Evidence from this study provides one possible mechanism for what Kendler and colleagues (2008) termed the socially mediated hypothesis linking early age of onset to later alcohol dependence and problems. This hypothesis proposes that the primarily genetic risk of later alcohol-related problems is mediated by exposure to environmental risk largely responsible for onset. Consistent with this view, our results suggest that undersupervised youth who begin drinking relatively early may provide the environmental point of entry for drinking behavior in early adolescent social ecologies predicted by genetically informative studies (e.g., Kendler et al., 2008; McGue et al., 2006), by exposing their friends to drinking. Further, drinking peer clusters are self-reinforcing, in that drinkers have an affinity for other drinkers as friends. This property of drinking behavior would tend to perpetuate consumption and lead to alcohol-related problems, especially among genetically vulnerable youth, for instance, those high on lack of harm avoidance and poor impulse control, or who require more alcohol to feel intoxicated (Schuckit et al., 2012). Such youth are quite possibly no more vulnerable to exposure-related risk for onset than their peers, but pay a price later on.
