Appendix A Behavioral Genetics

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1 A01_ /4/07 12:31 PM Page A-1 Appendix A Behavioral Genetics Think about some trait that distinguishes you from other people, a trait on which you feel you are well above or well below average. Perhaps it relates to your skill at sports, languages, or music, or maybe to your fearfulness, sociability, or other aspects of your personality. Have you ever wondered what made you the way you are? If you are shy, for example, it is easy to think of possible environmental explanations. Perhaps you are shy because as a child you had few opportunities to meet new children or because you had embarrassing or unpleasant experiences when you did meet them. Maybe you have shy parents who served as the role models you imitated. Such environmental explanations are certainly reasonable, but it is also possible that you inherited a disposition toward shyness from your parents. It is even more likely that both inheritance and environment contributed to your shyness. Topics such as these are addressed by researchers in the field of behavioral genetics, the study of how genes affect behavior (see the chapter on research in psychology). These researchers have developed methods to explore genetic, as well as environmental, origins of behavioral differences among people. The results of behavioral genetics research make it clear that heredity has a significant influence, not just on shyness but on personality more generally, on cognitive abilities, on psychological disorders, and on many other aspects of human behavior and mental processes. However, behavioral genetics is just as much the study of environment as of genetics. In the process of trying to disentangle genetic from environmental factors, researchers have made several important discoveries about the impact of the environment. In this appendix, we discuss behavioral genetics in more detail than we did in the chapter on research in psychology. We begin with a review of the biochemical mechanisms underlying genetics and heredity. We then offer a brief history of genetic research in psychology, followed by a discussion of what research on genetic influences can and cannot tell us about the origins of human differences. Finally, we describe some findings from behavioral genetics research that illuminate several important aspects of human behavior and mental processes. The Biology of Genetics and Heredity genetics The biology of inheritance. chromosomes Long, thin structures in every biological cell that contain genetic information. genes The biological instructions, inherited from both parents and located on the chromosomes, that provide the blueprint for physical development. deoxyribonucleic acid (DNA) The molecular structure of a gene that provides the genetic code. What does it mean to say that someone has inherited some physical feature or behavioral trait? The answer lies in genetics, the biology of inheritance. The story begins with the biochemistry of the human body and with the chromosomes contained within each of the body s cells. Most human cells contain forty-six chromosomes, arranged in twenty-three matching pairs. These chromosomes are long, thin structures that are made up of thousands of genes. Genes are the biochemical units of heredity that govern the development of an individual by controlling the synthesis of protein. They are composed of deoxyribonucleic acid (DNA) strands of sugar, phosphate, and four kinds of nitrogen-containing molecules twisted around each other in a double spiral (see Figure 1). It is the particular order in which the four nitrogen-containing molecules are arranged in the DNA that determines, through the production of ribonucleic acid (RNA), which protein each gene will produce. Protein molecules, in turn, form the physical structure of each cell and also direct the activity of the cell. So as a function of DNA, the genes contain a coded message that provides a blueprint for constructing every aspect of a physical human being, including eye color, height, blood type, inherited disorders, and the like and all in less space than the period that ends this sentence. A-1

2 A02_ /4/07 12:31 PM Page A-2 A-2 APPENDIX A Behavioral Genetics The human body contains 100 trillion cells. There is a nucleus inside each human cell (except red blood cells). Each nucleus contains 46 chromosomes, arranged in 23 pairs. One chromosome of every pair is from each parent. The chromosomes are filled with tightly coiled strands of DNA. Genes are segments of DNA that contain instructions to make proteins the building blocks of life. G C T A G C A T G C C G A T FIGURE 1 Genetic Building Blocks Only about 3 percent of DNA contains genes that guide the production of proteins, but it nevertheless determines all aspects of our physical bodies, and influences many of our behavioral characteristics, too. Source: Genetic Building Blocks from Time, January 17, Copyright 1994 TIME, Inc. Reprinted with permission. polygenic A term describing characteristics that are determined by more than one gene. genotype The full set of genes, inherited from both parents, contained in twenty-three pairs of chromosomes. phenotype How an individual looks and acts, which depends on how inherited characteristics interact with the environment. New cells are constantly being produced by the division of existing cells. Most of the body s cells divide through a process called mitosis, in which the cell s chromosomes duplicate themselves so that each new cell contains copies of the twenty-three pairs of chromosomes in the original. A different kind of cell division occurs when a male s sperm cells and a female s egg cells (called ova) are formed. This process is called meiosis. In meiosis, the chromosome pairs are not copied. Instead, they are randomly split and rearranged, leaving each new sperm and egg cell with just one member of each chromosome pair, or twenty-three single chromosomes. No two of these special new cells are quite the same, and none contains an exact copy of the person who produced it. So at conception, when a male s sperm cell fertilizes the female s ovum, a truly new cell is formed. This fertilized cell, called a zygote, carries twenty-three pairs of chromosomes half of each pair from the mother and half from the father. The zygote represents a unique heritage, a complete genetic code for a new person that combines genes from both parents. As described in the chapter on human development, the zygote divides first into copies of itself and then into the billions of specialized cells that form a new human being. Whether or not genes express themselves in the individual who carries them depends on whether they are dominant or recessive. Dominant genes are outwardly expressed whenever they are present. Recessive genes are expressed only when they are paired with a similar gene from the other parent. For example, phenylketonuria (PKU) a disorder seen in about 1 in 10,000 newborns is caused by a recessive gene. When inherited from both parents, this gene disrupts the body s ability to control phenylalanine, an amino acid found in milk and other foods. As a result, this acid is converted into a toxic substance that can cause severe mental retardation. (Discovery of this genetic defect made it possible to prevent retardation in children with PKU simply by making sure they did not consume foods high in phenylalanine.) PKU is one of more than 4,000 single-gene disorders, but in fact, relatively few human characteristics are controlled by just one gene. Most characteristics are polygenic, meaning that they are controlled by many genes. Even a person s eye color and height are affected by more than one gene. The genes contained in the forty-six chromosomes inherited from parents make up an individual s genotype. Because identical twins develop from one fertilized egg cell, they are described as monozygotic; they have exactly the same genotype. So why don t all identical twins look exactly alike? Because they do not have exactly the same environment. An individual s phenotype is the set of observable characteristics that result from the combination of heredity and environment. In twins and nontwins alike, the way people actually look and act is influenced by the combination of genes they carry, as well as by environmental factors in other words, by both nature and nurture.

3 A03_ /4/07 12:31 PM Page A-3 A Brief History of Genetic Research in Psychology A-3 A Brief History of Genetic Research in Psychology The field now known as behavioral genetics began in the late 1800s with the work of Sir Francis Galton. A cousin of Charles Darwin, Galton was so impressed with Darwin s book on evolution that he decided to study heredity in the human species, especially as it relates to human behavior. Galton suggested the family, twin, and adoption study designs that are the mainstays of human behavioral genetics research today (see the chapter on research in psychology). He even coined the phrase naturenurture to refer to genetic and environmental influences. Galton s most famous behavioral genetics study was one in which he showed that genius runs in families. Unfortunately, Galton went too far in interpreting the evidence from this family study when he concluded that nature prevails enormously over nurture (Galton, 1883, p. 241). As noted in the chapter on research in psychology, family members can be similar to each other because of environmental, as well as hereditary, factors. So similarity seen among family members with regard to characteristics such as genius could be traced to the environments family members share, to their shared genes, or both. Still, Galton s work helped to focus psychologists interest on the influence of genetics and on the need to separate nature from nurture in drawing conclusions about why people resemble or differ from each other. The first two studies aimed at separating nature and nurture by studying twins and adoptees were conducted in Both focused on IQ, and both suggested the existence of an important genetic contribution to intelligence. However, research on the influence of genetics on behavior and mental processes was inhibited for a while because of two factors. The first was the impact of John B. Watson s behaviorism, which, as mentioned in the introductory chapter, suggested that we are only what we learn. In 1925, Watson insisted that there is no such thing as an inheritance of capacity, talent, temperament, mental constitution and characteristics. These things again depend on training that goes on mainly in the cradle (Watson, 1925, pp ). The second factor that discouraged attention to human genetics was its association with the view proclaimed by Adolf Hitler and his Nazis that certain groups of people were genetically inferior. This view led to the Holocaust during World War II, a campaign of genocide during which millions of Jews and other allegedly inferior people were killed. Genetic research on human behavior was reduced to a trickle during the 1930s and 1940s, but research with animals led, in 1960, to publication of the first behavioral genetics textbook (Fuller & Thompson, 1960) and to signs of increased interest in human genetics. In 1963 an influential article reviewed family, twin, and adoption findings and concluded that genetic factors are an important influence on IQ (Erlenmeyer-Kimling & Jarvik, 1963). Around the same time, the first adoption study of schizophrenia pointed to a strong genetic contribution to that disorder (Heston, 1966). In the early 1970s, however, interest in human behavioral genetics among psychologists faded again, this time because of reactions to two publications. The first was a paper by Arthur Jensen in which he suggested that differences in average IQ between blacks and whites might be partly due to genetic factors (Jensen, 1969). The second was a book by Richard Herrnstein in which he argued that genetics might contribute to social class differences (Herrnstein, 1973). The furious public and scientific response to these publications which included branding the authors as racists inhibited genetic research in psychology, even though very few behavioral geneticists were studying ethnic or class differences. It was not until later in the 1970s and into the 1980s that major genetic studies were again conducted in psychology. Today most psychologists recognize the role of both genetics and environment in behavior and mental processes, including the controversial area of cognitive abilities (Snyderman & Rothman, 1987). In fact, in 1992 the American Psychological

4 A04_ /4/07 12:31 PM Page A-4 A-4 APPENDIX A Behavioral Genetics Association selected behavioral genetics as one of two themes best representing the past, present, and especially the future of psychological research (Plomin & McClearn, 1993). To some, though, the study of human behavioral genetics still carries a hint of racism and class elitism. These concerns were resurrected about a decade ago by a book titled The Bell Curve, which considers the role of genetics in ethnic differences in intelligence and the implications of intelligence for social class structure (Herrnstein & Murray, 1994). Fortunately, reaction to that book has not altered the balanced perspective that recognizes the importance of nature as well as nurture in psychology (Neisser, 1997; Pinker, 2002). The Focus of Research in Behavioral Genetics Much of the controversy about behavioral genetics and about nature and nurture in general comes from misunderstandings about what behavioral genetics researchers study and, more specifically, what it means to say that genes influence behavior. For one thing, behavioral genetics is the study of genetic and environmental factors that are responsible for differences among individuals or groups of individuals, not for the characteristics of any single individual. Consider height, for example. Identical twins are much more similar in height than are fraternal twins (who share no more genes than other siblings), and individuals who are genetically related but raised separately are just as similar in height as are relatives who are raised together. Further, genetically unrelated individuals who are raised together are no more similar in height than random pairs of individuals. These data suggest, not surprisingly, that height is highly heritable. This means that much of the variability in height that we see among people actually about 80 percent of it can be explained by genetic differences among them rather than by environmental differences. (It does not mean that a person who is six feet tall grew 80 percent of that height because of genes and the other 20 percent because of environment!) It also follows that if a person is, say, shorter than average, genetic reasons are probably the primary cause. We say probably because finding a genetic influence on height involves referring only to the origins of average individual differences in the population. So although the difference in people s heights is attributable mainly to genetic factors, a particular person s height could be due mainly to an early illness or other growth-stunting environmental factors. For example, Hattie and Samantha Peters, a pair of identical twins, were exposed to a rare condition in their mother s womb that deprived Samantha of vital nutrients. As a result, Hattie is 5 feet 4 inches tall, but Samantha is only 4 feet 8 inches (Taggart, 2004). To see how the logic of behavioral genetics applies to conclusions about psychological characteristics, suppose a researcher found that the heritability of a certain personality trait is 50 percent. This result would mean that approximately half of the differences among people on that trait are attributable to genetic factors. It would not mean that each person inherits half of the trait and gets the other half from environmental influences. As in our height example, behavioral geneticists want to know how much variability among people can be accounted for by genetic and environmental factors. The results of their research allow generalizations about the influence of nature and nurture on certain characteristics, but those generalizations do not necessarily apply to the origin of a particular person s characteristics. Another misconception about genetic influences is that they are hard-wired and thus have inevitable effects. Complex traits intelligence, for example are influenced by many genes, as well as by many environmental factors. So genetic influence means just that influence (Plomin, Owen, & McGuffin, 1994). Genes can affect a trait without completely determining whether or not it will appear.

5 A05_ /4/07 12:31 PM Page A-5 The Role of Genetic Factors in Psychology In the following sections, we consider behavioral genetics research results that tell a little more of the story about how genes can have an impact on behavior and mental processes. Genetic Influences over the Life Span One particularly interesting finding about genetic influences on general cognitive ability is that these influences continue to increase throughout the life span (McGue et al., 1993; Plomin, 1986). That is, the proportion of individual differences (variance) in IQ scores that can be explained by genetic factors increases from 40 percent in childhood to 60 percent in adolescence and young adulthood and then to 80 percent later in life. This increase in the magnitude of genetic influence can be seen, for example, in the expanding difference between IQ correlations for identical twins and those for fraternal twins: Identical twins become more similar in IQ over the life span, whereas fraternal twins become less similar as the years go by. This finding is all the more interesting for overturning the common assumption that environmental influences become increasingly important as accidents, illnesses, and other experiences accumulate throughout life. How can it be that genetic influences become more important over time? One possible explanation is that, as discussed later, genetic predispositions lead people to select, and even create, environments that foster the continued development of their genetically influenced abilities. Genes Affecting Multiple Traits Behavioral genetics research has also revealed that genes affecting one trait can sometimes affect others as well. For example, it appears that the same genetic factors that affect anxiety also affect depression (Kendler, Neale, et al., 1992; Kendler et al., 2003). So if we could identify specific genes responsible for genetic influences on anxiety, we would expect to find that the same genes were associated with the appearance of depression. Similarly, genetic factors affecting substance dependence are highly correlated with genetic factors affecting antisocial behavior and impulsive style (Kendler et al., 2003; Krueger et al., 2002). A similar finding has emerged for cognitive ability and scholastic achievement. Tests of scholastic achievement show almost as much genetic influence as do tests of cognitive ability. Moreover, tests of scholastic achievement correlate substantially with tests of cognitive ability. To what extent is a common set of genes responsible for this overlap? Research suggests that the answer is almost entirely. It appears that the genes that influence performance on mental ability tests are the same ones that influence students performance at school (Wadsworth, 1994). Identifying Genes Related to Behavior The Role of Genetic Factors in Psychology A-5 One of the most exciting new developments in behavioral genetics involves identifying the specific genes responsible for genetic influences in psychology (Plomin & Crabbe, 2000). For example, there are hundreds of rare, single-gene disorders that affect behavior. One of these is Huntington s disease, an ultimately fatal disorder that involves loss of motor control and progressive deterioration of the central nervous system. Huntington s disease emerges only in adulthood, beginning with personality changes, forgetfulness, and other behavioral problems. It is caused by a single dominant gene whose identification in 1983 made it possible to determine who will get this disease even though the biochemical mechanisms underlying the disorder are still not fully understood and prevention is not yet possible.

6 A06_ /4/07 12:32 PM Page A-6 A-6 APPENDIX A Behavioral Genetics Researchers are also tracking down the several genes involved in the appearance of Alzheimer s disease. (As described in the chapter on biological aspects of psychology, this disease causes memory loss and increasing confusion in many older people.) One of these, a gene that contributes to the risk for late-onset Alzheimer s disease, was identified in This gene increases the risk for Alzheimer s disease about fivefold, but its presence is neither necessary nor sufficient for the disease to appear. That is, many people with Alzheimer s disease do not have the gene, and many people with the gene do not have the disease. Nonetheless, this gene is by far the strongest risk factor known for Alzheimer s disease, and its discovery marks the beginning of a new era in which specific genes or regions of DNA near specific genes will be identified as influencing disorders and psychological traits. Additional examples include reports of linkages between DNA and reading disability (Cardon et al., 1994) and hyperactivity (Faraone et al., 2001). Progress in identifying specific genes in humans has been slower than expected, in part because research ethics and common sense prevent the use of selective breeding. Accordingly, human studies have lacked the statistical power needed to detect relatively weak but still potentially important genetic influences on behavior (Plomin et al., 2002). However, the more powerful genetic research techniques available in studies of animals have identified several genes associated, for example, with fearfulness (Flint et al., 1995), with sensitivity to drugs such as alcohol (Crabbe et al., 1999), and with various aspects of learning and memory (Wahlsten, 1999). Today s efforts to identify genes related to human behavior are being aided by advances flowing from the Human Genome Project, which in early 2001 succeeded in identifying the sequence of most of the 3 billion letters of DNA in the human genome. One of the most surprising findings of that project so far is that the human genome appears to contain only about 20,000 to 25,000 genes less than half the number expected, and a number that is similar to the estimates for animals such as mice and worms (International Human Genome Sequencing Consortium, 2004). Does this smaller-than-expected number of human genes mean that there are too few to influence all aspects of human behavior, and that the environment (nurture) must be even more important than we thought in this regard? Not necessarily. It may be that the greater complexity seen in human behavior versus, say, mouse behavior stems not from the number of genes we have but from the greater complexities involved in decoding our genes into proteins. Human genes, more than the genes of other species, are spliced in alternative ways to create a greater variety of proteins. It may be this more subtle variation in genes not the number of genes that is responsible for differences between mice and people. This possibility has important implications for behavioral genetics because if the obvious differences between humans and other species are due to subtle DNA differences, then individual differences within our species in other words, among people are likely to involve genetic factors that are even more subtle and hard to find. Fortunately, new techniques are available that make it possible to detect DNA differences for many thousands of genes simultaneously. These techniques will help in identifying genes related to behavior, a process which will fill in the causal picture about a variety of characteristics and disorders that are influenced by many genes and many environmental factors. But it might not just be the actions of multiple genes that have a major impact on behavior. It might also be a gene-environment interaction the combination of a specific gene in a specific environment that has the greatest influence on behavior. Examples of such interactions appear in research by Avshalom Caspi and his colleagues (Caspi et al., 2002; Caspi et al., 2003). In one study, they found that children living in an environment in which there is abuse or other maltreatment were at increased risk for displaying antisocial behavior in later life. But the at-risk children who also had a particular gene did not become antisocial. It was as if the gene protected them against this common consequence of childhood maltreatment (Caspi et al., 2002). A second study showed that, although depression and suicide are often associated with stressful life events, variation in a particular gene could predict

7 A07_ /4/07 12:32 PM Page A-7 Behavioral Genetics and Environmental Influences A-7 whether people became depressed and suicidal in response to such events (Caspi et al., 2003). Under-standing how multiple genes combine to influence behavior and analyzing gene-environment interactions will continue to be active and exciting areas of research in behavioral genetics. Behavioral Genetics and Environmental Influences TRY THIS As suggested earlier, research on genetic influences in psychology has also provided some of the best evidence for the importance of environmental influences. It has shown that even though genetic influences are important, they cannot explain everything about human behavior. For example, twin and adoption studies have provided evidence of the importance of genetic factors in schizophrenia (Gottesman, 1991; Sullivan, Kendler, & Neale, 2003), and as a result, many researchers are now trying to identify the specific genes responsible. Enthusiasm for genetic explanations of schizophrenia makes it easy to forget, however, that environmental factors can be at least as important as genes. As described in the chapter on psychological disorders, when one member of an identical-twin pair is schizophrenic, the chances are about 40 percent that the other member of the pair is also schizophrenic, a rate that is much higher than the 1 percent rate of schizophrenia in the general population. This result surely provides evidence of a strong genetic contribution to schizophrenia, but it also suggests that schizophrenia is strongly influenced by the environment. After all, most of the time, the identical twin of a person with schizophrenia will not display the disorder. Such differences within pairs of identical twins can be due only to the operation of environmental factors. In fact, research generally suggests that genetic factors account for only about half of the variance among individuals for psychological characteristics such as personality and psychopathology. This means that at least half of the variance among individuals on these characteristics is due to environmental factors. These environmental or more properly, nongenetic factors encompass everything other than genetic inheritance. They include such biological factors as prenatal events, nutrition, and illnesses, as well as more traditional environmental factors such as parenting, schooling, and peer influences. In short, one of the most important findings to emerge from behavioral genetics research has concerned the environment, not genetics. That research suggests that the most important environmental influences are likely to be those that different family members do not share (Plomin, Asbury, & Dunn, 2001). Psychologists want to find out more about these nonshared factors and how they act to create differences in children twins or not who grow up in the same family (Turkheimer & Waldron, 2000). So far, research on this topic has shown that children may grow up in the same family but that they experience quite different environments, especially in relation to their parents (Brody, 2004). Siblings perceive that their parents treat them very differently and observational studies back up these perceptions of differential treatment (Plomin, Asbury, & Dunn, 2001). Even events such as parental divorce, which would seem to be shared by all children in the family, appear to be experienced differently by each child, depending especially on age, personality, and the nature of the relationship with each parent. Research is also beginning to focus on environmental influences beyond the family such as relationships with teachers or friends which are even more likely than home-related factors to vary among siblings. If you have a brother or sister, think about a psychological trait on which you and your sibling differ confidence, for example. Why do you think you two are different on that trait?

8 A08_ /4/07 12:32 PM Page A-8 A-8 APPENDIX A Behavioral Genetics Perhaps one of you experienced a loss of self-confidence when faced with the demands of an impatient grade-school teacher or after being betrayed by a childhood friend. Did these differing experiences occur randomly and thus make you considerably more confident or less confident than your sibling? Or is it possible that differences in your genetic makeups helped bring about these different experiences? Unless you and your sibling are identical twins, you share only about 50 percent of your genes. Perhaps genetically influenced differences between the two of you in emotionality or other aspects of temperament, for example caused parents, peers, and others to respond to each of you differently. This brings us to the second major discovery about the environment to emerge from research on behavioral genetics: Environmental influences associated with differences between siblings might actually be the result of genetic differences between the siblings. Most of the measures used by psychologists to assess what might be thought of as environmental factors have now been shown to be influenced by genetic factors (Plomin & Bergeman, 1991). These include measures such as adolescents ratings of how their parents treated them, observations of parent-child interactions, and questionnaires about life events and social support. If scores on measures such as these reflected only environmental factors, then the scores of identical twins should be no more similar to each other than those of fraternal twins. By the same token, there should be little similarity in environmental-experience measures for genetically related individuals who grew up in different families. The results reported by behavioral geneticists do not fit these expectations (Plomin, 1994; Reiss et al., 2000). For example, parents differ in terms of how responsive they are to their children, but these differences in responsiveness correlate with the children s cognitive ability a trait that has a clear genetic component. So, as described in the chapters on cognitive abilities and human development, parental responsiveness can influence cognitive development, and as behavioral genetics research suggests children s inherited cognitive abilities can alter the responsiveness of their parents. That is, parents tend to be more responsive to bright children who ask lots of questions and are interested in the answers. Outside the family, too, genetic factors appear to play a role in generating environmental experiences (Harris, 1998). For example, research on the characteristics of children s peer groups shows that children tend to choose their friends and to be chosen as friends partly on the basis of genetically influenced traits, such as mental ability and temperament (Manke et al., 1995). Several studies also suggest that genetic factors can increase or decrease the likelihood of family conflicts and other social stressors that threaten one s physical and psychological well-being (Reiss et al., 2000). In addition, genetic influences on personality can account for the genetic effects seen in adults reports about their family environments when growing up (Krueger, Markon, & Bouchard, 2003). An important implication of genetic influences on environmental events is that measuring the impact of family relationships, peer influences, and other environmental factors on behavior and mental processes may be less straightforward than psychologists thought. A measure that is aimed at assessing an environmental factor may nonetheless be affected by the genetic characteristics of the people being studied. In human development, nature and nurture work together. Children select, modify, and create environments that are correlated with their genetic inclinations. As developmental psychologists have long argued, children are not formless blobs of clay passively molded by the environment. Rather, they are active participants in their experiences. The new findings we have described here suggest that genetics plays an important role in those experiences (Plomin, 1994).

9 A09_ /4/07 12:32 PM Page A-9 Summary A-9 SUMMARY Behavioral genetics is the study of how genes affect behavior. The Biology of Genetics and Heredity Research on the ways in which nature and nurture interact to shape behavior and mental processes requires a knowledge of genetics, the biology of inheritance. The genetic code that transmits characteristics from one generation to the next is contained in the deoxyribonucleic acid (DNA) that makes up the genes that in turn make up chromosomes. Dominant genes are expressed whenever they are present; recessive genes are expressed only when inherited from both parents. Most human characteristics are controlled by more than one gene; they are polygenic. The genes in a person s forty-six chromosomes make up the genotype. The phenotype how people actually look and act is influenced by genes and the environment. A Brief History of Genetic Research in Psychology Sir Francis Galton s work in the nineteenth century helped to stimulate psychologists interest in the influence of genetics on behavior. The popularity of research in this area has waxed and waned over the years, but today most psychologists recognize the role of genetic, as well as environmental, influences on many aspects of behavior and mental processes. The Focus of Research in Behavioral Genetics Behavioral genetics research identifies the genetic and environmental factors responsible for differences among individuals, not for the characteristics of a particular person. Although genes can influence a trait, they may not completely determine whether that trait appears. The Role of Genetic Factors in Psychology Genetic factors probably influence, to some extent, every aspect of behavior and mental processes. Genetic Influences over the Life Span Genetic influences on general cognitive ability appear to increase over time, possibly because genetic predispositions lead people to select and even create environments that foster the continued development of abilities that are in line with those predispositions. Genes Affecting Multiple Traits Genes that affect one trait, such as anxiety, can sometimes also affect other traits, such as depression. Identifying Genes Related to Behavior Current research in behavioral genetics, aided by findings from the Human Genome Project, is identifying specific genes responsible for specific characteristics especially rare, single-gene disorders such as Huntington s disease. It is also illuminating gene-environment interactions. Behavioral Genetics and Environmental Influences Research in behavioral genetics has actually provided evidence for the importance of environmental influences, too, because the research shows that genetics alone cannot account for such characteristics as intelligence, personality, and psychological disorders. Some of the most important environmental influences are likely to be those that members of the same family do not share. In short, neither nature nor nurture is conducting the performance of the other: They are playing a duet.