The Relationship Code is the report of a longitudinal study, conducted over a ten-year period, of the influence of family relationships and genetic factors on competence and psychopathology in adolescent development. The sample for this landmark study included 720 pairs of same-sex adolescent siblings--including twins, half siblings, and genetically unrelated siblings--and their parents.

Using a clear expressive style, David Reiss and his coinvestigators identify specific mechanisms that link genetic factors and the social environment in psychological development. They propose...