While there are a number of significant associations in the results, it is important to revisit the question of whether they represent the spread of alcohol consumption behavior (induction) as opposed to reflecting selection effects (homophily) or shared environmental effects (confounding) (39, 40, 41). While it is impossible to completely rule out these alternative explanations, a number of our findings strongly suggest that induction plays an important role. For example, the directionality of friendship ties is significant in predicting the spread of alcohol consumption behavior; these effects provide some evidence for the inter-personal induction of alcohol consumption behavior and suggest that covariance in drinking between friends is not the result of unobserved contemporaneous exposures to both principal and contact. If it were, there should be an equally strong influence, regardless of the directionality of friendship. Moreover, the results show that neither immediate neighbors nor geographic distance modify alcohol consumption behavior. If shared exposure (such as proximity to liquor stores or local economic hardship) were key, the effects would decay with distance. Moreover, since the models control for a principal's prior
