The current study examines how a person’s alcohol consumption is related to their peers’ alcohol use from early adolescence through early adulthood, in a population-based sample of male twins. We fit three longitudinal models that represent alternative causative and correlative relationships between individual and peer alcohol use. These models capitalized on the genetically informative nature of twin samples in that we were able to investigate whether different sources of covariance – genetic and/or environmental – were operating in a causal or correlated manner. Unless otherwise noted, for the purposes of this report, we restrict our use of the terms social influence and social selection to theoretically causal processes that arise apart from correlation/shared liability processes. In other words, we explicitly examine both social influence and social selection, while controlling for genetic and environmental influences on one’s own alcohol use and perceived peer alcohol use. This is accomplished by including causal pathways in both directions in a subset of our models (available in the Supplementary Material). A critical implication of causal processes is that, if the expressed phenotype changes, the downstream
