The comparative study of identical and fraternal twins is a powerful method for identifying the relative contributions of genes and environments to individual differences in behavioral, physical, and health-related characteristics. Studies of reared-apart twins provide even more compelling ways to approach this same class of questions, given that the co-twins were raised in different homes, communities and/or countries, so were unable to influence one another.

This talk first reviews the different types of twins and the logic behind twin designs before presenting findings from past, recent and current studies of twins who grew up separately. Here, the focus will be on the Minnesota Study of Twins Reared Apart and the Fullerton Study of Chinese Twins; however, research using reared together twins, virtual twins (same-age-unrelated individuals reared together who replicated twinship but without a genetic link) and unrelated look-alikes will also be described with reference to family relations, tacit coordination, personality similarity, and bereavement.

The vast body of evidence indicates that genetic influence is pervasive, affecting virtually every measured human trait, but environmental factors also play a role. The occasional abuse of twin studies must be recalled to prevent their future occurrence.