Family members create for each other the social context in which family behavior and personality development take place. The importance of social influence in families is evidenced empirically by family members' great similarity on a wide variety of characteristics. Focusing on intrafamily...

Consistent with inclusive fitness theory, evolutionary biologists predict that individuals care more for their biological than their social children and hence that biological children assess the relationships to their parents better than stepchildren. To test this assumption, we use data from...