In general, behavioral geneticists study our differences, such as those originating in heredity. Genes, the basic units of heredity, are located on chromosomes, which consist of strands of DNA. Our genes, together with noncoding DNA, make up the human genome. Most human traits depend on more than one gene pair, which makes tracking down the genetic contributions to a trait extremely difficult. However, advances in technology now permit scientists to carry out genome-wide association studies, examining variations in many DNA elements at once, and even whole-genome sequencing, which examines the entire 3 billion base pairs of DNA. But locating a gene does not automatically tell us what it does or how it does it, or how multiple genes interact and influence behavior.
The genome changes over time, because of mutations that arise before or after birth, and because of epigenetic changes that affect the expression (activity) of specific genes without altering the sequence of bases in those genes. Mutations and epigenetic changes can be affected by environmental factors.
In general, evolutionary psychologists study our commonalities and argue that many fundamental human similarities can be traced to the processes of evolution, especially the process of natural selection. They draw inferences about the behavioral tendencies that might have been selected because they helped our forebears solve survival problems and enhanced reproductive fitness; they then conduct research to see if such tendencies actually exist throughout the world.
Many evolutionary psychologists believe that the mind is not a general-purpose computer, but instead evolved as a collection of specialized mental modules to handle specific survival problems. Among the candidates for such modules are inborn reflexes, an attraction to novelty, a motive to explore and manipulate objects, an impulse to play, and the capacity for certain basic cognitive skills, including a rudimentary understanding of number. However, because some behavior or trait exists does not necessarily mean that it is adaptive or the product of natural selection.
Sociobiologists and evolutionary psychologists argue that males and females have evolved different sexual and courtship strategies in response to survival problems faced in the distant past. In this view, it has been adaptive for males to be promiscuous, to be attracted to young partners, and to want sexual novelty; and for females to be monogamous, to be choosy about partners, and to prefer security to novelty.
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; and that the evolutionary emphasis on the Pleistocene Age may not be warranted. 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 and proximity. The central issue dividing evolutionary theorists and their critics is the length of the “genetic leash.”
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; for example, heritability is higher for children in affluent families than in impoverished ones. And even highly heritable traits can often be modified by the environment.
Behavioral geneticists often study differences among individuals by using data from studies of adopted children and of identical and fraternal twins. By comparing the genetic and environmental “overlap” across these groups, researchers can estimate the heritability of a trait.
Heritability estimates for intelligence (as measured by tests of one's intelligence quotient, or IQ) average about .40 to .50 for children and adolescents and .60 to .80 for adults. Identical twins are more similar in IQ-test performance than fraternal twins, and adopted children's scores correlate more highly with those of their biological parents than with those of their nonbiological relatives. These results do not mean that genes determine intelligence; the remaining variance in IQ scores must be due largely to environmental influences.
It is a mistake to draw conclusions about group differences from heritability estimates based on differences within a group. The available evidence fails to support genetic explanations of black–white differences in performance on IQ tests.
Environmental factors such as poor prenatal care, malnutrition, exposure to toxins, and stressful family circumstances are associated with lower performance on intelligence tests. Conversely, a healthy and stimulating environment, as well as certain kinds of enrichment activities, can improve performance. IQ scores have been rising in many countries for several generations, most likely because of improved education, better health, and the increase in jobs requiring abstract thought.
The interaction between genes and environment is far more complex than anyone once imagined. Genes influence which environments people find most congenial, and environmental factors influence the genome by their effects on mutations and epigenetic changes. Development of a person is the result of a dynamic dialogue between genes and the environment—plus the addition of chance events. Genetic and environmental influences blend and become indistinguishable in the development of any one person.