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

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.