Mental Ability Quotes

“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

“Another socially important variable associated with mental ability is that of marriage, or more specifically that of who marries whom. A consistent finding in several studies of the characteristics of spouses is that there is a tendency for spouses to be similar in some—but not all—aspects of mental ability; in other words, some aspects of mental ability do show substantial “assortative mating.”

For example, in a study of married couples, Watson et al. (2004) examined spouses' scores on two mental ability tasks—a vocabulary test and a matrix reasoning test. Interestingly, even though vocabulary and matrix reasoning tend to be correlated with each other (both are strongly g-loaded tests), they revealed quite different results when correlations between spouses were considered. On the one hand, wives' and husbands' levels of vocabulary showed a fairly strong positive correlation, about .45.

But, on the other hand, wives' and husbands' levels of matrix reasoning were correlated only about .10. This result is consistent with previous findings, in which spouses have tended to show quite similar levels of verbal comprehension ability, but no particular similarity in mathematical reasoning ability (e.g., Botwin, Buss, & Shackelford, 1997).

Why should it be the case that spouses tend to be similar in verbal abilities, but not so similar in (equally g-loaded) nonverbal reasoning abilities? One likely explanation—as you might guess—is that two people will tend to have more rewarding conversations if they have similar levels of verbal ability, but that similar levels of nonverbal or mathematical reasoning ability are unlikely to contribute in an important way to any aspect of relationship quality.”
― Michael C. Ashton, Individual Differences and Personality

“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