Twin Studies (Psychology)

The scientific study that attempts to determine the relative contributions of genes and the environment to behavior and/or personality characteristics. Because twins usually share the same environment and are treated similarly by parents, investigators can compare the similarities seen in identical twins, who share both genes and the environment, with those in nonidentical twins, who share the same environment but have different genetic makeups. If identical twins are more alike in some characteristics than are nonidentical twins, such characteristics may have a strong genetic component. In studies of intelligence, it has generally been found that identical twins are more similar to each other than they are to other siblings. Likewise, fraternal twins are less similar to each other than are identical twins. Twin studies also include adoption studies of identical twins reared apart, which allow the role of heredity to be highlighted. The most famous of such studies was the Minnesota Study of Twins Reared Apart, published in 1990. The Minnesota study reported that heredity was found to account for 70% of the variance in intelligence scores; however, these findings are limited to the extent that the researchers could not control for differences across adopted families. Most researchers consider the variance from inheritance to be closer to 50%. For more information, see Bouchard, Lykken, McGue, Segal, and Tellegan (1990).