CHAPTER 4

NATURE, NURTURE, AND HUMAN DIVERSITY

Behavior Genetics: Predicting Individual Differences

4-1 What are chromosomes, DNA, genes, and the human genome? How do behavior geneticists explain our individual differences?

Genes are the biochemical units of heredity that make up chromosomes, the thread-like coils of DNA. When genes are “turned on” (expressed), they provide the code for creating the proteins that form our body’s building blocks. Most human traits are influenced by many genes acting together. The human genome is the shared genetic profile that distinguishes humans from other species, consisting at an individual level of all the genetic material in an organism’s chromosomes. Behavior geneticists study the relative power and limits of genetic and environmental influences on behavior.

4-2 How do twin and adoption studies help us understand the effects and interactions of nature and nurture?

Studies of identical (monozygotic) twins versus fraternal (dizygotic) twins, separated twins, and biological versus adoptive relatives allow researchers to tease apart the influences of heredity and environment. Research studies on separated identical twins maintain the same genes while testing the effects of different home environments. Studies of adoptive families let researchers maintain the same home environment while studying the effects of genetic differences. Heritable individual differences (in traits such as height and weight) do not necessarily explain gender or ethnic group differences. Shared family environments have little effect on personality.

C-10

4-3 What have psychologists learned about temperament?

The stability of temperament, a person’s characteristic emotional reactivity and intensity, from the first weeks of life suggests a genetic predisposition. The genetic effect appears in physiological differences such as heart rate and nervous system reactivity.

4-4 What is heritability, and how does it relate to individuals and groups?

Heritability describes the extent to which variation among members of a group can be attributed to genes. Heritable individual differences (in traits such as height or intelligence) need not imply heritable group differences. Genes mostly explain why some people are taller than others, but not why people are taller today than they were a century ago.

4-5 How is molecular genetics research changing our understanding of the effects of nature and nurture?

Molecular genetics research on structure and function of genes is building new understandings of how teams of genes influence many human traits. One goal of molecular behavior genetics, the study of how the structure and function of genes interact with our environment to influence behavior, is to find some of the many genes that together orchestrate complex traits (such as body weight, sexual orientation, and impulsivity). Environments can trigger or block genetic expression. The field of epigenetics studies the influences on gene expression that occur without changes in DNA.

4-6 What are some benefits and risks of prenatal genetic testing?

Genetic tests can now reveal at-risk populations for dozens of diseases, and the search is on to discover the markers of genetically influenced disorders. But prenatal screening poses ethical dilemmas. For example, testing for an offspring’s sex has enabled selective abortions, which in some cultures has resulted in millions more male births. And future screening for vulnerability to psychological disorders could deprive the world of great talents. (Handel, van Gogh, Churchill, Lincoln, Tolstoy, and Dickinson were all troubled people, for example.)

Evolutionary Psychology: Understanding Human Nature

4-7 How do evolutionary psychologists use natural selection to explain behavior tendencies?

Evolutionary psychologists seek to understand how our traits and behavior tendencies are shaped by natural selection, as genetic variations increasing the odds of reproducing and surviving in their particular environment are most likely to be passed on to future generations. Some variations arise from mutations (random errors in gene replication), others from new gene combinations at conception. Humans share a genetic legacy and are predisposed to behave in ways that promoted our ancestors’ surviving and reproducing. Charles Darwin’s theory of evolution is an organizing principle in biology. He anticipated today’s application of evolutionary principles in psychology.

4-8 How might an evolutionary psychologist explain male-female differences in sexuality and mating preferences?

Men tend to have a recreational view of sexual activity; women tend to have a relational view. Evolutionary psychologists reason that men’s attraction to multiple healthy, fertile-appearing partners increases their chances of spreading their genes widely. Because women incubate and nurse babies, they increase their own and their children’s chances of survival by searching for mates with the potential for long-term investment in their joint offspring.

C-11

4-9 What are the key criticisms of evolutionary explanations of human sexuality, and how do evolutionary psychologists respond?

Critics argue that evolutionary psychologists start with an effect and work backward to an explanation. They also charge that evolutionary psychologists try to explain today’s behavior based on decisions our distant ancestors made thousands of years ago, noting that a better, more immediate explanation takes learned social scripts into account. And, the critics wonder, does this kind of explanation absolve people from taking responsibility for their sexual behavior? Evolutionary psychologists respond that understanding our predispositions can help us overcome them. They recognize the importance of social and cultural influences, but they also cite the value of testable predictions based on evolutionary principles.

Culture, Gender, and Other Environmental Influences

4-10 How do early experiences modify the brain?

Our genetic predispositions and our specific environments interact. Environments can trigger gene activity, and genetically influenced traits can evoke responses from others.

As a child’s brain develops, neural connections grow more numerous and complex. Experiences then prompt a pruning process, in which unused connections weaken and heavily used ones strengthen. Early childhood is an important period for shaping the brain, but throughout our lives our brain modifies itself in response to our learning.

4-11 In what ways do parents and peers shape children’s development?

Parents influence their children in areas such as manners and political and religious beliefs, but not in other areas, such as personality. As children attempt to fit in with their peers, they tend to adopt their culture—styles, accents, slang, attitudes. By choosing their children’s neighborhoods and schools, parents exert some influence over peer group culture.

4-12 How does culture affect our behavior?

A culture is an enduring set of behaviors, ideas, attitudes, values, and traditions shared by a group and transmitted from one generation to the next. Cultural norms are understood rules that inform members of a culture about accepted and expected behaviors. Cultures differ across time and space.

4-13 How do individualist and collectivist cultures differ in their values and goals?

Within any culture, the degree of individualism or collectivism varies from person to person. Cultures based on self-reliant individualism, like those found in North America and Western Europe, tend to value personal independence and individual achievement. They define identity in terms of self-esteem, personal goals and attributes, and personal rights and liberties. Cultures based on socially connected collectivism, like those in many parts of Asia and Africa, tend to value interdependence, tradition, and harmony, and they define identity in terms of group goals, commitments, and belonging to one’s group.

4-14 How does the meaning of gender differ from the meaning of sex?

In psychology, gender is the socially influenced characteristics by which people define men and women. Sex refers to the biologically influenced characteristics by which people define males and females. Our gender is thus the product of the interplay among our biological dispositions, our developmental experiences, and our current situation.

C-12

4-15 What are some ways in which males and females tend to be alike and to differ?

We are more alike than different, thanks to our similar genetic makeup—we see, hear, learn, and remember similarly. Males and females do differ in body fat, muscle, height, age of onset of puberty, life expectancy, and vulnerability to certain disorders. Men admit to more aggression than women do, and they are more likely to be physically aggressive. Women’s aggression is more likely to be relational. In most societies, men have more social power, and their leadership style tends to be directive, whereas women’s is more democratic. Women focus more on social connectedness, and they “tend and befriend.”

4-16 How do sex hormones influence prenatal and adolescent sexual development, and what is a disorder of sexual development?

Both sex chromosomes and sex hormones influence development. Biological sex is determined by the father’s contribution to the twenty-third pair of chromosomes. The mother always contributes an X chromosome. The father may also contribute an X chromosome, producing a female, or a Y chromosome, producing a male by triggering additional testosterone release and the development of male sex organs. During puberty, both primary and secondary sex characteristics develop. Sex-related genes and physiology influence behavioral and cognitive differences between males and females. Disorders of sexual development are inherited conditions that involve unusual development of sex chromosomes and anatomy.

4-17 How do gender roles and gender identity differ?

Gender roles, the behaviors a culture expects from its males and females, vary across place and time. Social learning theory proposes that we learn gender identity—our sense of being male, female, or some combination of the two—as we learn other things: through reinforcement, punishment, and observation. Critics argue that cognition also plays a role because modeling and rewards cannot explain gender typing. Some children organize themselves into “boy worlds” and “girl worlds”; others prefer androgyny. Transgender people’s gender identity or expression differs from their birth sex. Their sexual orientation may be heterosexual, homosexual, bisexual, or asexual.

4-18 What is included in the biopsychosocial approach to development?

Individual development results from the interaction of biological, psychological, and social-cultural influences. Biological influences include our shared human genome; individual variations; prenatal environment; and sex-related genes, hormones, and physiology. Psychological influences include gene-environment interactions; the effect of early experiences on neural networks; responses evoked by our own characteristics, such as gender and temperament; and personal beliefs, feelings, and expectations. Social-cultural influences include parental and peer influences; cultural traditions and values; and cultural gender norms.