Heredity and individual differences in behavior
The preceding five chapters have documented the evidence for the inheritance of behavior. Data from many species and many types of investigation support the view that hereditary influences upon behavior are not exceptional but are almost universal. The exact nature of the relationship between nature and nurture in the production of individual differences is, however, by no means settled. Behavioral characters are not evoked by particular genes as neatly as blood types or coat colors, although the notion of congruence between particular genes and particular modes of behavior still persists in some quarters.
The general issue of the significance of genetic contributions to individual differences may be approached in two ways, through population genetics and through physiological genetics. The first method seeks an answer to the question, "How much does genetics contribute?"; the second is oriented to the inquiry, "How does genetics make its contribution?" (See Anastasi, 1958b.) The first question has no logical meaning when applied to an individual, for his whole genotype and total life experience contribute to every aspect of his behavior, and their influences cannot be separated. With respect to populations, however, one may well ask how much of the observed variation in behavior is attributable to genetic differences and how much to environment. In the language of genetics this is equivalent to determining the heritability of a trait. The fact that heritability is an attribute of populations rather than of traits has been emphasized earlier (page 64). In a heterogeneous colony of laboratory rats, for example, one might expect to find significant heritability of activity. The heritability of the same trait in a closely inbred colony would probably be zero, though the rats might be quite variable. Yet in both colonies genes would affect activity level through the same physiological mechanisms, and the activity of the inbred animals would