of thought and behavior.

The Human Genome (1 of 8)

• In general, behavioral geneticists study our

differences, such as those originating in heredity.

• Researchers attempt to tease apart the relative

contributions of:

– heredity

– environment

• They adopt a nature and nurture approach in their

investigations.

The Human Genome (2 of 8)

• Genes, the basic units of heredity, are located on

chromosomes, which consist of strands of DNA.

• Each sperm cell and each egg cell (ovum)

contains 23 chromosomes.

• At conception, the fertilized egg and all the body

cells that eventually develop from it (except for

sperm cells and ova) contain:

– 46 chromosomes

– arranged in 23 pairs

The Human Genome (3 of 8)

• Our genes, together with noncoding DNA, make

up the human genome.

– Many genes contribute directly to a particular trait.

– Others work indirectly by switching other genes on or

off.

– Many genes are inherited in the same form by

everyone.

– Others vary, contributing to our individuality.

The Human Genome (4 of 8)

• Most human traits depend on more than one gene

pair.

• This makes tracking down the genetic

contributions to a trait extremely difficult.

The Human Genome (5 of 8)

• However, advances in technology now permit

scientists to carry out:

– genome-wide association studies

 examining variations in many DNA elements at once

– whole-genome sequencing

 examines the entire 3 billion base pairs of DNA

• The researchers start by looking for DNA

differences called genetic markers.

The Human Genome (6 of 8)

• Locating a gene does not automatically tell us:

– what it does

– how it does it

– how multiple genes interact and influence behavior

• Usually, locating a gene is just the first small step

in understanding exactly what it does and how it

works.

The Human Genome (7 of 8)

Figure 3.1

Genes and Chromosomes

Science Source

The Human Genome (8 of 8)

Figure 3.2

DNA Double Helix

Africa Studio/Fotolia

Epigenetics (1 of 2)

• Many people think of the genome as a static

blueprint, a set of coded messages that never

changes over a person’s lifetime.

• But this is a big misconception.

• The genome changes over time because of:

– mutations that arise before or after birth

– epigenetic changes that affect the expression (activity)

of specific genes without altering the sequence of

bases in those genes

Epigenetics (2 of 2)

• Mutations and epigenetic changes can be affected

by environmental factors.

– Example: Epigenetic changes may help explain why

one identical twin might get a disease and the other not

get it.

• Epigenetic changes affect:

– behavior

– learning and memory

– vulnerability to mental disorders

The Genetics of Similarity

• LO 3.2.A Explain how natural selection contributes

to changes in gene frequencies in a population.

• LO 3.2.B List and describe five innate human

characteristics.

Evolution and Natural Selection (1 of 3)

• Evolutionary psychologists study our

commonalities:

– personality

– emotion

– sexual behavior

– reasoning

• They trace these to the processes of evolution,

especially the process of natural selection.

Evolution and Natural Selection (2 of 3)

• They draw inferences about the behavioral

tendencies that might have been selected.

• These tendencies:

– helped our forebears solve survival problems

– enhanced reproductive fitness

• They then conduct research to see if such

tendencies actually exist throughout the world.

Evolution and Natural Selection (3 of 3)

• Many evolutionary psychologists believe that the

mind is not a general-purpose computer.

• It is viewed as a collection of specialized mental

modules to handle specific survival problems.

– a module does not have to correspond to one specific

brain area

• Critics are concerned that the notion of mental

modules might lead to misguided assumptions.

– namely, that virtually every human activity and capacity

is innate

Innate Human Characteristics (1 of 2)

• Because of the way our species evolved, many

abilities, tendencies, and characteristics are:

– either present at birth in all human beings, or

– develop rapidly as a child matures

• Examples of traits:

– inborn reflexes

– an attraction to novelty

– a motive to explore and manipulate objects

– an impulse to play

– the capacity for certain basic cognitive skills

Innate Human Characteristics (2 of 2)

• There are adaptive and evolutionary aspects of:

– sensory and perceptual abilities

– learning

– ethnocentrism

– cognitive biases

– memory

– emotions and emotional expressions

– stress reactions

– the tendency to gain weight when food is plentiful

– attachment to others

Our Human Heritage: Courtship and

Mating

• LO 3.3.A Compare the sexual strategies of

females and males, according to the

sociobiological perspective.

• LO 3.3.B Discuss four challenges to the

evolutionary view of human mating strategies.

Evolution and Sexual Strategies (1 of 3)

• Sociobiologists and evolutionary psychologists

argue that males and females have evolved

different sexual and courtship strategies.

• These have evolved in response to survival

problems faced in the distant past.

Evolution and Sexual Strategies (2 of 3)

• In this view, it has been adaptive for:

– males to be promiscuous, to be attracted to young

partners, and to want sexual novelty

– females to be monogamous, to be choosy about

partners, and to prefer security to novelty

• Evolutionary psychologists research

commonalities in human mating and dating

practices around the world.

Evolution and Sexual Strategies (3 of 3)

Figure 3.3

Preferred Age in a Mate

The “Genetic Leash” (1 of 3)

• Critics argue that evolutionary explanations of

infidelity and monogamy:

– are based on simplistic stereotypes of gender

differences

– that they rely too heavily on answers to questionnaires,

which often do not reflect real-life choices

– that convenience samples used in questionnaire

studies are not necessarily representative of people in

general

– that the evolutionary emphasis on the Pleistocene Age

may not be warranted

The “Genetic Leash” (2 of 3)

• Moreover, our ancestors probably did not have a

wide range of partners to choose from.

• Evidence suggests that what may have evolved is

mate selection based on:

– similarity

– proximity

• The central issue dividing evolutionary theorists

and their critics is the length of the “genetic leash.”

The “Genetic Leash” (3 of 3)

Figure 3.4

Attitudes toward Chastity

The Genetics of Difference

• LO 3.4.A Explain what heritability refers to, and

discuss three important facts about heritability that

should be kept in mind when discussing genetic

contributions to behavior.

• LO 3.4.B Outline the basic design of a heritability

study that involves twins and adoptees.

The Meaning of Heritability (1 of 2)

• Heritability refers to the extent to which differences

in a trait or ability within a group of individuals are

accounted for by genetic differences.

• Heritability estimates do not apply to specific

individuals or to differences between groups.

• They apply only to differences within a particular

group living in a particular environment.

– Example: Heritability is higher for children in affluent

families than in impoverished ones.

The Meaning of Heritability (2 of 2)

• Even highly heritable traits can often be modified

by the environment.

• Behavioral geneticists have found many examples

of how genes interact with the environment.

– Although height is highly heritable, malnourished

children may not grow to be as tall as they would with

sufficient food.

– Children who eat an extremely nutritious diet may grow

to be taller than anyone thought they could.

– The same principle applies to psychological traits and

skills.

Computing Heritability (1 of 3)

• Behavioral geneticists often study differences

among individuals by using data from studies of:

– adopted children

– identical and fraternal twins

• By comparing the genetic and environmental

“overlap,” researchers can estimate the heritability

of a trait.

– Example: If identical twins are more alike than fraternal

twins, then the increased similarity must be due to

genetic influences.

Computing Heritability (2 of 3)

Figure 3.5

Heritability and Adoption

Computing Heritability (3 of 3)

Figure 3.6

Twins and Genetics

Our Human Diversity: The Case of

Intelligence

• LO 3.5.A Discuss the extent to which intelligence may

be heritable.

• LO 3.5.B Explain why both between-group and within

group variability are important in arguments about

group differences in intelligence.

• LO 3.5.C List four ways that the environment nurtures

or thwarts mental ability, and give an example of each.

• LO 3.5.D Explain how both nurture and nature play an

interactive role in shaping behavior.

Genes and Individual Differences (1 of 4)

• Heritability estimates for intelligence (as measured

by tests of one’s intelligence quotient, or IQ)

average about:

– .40 to .50 for children and adolescents

– .60 to .80 for adults

Genes and Individual Differences (2 of 4)

• Identical twins are more similar in IQ-test

performance than fraternal twins.

• Adopted children’s scores correlate more highly

with those of their biological parents than with

those of their nonbiological relatives.

Genes and Individual Differences (3 of 4)

• These results do not mean that genes determine

intelligence.

• The remaining variance in IQ scores must be due

largely to environmental influences.

Genes and Individual Differences (4 of 4)

Figure 3.7

Correlations in Siblings’ IQ Scores

The Question of Group Differences (1 of 3)

• If genes influence individual differences in

intelligence, do they also help account for

differences between groups?

• This question has enormous political and social

importance.

• It is a mistake to draw conclusions about group

differences from heritability estimates based on

differences within a group.

The Question of Group Differences (2 of 3)

• The available evidence fails to support genetic

explanations of black–white differences in

performance on IQ tests.

• Even among groups popularly thought to be high

achievers, purely genetic explanations are

unsatisfactory.

– Ashkenazi Jews

The Question of Group Differences (3 of 3)

Figure 3.8

The Tomato Plant Experiment

The Environment and Intelligence (1 of 5)

• Environmental factors are associated with lower

performance on intelligence tests:

– poor prenatal care

– malnutrition

– exposure to toxins

– stressful family circumstances

The Environment and Intelligence (2 of 5)

• Conversely, a healthy and stimulating environment

can raise IQ scores.

• Certain kinds of enrichment activities can improve

performance.

– Example: Attending a good-quality preschool increases

the reading and math skills of children from racial and

ethnic minorities.

The Environment and Intelligence (3 of 5)

• Children’s mental abilities improve when their

parents:

– talk to them about many topics

– describe things accurately and fully

– encourage them to think things through

– read to them

– expect them to do well

• Children’s abilities also improve when their peers

value and strive for intellectual achievement.

The Environment and Intelligence (4 of 5)

• IQ scores have been rising in many countries for

several generations, most likely because of:

– improved education

– better health

– increase in jobs requiring abstract thought

The Environment and Intelligence (5 of 5)

Figure 3.9

Climbing IQ Scores

Beyond Nature versus Nurture (1 of 3)

• The development of a human being (or other

animal) is the result of a constant dialogue

between:

– the genome and

– its environment

• The interaction between genes and environment

is far more complex than anyone previously

imagined.

Beyond Nature versus Nurture (2 of 3)

• Genes influence which environments people find

most congenial.

• Environmental factors influence the genome by

their effects on mutations and epigenetic changes.

Beyond Nature versus Nurture (3 of 3)

• Development of a person is the result of:

– a dynamic dialogue between genes and the

environment

– the addition of chance events

• Genetic and environmental influences blend and

become indistinguishable in the development of

any one person.