THE HERITABILITY OF IQ

IQ heritability, the portion of a population's IQ variability attribut able to the effects of genes', has been investigated for nearly a cent ury, yet it remains controversial. Covariance between relatives may be due not only to genes, but also to shared environments, and most prev ious models have assumed different degrees of similarity induced by en vironments specific to twins, to non-twin siblings (henceforth sibling s), and to parents and offspring. We now evaluate an alternative model that replaces these three environments by two maternal womb environme nts, one for twins and another for siblings, along with a common home environment. Meta-analysis of 212 previous studies shows that our 'mat ernal-effects' model fits the data better than the 'family-environment s' model. Maternal effects, often assumed to be negligible, account fo r 20% of covariance between twins and 5% between siblings, and the eff ects of genes are correspondingly reduced, with two measures of herita bility being less than 50%. The shared maternal environment may explai n the striking correlation between the IQs of twins, especially those of adult twins that were reared apart. IQ heritability increases durin g early childhood, but whether it stabilizes thereafter remains unclea r. A recent study of octogenarians(2), for instance, suggests that IQ heritability either remains constant through adolescence and adulthood (3), or continues to increase with age(2). Although the latter hypothe sis has recently been endorsed(4), it gathers only modest statistical support in our analysis when compared to the maternal-effects hypothes is. Our analysis suggests that it will be important to understand the basis for these maternal effects if ways in which IQ might be increase d are to be identified.