Twin Studies , Heritability , and Intelligence Letters from

Unraveling the sources of phenotypic variability for human behavioral traits has been a notoriously elusive goal. Attempts to determine the influence of genes on traits such as general cognition are frustrated by the fact that we know little about how genes and environment combine to shape a developing individual. Although this influence may be quantified in a number of ways, we will use heritability in the narrow sense ( ), which measures the additive contribution of genes to the variation in a trait observed among members of a specific population. Given a particular model, the heritability of a trait may be estimated from familial correlations, as in the recent study of 240 pairs of elderly twins by Gerald E. McClearn . (Reports, ). However, there are many possible models, and we have little information about which is the most appropriate for a given trait. 1