Mental Ability Books

“When studies using mental ability test scores from children are considered, the heritability of mental ability is typically found to be about .40, and the effect of the common or shared environment is found to be almost as strong, about .35. In contrast, when studies using mental ability test scores from adults (or older adolescents) are considered, estimates of the heritability of mental ability are much higher, typically about .65, whereas estimates of common or shared environment effects are much lower, probably under .20 (see review by Haworth et al., 2010). These findings indicate that differences among children in their levels of mental ability are attributable almost as much to their common environment—that is, to features of their family or household circumstances—as to their genetic inheritances. However, the findings also suggest that as children grow up, the differences among them in mental ability become less strongly related to the features of their common environments, and more strongly related to their genetic inheritances. In other words, the effect on one's mental ability of the family or household in which one is reared tends to become less important as one grows up, so that by adulthood one's level of mental ability is heavily dependent on one's genetic characteristics. It is as if one's level of mental ability—relative to that of other persons of the same age—can be raised (or lowered) during childhood by a particularly good (or poor) home environment, but then gradually returns to the level that one's genes tend to produce.

The aforementioned findings are based mainly on samples of participants who belong to the broad middle class of modern Western countries. There is some evidence, though, that the heritability of IQ tends to be somewhat lower (at least until young adulthood, and perhaps beyond) when studies are conducted using participants of less enriched environments, such as those in economically underdeveloped countries or in the lowest socioeconomic classes of some Western countries (see review by Nisbett et al., 2012). One recent study (Tucker-Drob & Bates, 2016) found that in the United States, additive genetic influences had a weaker influence on IQ among persons of low socioeconomic status than among persons of high socioeconomic status. (Interestingly, Tucker-Drob and Bates did not find this effect in western European countries or in Australia, where socioeconomic status differences tend to be smaller.) The above findings suggest that whenever the heritability of IQ is discussed, it is important to consider the ages of the persons being examined as well as their socioeconomic status and their country.”
― Michael C. Ashton, Individual Differences and Personality