By the age of 2 years, children with a manic-depressive parent were already found to be experiencing substantial psychiatric problems. The authors examined the ways in which these early problems then become embedded in social relationships. Naturalistic observations and experimental manipulations of the emotional environment were used to measure 2-year-old children's regulation of emotion, aggression, and altruism during peer interactions. Children with a manic-depressive parent had difficulty in sharing with their friends and in handling hostility, showing maladaptive patterns of aggression. The social and emotional problems of these children were similar to the interpersonal problems of their manic-depressive parents.