Sibling effects on substance use in adolescence: social contagion and genetic relatedness.

Prior research on sibling contagion for substance use has not attended to individual differences in the sibling relationship that may be influenced by genetic similarity. The authors utilizing data on a sample of twin and nontwin siblings participating in the National Longitudinal Study of Adolescent Health (Add Health). Although monozygotic twins had the highest levels of sibling contact and mutual friendships, the pattern of results for other sibling types were not consistent with genetic models, and biometric analysis indicated that shared environmental factors influenced these sibling relationship features. Application of DeFries-Fulker regression models provided evidence that sibling contact and mutual friendships represent a source of social contagion for adolescent smoking and drinking independent of genetic relatedness. The results are interpreted using a social contagion framework and contrasted with other competing models such as those focused on the equal environments assumption and niche selection.