The etiology of high and low cognitive ability during the second half of the life span

Abstract

Although there is much evidence of genetic influence on individual differences in cognitive ability, little is known about genetic and environmental contributions to the high and low extremes of intelligence. The etiology of high and low cognitive ability in adulthood was explored by applying the DeFries and Fulker (1985, 1988) multiple regression analysis to data from subjects selected from a sample of 302 twin pairs ( M age = 65.6) for scores ± 1 standard deviation on a measure of general cognitive ability in the Swedish Adoption/Twin Study of Aging (SATSA). Estimates of group heritability, the extent to which a mean difference on a quantitative measure between a selected group and the unselected population is heritable, were significant for both high and low ability ( h 2 g =.73 High; h 2 g =.77 Low); and were similar in magnitude to the heritability of individual differences in the full sample. However, the estimate of group heritability for low ability was constrained using a nonadditive genetic model. These results replicated in an analysis of follow-up test scores obtained 3 years later. Thus, the etiology of high and low ability may be the same as the etiology in the normal range for this Swedish sample of adult twins.