Genetic Influence on Human Psychological Traits A Survey

There is now a large body of evidence that supports the conclusion that individual differences in most, if not all, reliably measured psychological traits, normal and abnormal, are substantively influenced by genetic factors. This fact has important implications for research and theory building in psychology, as evidence of genetic influence unleashes a cascade of questions regarding the sources of variance in such traits. A brief list of those questions is provided, and representative findings regarding genetic and environmental influences are presented for the domains of personality, intelligence, psychological interests, psychiatric illnesses, and social attitudes. These findings are consistent with those reported for the traits of other species and for many human physical traits, suggesting that they may represent a general biological phenomenon. KEYWORDS—behavior genetics; heritability; individual differences Among knowledgeable researchers, discussions regarding genetic influences on psychological traits are not about whether there is genetic influence, but rather about how much influence there is, and how genes work to shape the mind. As Rutter (2002) noted, ‘‘Any dispassionate reading of the evidence leads to the inescapable conclusion that genetic factors play a substantial role in the origins of individual differences with respect to all psychological traits, both normal and abnormal’’ (p. 2). Put concisely, all psychological traits are heritable. Heritability (h) is a descriptive statistic that indexes the degree of population variation in a trait that is due to genetic differences. The complement of heritability (1 h) indexes variation contributed by the environment (plus error of measurement) to population variation in the trait. Studies of human twins and adoptees, often called behavior genetic studies, allow us to estimate the heritability of various traits. The name behavior genetic studies is an unfortunate misnomer, however, as such studies are neutral regarding both environmental and genetic influences. That they repeatedly and reliably reveal significant heritability for psychological traits is an empirical fact and one not unique to humans. Lynch and Walsh (1998) pointed out that genetic influence on most traits, as indexed by estimates of heritability, is found for all species and observed that ‘‘the interesting questions remaining are, How does the magnitude of h differ among characters and species and why?’’ (p. 175). WHY STUDY GENETIC INFLUENCES ON HUMAN