Intergenerational closure refers to parents' knowing the parents of their adolescents' friends. This study treated intergenerational closure - as reported by the parents of adolescent twins - as the dependent variable in a behavioral genetic analysis. The sample consisted of identical and fraternal twin pairs in the National Longitudinal Study of Adolescent Health (Add Health). Models that were fit to twin covariance matrices estimated that additive genetic influences accounted for 44% of the variance in intergenerational closure and shared environmental influences for 44%. These findings provide evidence that the heritable characteristics of adolescents influence the connections between their families and their extrafamilial social networks.