Genetic and environmental influences on adult intelligence and special mental abilities.

I review representative biometric studies of adult twins and adoptees that have been used to generate estimates of genetic and environmental influence on intelligence (IQ) and special mental abilities. The various studies converge on a heritability estimate between 0.60 and 0.80 for IQ. Estimates of common environmental influence from the same studies are near zero. Studies of twins reared together and studies of unrelated individuals reared together yield sizable estimates of common family environmental influence in childhood but also demonstrate that this influence dissipates with age and approaches zero in adulthood. Twin studies of the major special mental abilities (verbal, spatial, perceptual speed and accuracy, memory) yield heritability estimates of about 0.50 and modest estimates of common environmental influence.