Abstract
This paper examines how genetic, shared environment, and nonshared environment influence individual differences in close relationships. First, a brief explanation of behavioural genetic methods is given. Next, we review behavioural genetic studies of the close relationships in which people are typically involved (parent-child, sibling, friend, and spouse). Many of the studies reviewed indicate a modest to moderate role for genetic factors. This suggests that genetic make-up can affect the nature of close relationships. It also implies that genetic factors may be involved in the influence of close relationships on individual adaptation. Behavioural genetic research has begun to show that this is the case. Close relationships may be especially important sources of nonshared environment. Behavioural genetic research consistently shows that environmental factors that have a functional effect on psychological traits work to make siblings in the same family different from one another rather than similar. Attempts to pinpoint specific aspects of the nonshared environment are under way. Parent-child relationships have been systematically explored, and found not to play an important role in nonshared environment. This leaves open the possibility that extrafamilial relationships, in particular friendships, may be important contributors to the nonshared environment.