Genetic and environmental influences on adult intelligence and special mental abilities

Human Biology, Apr 1998 by Bouchard, Thomas J Jr

The special mental ability findings from MISTRA based on MZ and DZ twins reared apart are shown in Table 4, where they are analyzed as a fourgroup design using data from MZ and DZ twins reared together. The MZ and DZ twin data for twins reared together are from an update of the meta-analysis initially carried out by Nichols (1978) and updated by Bouchard, Segal et al. (1990). The analysis was carried out on the weighted mean of the correlations using the mean sample size of the studies [see Bouchard, Segal et al. (1990, Tables 5-8)]. There were 43-45 samples (some studies contributed more than 1 sample) for verbal ability, 54 or 55 samples for spatial ability, 20 or 21 samples for perceptual speed and accuracy, and 16 samples for memory ability.

The data suggest that the heritability of the major mental abilities hovers between 0.50 and 0.55. Both studies of twins reared apart, however, suggest somewhat lower heritabilities for memory abilities. Different memory tests appear to yield different heritabilities [see Thapar, Petrill et al. (1994) for a brief review]. A detailed analysis of four Wechsler Adult Intelligence Scale (WAIS) subtests, including digit span using the Minnesota Study of Aging twin sample and the SATSA twins reared together, also suggests a lower heritability for memory relative to other special mental abilities (Finkel and McGue 1993; Finkel et al. 1995). More research is clearly needed in this area, given the theoretical importance of memory for understanding g (Kyllonen 1996; Kyllonen and Christal 1990).

Cross-Cultural Studies of Special Mental Abilities. A recent study of twins, aged 15-19 years old, in Croatia suggests that the heritability of verbal and spatial abilities is similar to that found in the United States (Bratko 1996). A comparable study in Egypt, however, reports lower heritabilities (AbdelRahim et al. 1990). Cross-cultural work in this area is still in its infancy. Other relevant studies include Park et al. (1978), Wilson and Vandenberg (1978), DeFries et al. (1974) and Johnson et al. (1976).

Genetic Influences on Special Mental Abilities As a Function of Their g Loading

Inbreeding Depression. There is now considerable evidence that, as predicted from genetic theory, inbreeding depresses the mean scores of inbred individuals on mental ability tests (Afzal 1988; Agrawa et al. 1984; Badaruddoza and Afzal 1993; Bashi 1977). Neel and his colleagues have also shown that inbreeding effects are as strong for mental abilities as they are for physical traits (Neel et al. 1970; Schull 1995; Schull and Neel 1965, 1972). A number of reports show that there is a significant correlation between the degree to which a cognitive ability loads on g and the degree of inbreeding depression that it shows (Jensen 1983, 1987; Nagoshi and Johnson 1986; Vernon 1989). Kamin (1980) severely criticized the early literature on inbreeding and IQ, and the newer studies are far from perfect (Bouchard 1993), but they are consistent.

Resemblance of Collateral Relatives on Special Mental Abilities As a Function of g Loadings. In line with the inbreeding findings, Johnson and Nagoshi (1990), using the large Hawaii Family Study of Cognition (HFSC) sample (DeFries et al. 1976, 1979; DeFries and Plomin 1978), showed that resemblance of biological collateral relatives (uncles and aunts and nephews and nieces with a one-fourth additive genetic relationship) is correlated with the g loading of special mental abilities, whereas there is no significant correlation for the same unrelated collateral relatives. This same study also showed that the correlations for cousins (who have only a one-eighth additive genetic relationship) are somewhat higher (mean of 0.16 vs. 0.20). Johnson and Nagoshi (1990) suggested that this latter finding is perhaps due to a strong environmental cohort effect.