Heredity and familial environment in intelligence and educational level--a sibling study.

Although it is well established that family members resemble each other in intelligence , the extent to which this results from either shared genes or a shared environment remains controversial, perhaps especially since the relevant evidence presented by Burt has been shown probably to have been fabricated. The influence of heredity and familial environment may be distinguished by studying adoptees. Here we present correlations in intelligence and educational level between genetically related pairs of adult adoptees who have been reared separately, and, conversely, between genetically unrelated pairs of adult adoptees who have been reared together. We are unaware of any previous study of adults which has reported on both of these types of relationship. The results for intelligence conform closely to what would be predicted by a simple polygenic model of genetic transmission whereas those for educational attainment imply both genetic and familial environmental components.