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“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.”
― Michael C. Ashton, Individual Differences and Personality