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 a striking hypothesis: family relationships are crucial to the expression of genetic influences on a broad array of complex behaviors in adolescents. Moreover, this role of family relationships may be very specific: some genetic factors are linked to mother–child relationships, others to father–child relations, some to relationship warmth, while others are linked to relationship conflict or control. The specificity of these links suggests that family relationships may constitute a code for translating genetic influences into the ontogeny of behaviors, a code every bit as important for behavior as DNA-RNA.