The Genetic Determination of Differences in Intelligence: A Study of Monozygotic Twins Reared Together and Apart

Abstract

In the course of a series of investigations into various aspects of mental inheritance an intensive study has been made of so-called 'identical' twins. The cases examined fall into two main groups: first, those reared together in their parents' homes; secondly, those separated in early infancy, and brought up apart. With the latter, despite wide differences in environmental conditions, the correlations for intelligence, unlike those for school attainments, prove to be surprisingly high. It is argued that this implies that 'intelligence', when adequately assessed, is largely dependent on genetic constitution. Supplementary correlations from other groups (dizygotic twins, siblings reared together and apart, first and second cousins, and other related pairs) confirm this conclusion.

Model-fitting approaches to the analysis of human behaviour

“…However, there are several reasons for caution. Firstly, the similarity of adopted individuals and their foster-parents is not zero either in the earlier studies (Burt, 1966;Jencks et al, 1972) nor in the more recent studies of Scarr and Weinberg (1976). If such similarity is not entirely due to placement, then there must be environmental factors in parentoffspring similarity which will lead to the natural parent-offspring correlation being an overestimate of the hereditary similarity of parents and offspring.…”

“…Eaves ' (1975) reanalysis of the correlations of Burt (1966) demonstrated that there was little reason to treat the small foster-parent-foster-child correlation as differing significantly from zero and thus to regard the whole of the natural parent-child similarity as an estimate of genetical covariation. All family environmental effects on siblings and twins were thereby assigned to factors which did not depend on the IQ of parents.…”

Testing models for variation in intelligence

“…No such limit exists in fact so his conclusions cannot be discounted on this basis. More critical, however, was Jinks and Eaves' re-analysis of published correlations (Burt, 1966; Jencks, 1973) which demonstrated that any signficant heterogeneity of heritability estimates obtained from different degrees of relationship can be removed if the contribution of dominance is precisely specified and a weighted least squares procedure is adopted.The correlations analysed have already been tabulated (Jinks and Eaves, 1974), but details of the model and estimation procedure were necessarily omitted and are considered here. The model (table I) is that of Fisher (1918) for the correlations between relatives for a population in equilibrium under assortative mating with additions to specify the contribution of certain plausible environmental components.…”

“…No such limit exists in fact so his conclusions cannot be discounted on this basis. More critical, however, was Jinks and Eaves' re-analysis of published correlations (Burt, 1966; Jencks, 1973) which demonstrated that any signficant heterogeneity of heritability estimates obtained from different degrees of relationship can be removed if the contribution of dominance is precisely specified and a weighted least squares procedure is adopted.…”

Editorial Commentary

“…Much has happened in the field of intelligence genetics since the days (and failings) of Burt (1966) and Kamin (1974), Tryon (1940) and Searle (1949), as consulting a recent genetics textbook readily reveals (Vogel and Motulsky 1986). In case there is after all genetic variability, Macphail seems to have a ready backup argument: since the laws of causality are the same for all nonhuman vertebrates (indeed for all organisms), convergent selection pressure has seen to it that all are equally good at detecting them.…”