But it is not meaningful to speak of the heritability of people’s having two eyes, because there are no individual differences in the trait. Heritability is not 1; rather, it is meaningless (because there is a 0 in the denominator of the ratio) and cannot be sensibly calculated. Now consider a second complementary example, occupational status. Tills attribute has a statistically significant

heritability coefficient,37 but it is certainly not under direct INCB018424 mw genetic control. Clearly there Inhibitors,research,lifescience,medical is no gene or set of genes for occupational status. Rather, the effect is indirect and mediated by attributes such as intelligence, personality, and Interpersonal attractiveness that themselves are under some degree of Inhibitors,research,lifescience,medical genetic control. The effects of genes thus are, at best, indirect. Other attributes, such as divorce, may run in families—that is, show familiality—but again,

they also are not under direct but rather under indirect genetic control. Heritability has no fixed value for a given attribute such as intelligence. Although we may read about “the heritability of IQ” (which, according to most theories, is not exactly the same as intelligence), there is no single fixed value of heritability that represents some true, constant value for the heritability of IQ or anything else. Heritability is dependent on Inhibitors,research,lifescience,medical numerous factors, but the most important single factor is the range of environments. Because heritability represents a proportion of variation, its value will depend on the amount of variation. As Herrnstein pointed out, if there were no variation at all in the environments in which people lived, heritability would be 1, because there Inhibitors,research,lifescience,medical would be no other source of variation. If there is wide variation in environments, however, heritability is likely to decrease. In speaking of heritability, we must remember that genes always operate within environment contexts. All genetic effects occur within a reaction range. As a result, environment will likely have differential effects Inhibitors,research,lifescience,medical on

the same genetic structure. The reaction range is the range of phenotypes (all observable effects) that a given genotype (latent structure of genes) for any these particular attribute can produce, given the interaction of environment with that genotype. For example, genotype determines a reaction range for the possible heights a particular person can attain, but other factors, such as diseases, childhood nutrition, and the like may affect the adult height that is attained. Furthermore, If different genotypes respond differently to environmental variation, heritability will differ depending on the mean and variance of relevant factors in the environment.38 Thus, the statistic is not a fixed, constant value. There exist no purely genetic effects on behavior, as would be demonstrated dramatically if a child were raised In a small closet with no stimulation.