A rich literature in psychology suggests that teenagers may be influenced by the behavior of older siblings. A number of studies have found

substantial correlations in risky behavior between siblings, raising the possibility that adolescents may directly influence the actions of their brothers or sisters. We assess the extent to which these correlations are due to causal effects. Our identification strategy is based on the fact that the behavior of a child at a point in time cannot directly influence a sibling's actions in a prior year. We also assume that the direction of influence is from older siblings to younger siblings. Under this assumption as well as a number of other restrictions on dynamics, one can identify the causal effect from a regression of the behavior of the younger sibling on the past behavior and the future behavior of the older sibling. We also estimate a joint dynamic model of the behavior of older and younger sibling that allows for family effects, individual specific heterogeneity, and state dependence. We use the model to simulate the dynamic response of substance use to the behavior of the older sibling. Our results suggest smoking, drinking, and marijuana use are influenced by the example of older siblings, although most of the link between siblings reflects correlation rather than causation. We also explore the extent to which the link between siblings depends on the gender match, the age gap, and parenting variables, with mixed results.