GENETIC-INFLUENCES AND ENVIRONMENTAL-INFLUENCES ON ADULT-INTELLIGENCEAND 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 vari ous studies converge on a heritability estimate between 0.60 and 0.80 for IQ. Estimates of common environmental influence from the same stud ies are near zero. Studies of twins reared together and studies of unr elated individuals reared together yield sizable estimates of common f amily environmental influence in childhood but also demonstrate that t his influence dissipates with age and approaches zero in adulthood. Tw in studies of the major special mental abilities (verbal, spatial, per ceptual speed and accuracy, memory) yield heritability estimates of ab out 0.50 and modest estimates of common environmental influence.