The authors examined genetic and environmental influences, including the contributions of 2 measured aspects of the shared environment of twins (paternal absence, socioeconomic disadvantage) on the development of mother-reported separation anxiety disorder (SAD) history in a sample of 1,887 female twin pairs. Four different symptom categories of SAD were considered. Results revealed that all 4 SAD symptom categories were significantly heritable, whereas the contribution of shared environmental influences to the variation in risk was significant for only 2 of the 4 SAD categories. Paternal absence was found to have an important influence in vulnerability for SAD, whereas the effect of socioeconomic disadvantage was less robust. Evidence for race differences in the etiology of SAD was not found.