Genes thus express themselves through covariation and Interaction with the environment. Because the value of a given heritability statistic is relevant only under
existing circumstances, the statistic does not and cannot address the modifiability of a trait. A trait could have a high level of heritability and nevertheless be highly modifiable. The heritability statistic pertains to correlations, whereas modifiability Inhibitors,research,lifescience,medical pertains to mean effects. For example, height has a heritability of over .90. Yet height also is highly modifiable, as shown by the fact that average heights have risen substantially over the past several generations. The heritability of intelligence is typically estimated as between .4 and .8.39 The value typically depends on the method used to estimate heritability, such as studies of degrees of relatedness (eg, identical vs fraternal twins) or identical twins reared apart. The studies are hard to interpret, in part because their assumptions are not always met. For example, identical Inhibitors,research,lifescience,medical twins reared apart are not randomly assigned to environments, so one cannot cleanly separate genetic from environmental variation. Matters are complicated by the fact that heritability estimates vary across populations. For example, estimates of the heritability of IQ in twin studies
carried out in the former Inhibitors,research,lifescience,medical Soviet Union tended to be higher than they were in comparable studies conducted within the United States.40 This observation
made sense in terms of our discussion above. Environmental variation in Russia under the Soviet regime was relatively constrained; Inhibitors,research,lifescience,medical most people lived in Cisplatin mouse roughly comparable environments. As a result, heritability estimates were higher. Most of the IQ heritability studies up to today have been carried out in nations within the developed world. Relatively little information exists regarding the heritability of IQ in the developing world, although what evidence there is suggests moderate heritability Inhibitors,research,lifescience,medical in these nations as well.41 Heritability also varies as a function of socioeconomic status (SES). Turkheimer and his colleagues have found that heritability is very substantially higher in higher Mephenoxalone SES families than in lower SES families. In particular, at the lowest levels of SES, shared environment accounted for almost all of the variation in IQs, whereas at the highest levels shared environment accounted for practically no variation.42 In sum, heritability estimates do not explain in any meaningful sense genetic regulation of human behavior. Furthermore, they do not provide accurate estimates of the strength of the genetic regulation. Rather, genes act within the context of environments and their effects must be understood within these contexts. Racial differences in intelligence Where does race fit into the genetic pattern we have been discussing above? (See refs 29,30; this section draws on collaborations with Elena Grigorenko, Kenneth Kidd, and Steven Stemler).
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