The hypothesis that the association between parental divorce and children's adjustment is mediated by genetic factors was examined in the Colorado Adoption Project, a prospective longitudinal study of 398 adoptive and biological families. In biological families, children who experienced their parents' separation by the age of 12 years exhibited higher rates of behavioral problems and substance use, and lower levels of achievement and social adjustment, compared with children whose parents' marriages remained intact. Similarly, adopted children who experienced their (adoptive) parents' divorces exhibited elevated levels of behavioral problems and substance use compared with adoptees whose parents did not separate, but there were no differences on achievement and social competence. The findings for psychopathology are consistent with an environmentally mediated explanation for the association between parent divorce and children's adjustment; in contrast, the findings for achievement and social adjustment are consistent with a genetically mediated explanation involving passive genotype-environment correlation.