Reconsidering the heritability of general cognitive ability in adults: taking into account assortative mating and cultural transmission

AbstRAct Heritability estimates of general cognitive ability in adulthood generally range from 75% to 85%, with all heritability due to additive genetic influences, while genetic dominance and shared environmental factors are absent or too small to be detected. These estimates are derived from studies based on the classical twin design (CTD) and are based on the assumption of random mating among others. Yet, considerable positive assortative mating has been reported for general cognitive ability. Unmodelled assortative mating increases the DZ twin correlation and thereby inflates the estimates of shared environmental factors and deflates estimates of genetic dominance in a CTD. The reported absence of both these effects for general cognitive ability in adulthood may be due to the presence of assortative mating that is not accounted for. In the present study, an extended twin family design was used to allow modeling of effects of assortative mating, additive genetic factors, genetic dominance and shared and non-shared environmental factors. Psychometric IQ data were available for adult monozygotic and dizygotic twins, their siblings, the partners of the twins and siblings, and either the parents or the adult offspring of the twins and siblings (N=1314). Results showed that variation of general cognitive ability in adulthood is not only due to additive genetic factors (44%) and non-shared environmental factors (18%), but also to genetic dominance (27%) and positive assortative mating (11%). Considering non-additive effects in future gene finding studies might enhance their success for general cognitive ability. heritability of general cognitive ability in adults: taking into account assortative mating and cultural transmission. In revision. reconsidering the heritability of general cognitive ability in adults 27 IntRoductIon Considerable evidence from classical twin studies shows that individual differences in general cognitive ability in adults are largely explained by genetic factors. These estimates are based on the classical twin design (CTD) in which the phenotypic resemblances of monozygotic (MZ) and dizygotic (DZ) twins are compared (Plomin et al., 2001a). MZ twin correlations of ~.80 and DZ twin correlations of ~.40 are typically reported and suggest absence of both shared environmental influences and genetic dominance. At the same time there is evidence for strong assortative mating on general cognitive ability, i.e., non-random mating of spouse pairs. Spousal correlations for general cognitive ability range from. Assortative mating for a heritable trait leads to a non-random distribution of genetic variants important for that trait as spouses will be more similar genetically than …