Genetic Influence on Human Psychological Traits A Survey

C U R RE N T DI R EC TIO N S I N P SY CH O L O G I CA L SC I EN C E Genetic Influence on Human Psychological Traits A Survey Thomas J. Bouchard, Jr. University of Minnesota, Minneapolis ABSTRACT—There is now a large body of evidence that supports empirical fact and one not unique to humans. Lynch and Walsh (1998) the conclusion that individual differences in most, if not all, pointed out that genetic influence on most traits, as indexed by esti- reliably measured psychological traits, normal and abnormal, mates of heritability, is found for all species and observed that ‘‘the are substantively influenced by genetic factors. This fact has interesting questions remaining are, How does the magnitude of h2 important implications for research and theory building in differ among characters and species and why?’’ (p. 175). psychology, as evidence of genetic influence unleashes a cascade of questions regarding the sources of variance in such traits. A WHY STUDY GENETIC INFLUENCES ON HUMAN brief list of those questions is provided, and representative BEHAVIORAL TRAITS? findings regarding genetic and environmental influences are presented for the domains of personality, intelligence, psycho- A simple answer to the question of why scientists study genetic in- logical interests, psychiatric illnesses, and social attitudes. fluences on human behavior is that they want a better understanding These findings are consistent with those reported for the traits of of how things work, that is, better theories. Not too many years ago, other species and for many human physical traits, suggesting Meehl (1978) argued that ‘‘most so-called ‘theories’ in the soft areas of that they may represent a general biological phenomenon. psychology (clinical, counseling, social, personality, community, and KEYWORDS—behavior genetics; heritability; individual differ- school psychology) are scientifically unimpressive and technologi- ences cally worthless’’ (p. 806). He listed 20 fundamental difficulties faced by researchers in the social sciences. Two are relevant to the current discussion: heritability and nuisance variables. The two are closely Among knowledgeable researchers, discussions regarding genetic related. Nuisance variables are variables assumed to be causes of influences on psychological traits are not about whether there is ge- group or individual differences irrelevant to the theory of an investi- netic influence, but rather about how much influence there is, and how gator. Investigators seldom provide a full theoretical rationale in genes work to shape the mind. As Rutter (2002) noted, ‘‘Any dis- support of their choice of nuisance variables to control. As Meehl passionate reading of the evidence leads to the inescapable conclu- pointed out, removing the influence of parental socioeconomic status sion that genetic factors play a substantial role in the origins of (SES; i.e., treating it as a nuisance variable) on children’s IQ, when individual differences with respect to all psychological traits, both studying the causes of individual differences in IQ, makes the as- normal and abnormal’’ (p. 2). Put concisely, all psychological traits are sumption that parental SES is exclusively a source of environmental heritable. Heritability (h2) is a descriptive statistic that indexes the variance, as opposed to being confounded with genetic influence.1 degree of population variation in a trait that is due to genetic differ- Meehl argued that this example ‘‘is perhaps the most dramatic one, ences. The complement of heritability (1 h2) indexes variation but other less emotion-laden examples can be found on all sides in the contributed by the environment (plus error of measurement) to pop- behavioral sciences’’ (p. 810). His point was that knowledge of how ulation variation in the trait. Studies of human twins and adoptees, genetic factors influence any given measure (e.g., SES) or trait (e.g., often called behavior genetic studies, allow us to estimate the herit- IQ) will allow scientists to develop more scientifically impressive and ability of various traits. The name behavior genetic studies is an worthwhile theories about the sources of individual differences in unfortunate misnomer, however, as such studies are neutral regarding psychological traits. both environmental and genetic influences. That they repeatedly and Evidence of genetic influence on a psychological trait raises a se- reliably reveal significant heritability for psychological traits is an ries of new questions regarding the sources of population variance for that trait. All the questions addressed in quantitative genetics (Lynch Address correspondence to Thomas J. Bouchard, Jr., Psychology & Walsh, 1998) and genetic epidemiology (Khoury, 1998) become Department, University of Minnesota, 75 East River Rd., Min- 1 neapolis, MN 55455-0344; e-mail: [email protected]. See Evans (2004, Fig. 1) for a recent commission of this error. 148 Copyright r 2004 American Psychological Society Volume 13—Number 4  Thomas J. Bouchard, Jr. relevant. What kind of gene action is involved? Is it a simple additive sults from two twin studies of IQ in old age (over 75) are reported in influence, with the effects of genes simply adding up so that more Table 1. Both studies found a substantial level of genetic influence genes cause greater expression of the trait, or is the mode of action and little shared environmental influence. The results do, however, more complex? Are the effects of genes for a particular trait more suggest some decline in heritability when compared with results for pronounced in men or women? Are there interactions between genes earlier ages. There is no evidence for sex differences in heritability for and the environment? For example, it has been known for a long time IQ at any age. that stressful life events lead to depression in some people but not others. There is now evidence for an interaction. Individuals who carry a specific genetic variant are more susceptible to depression when Psychological Interests exposed to stressful life events than individuals who do not carry the Heritabilities for psychological interests, also called vocational or genetic variant (Caspi et al., 2003). Are there gene-environment occupational interests, are also reported in Table 1. These heritabil- correlations? That is, do individuals with certain genetic constitutions ities were estimated using data gathered in a single large study that seek out specific environments? People who score high on measures of made use of a variety of samples (twins, siblings, parents and their sensation seeking certainly, on average, tend to find themselves in children, etc.) gathered over many years. All respondents completed more dangerous environments than people who score low for this trait. one form or another of a standard vocational interest questionnaire. McGue and I have provided an extended list of such questions There is little variation in heritability for the six scales, with an av- (Bouchard & McGue, 2003). erage of .36. As with personality traits, there is evidence for nonad- ditive genetic influence. Unlike personality, psychological interests show evidence for shared environmental influence, although this in- ESTIMATES OF THE MAGNITUDE OF GENETIC INFLUENCE fluence is modest, about 10% for each trait. ON PSYCHOLOGICAL TRAITS Table 1 reports typical behavior genetic findings drawn from studies of Psychiatric Illnesses broad and relatively representative samples from affluent Western Schizophrenia is the most extensively studied psychiatric illness, and societies. In most, but not all, of these studies, estimates of genetic the findings consistently suggest a very high degree of genetic influ- and environmental influences were obtained from studies of twins. ence (heritability of about .80), mostly additive genetic influence, with Because the studies probably undersampled people who live in the no shared environmental influence. There do not appear to be gender most deprived segment of Western societies, the findings should not be differences in the heritability of schizophrenia. Major depression is considered as generalizable to such populations. (Documentation for less heritable (about .40) than schizophrenia. Men and women share most of the findings can be found in Bouchard & McGue, 2003.) most, but not all, genetic influences for depression. Panic disorder, generalized anxiety disorder, and phobias are moderately heritable, Personality and the effect is largely additive, with few if any sex differences. The Psychologists have developed two major schemes for organizing spe- heritability of alcoholism is in the range of .50 to .60, mostly because cific personality traits into a higher-order structure, the Big Five and of additive genetic effects. Findings regarding the possibility of sex the Big Three. As Table 1 shows, the findings using the two schemes differences in the heritability of alcoholism are mixed. are much the same. Genetic influence is in the range of 40 to 50%, Antisocial behavior has long been thought to be more heritable in and heritability is approximately the same for different traits. There is adulthood than childhood. The results of a recent analysis do not evidence of nonadditive genetic variance. That is, genes for person- support that conclusion. The genetic influence is additive and in the ality, in addition to simply adding or subtracting from the expression range of .41 to .46. Shared environmental influences decrease from of a trait, work in a more complex manner, the expression of a relevant childhood to adulthood, but do not entirely disappear in adulthood. gene depending to some extent on the gene with which it is paired on a There are no sex differences in heritability. chromosome or on genes located on other chromosomes. Research has yielded little evidence for significant shared environmental influence, that is, similarity due to having trait-relevant environmental influ- Social Attitudes ences in common. Some large studies have investigated whether the Twin studies reveal only environmental influence on conservatism up genes that influence personality traits differ in the two sexes (sex to age 19; only after this age do genetic influences manifest them- limitation). The answer is no. However, sometimes there are sex dif- selves. A large study (30,000 adults, including twins and most of their ferences in heritability. first-degree relatives) yielded heritabilities of .65 for males and .45 for females. Some of the genetic influence on conservatism is nonadditive. Recent work with twins reared apart has independently replicated Mental Ability these heritability findings. Conservatism correlates highly, about .72, Early in life, shared environmental factors are the dominant influence with right-wing authoritarianism, and that trait is also moderately on IQ, but gradually genetic influence increases, with the effects of heritable. shared environment dropping to near zero (see the twin studies in Religiousness is only slightly heritable in 16-year-olds (.11 for girls Table 1). Although not reported here, adoption studies of (a) unrelated and .22 for boys in a large Finnish twin study) and strongly influenced individuals reared together and (b) adoptive parents and their adopted by shared environment (.60 in girls and .45 in boys). Religiousness is offspring have reported similar results—increasing genetic influence moderately heritable in adults (.30 to .45) and also shows some shared on IQ with age and decreasing shared environmental influence. Re- environmental influence. Good data on sex differences in heritability Volume 13—Number 4 149  Genetic Influence on Human Psychological Traits TABLE 1 Estimates of Broad Heritability and Shared Environmental Influence and Indications of Nonadditive Genetic Effects and Sex Differences in Heritability for Representative Psychological Traits Nonadditive Shared Sex genetic environmental differences Trait Heritability effect effect in heritability Personality (adult samples) Big Five Extraversion .54 Yes No Perhaps Agreeableness (aggression) .42 Yes No Probably not Conscientiousness .49 Yes No Probably not Neuroticism .48 Yes No No Openness .57 Yes No Probably not Big Three Positive emotionality .50 Yes No No Negative emotionality .44 Yes No No Constraint .52 Yes No No Intelligence By age in Dutch cross-sectional twin data Age 5 .22 No .54 No Age 7 .40 No .29 No Age 10 .54 No .26 No Age 12 .85 No No No Age 16 .62 No No No Age 18 .82 No No No Age 26 .88 No No No Age 50 .85 No No No In old age ( > 75 years old) .54–.62 Not tested No No Psychological interests Realistic .36 Yes .12 NA Investigative .36 Yes .10 NA Artistic .39 Yes .12 NA Social .37 Yes .08 NA Enterprising .31 Yes .11 NA Conventional .38 Yes .11 NA Psychiatric illnesses (liability estimates) Schizophrenia .80 No No No Major depression .37 No No Mixed findings Panic disorder .30–.40 No No No Generalized anxiety disorder .30 No Small female only No Phobias .20–.40 No No No Alcoholism .50–.60 No Yes Mixed findings Antisocial behavior Children .46 No .20 No Adolescents .43 No .16 No Adults .41 No .09 No Social attitudes Conservatism Under age 20 years .00 NR Yes NR Over age 20 years .45–.65 Yes Yes in females Yes Right-wing authoritarianism (adults) .50–.64 No .00–.16 NA Religiousness 16-year-olds .11–.22 No .45–.60 Yes Adults .30–.45 No .20–.40 Not clear Specific religion Near zero NR NA NR Note. NA 5 not available; NR 5 not relevant. 150 Volume 13—Number 4  Thomas J. Bouchard, Jr. of religiousness in adults are not available. Membership in a specific & Wade, 2000). We also do not understand why most psychological religious denomination is largely due to environmental factors. traits are moderately heritable, rather than, as some psychologists expected, variable in heritability, with some traits being highly her- itable and others being largely under the influence of the environment. A Note on Multivariate Genetic Analysis It seems reasonable to suspect that moderate heritability may be a In this review, I have addressed only the behavior genetic analysis of general biological phenomenon rather than one specific to human traits taken one at a time (univariate analysis). It is important to psychological traits, as the profile of genetic and environmental in- recognize that it is possible to carry out complex genetic analyses of fluences on psychological traits is not that different from the profile of the correlations among traits and compute genetic correlations. These these influences on similarly complex physical traits (Boomsma, correlations tell us the degree to which genetic effects on one score Busjahn, & Peltonen, 2002) and similar findings apply to most or- (trait measure) are correlated with genetic effects on a second score, at ganisms. one or at many points in time. The genetic correlation between two traits can be quite high regardless of whether the heritability of either trait is high or low, or whether the correlation between the traits is Recommended Reading high or low. Consider the well-known positive correlation between Bouchard, T.J., Jr., & McGue, M. (2003). 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