Burk and colleagues (2012) compared selection and exposure effects associated with changes in drinking behavior for three age cohorts, addressing variation in exposure effects at different ages. They found that peer drinking clusters are mainly explained by selection effects up to about age 15, but exposure effects become important in midadolescence. In contrast, Mercken and colleagues (2012) found evidence of exposure effects at younger ages (ages 13–14) but not for older youth (ages 14–16), whereas alcohol-related selection appeared stronger for the older youth, although the alcohol selection effect approached significance for the youngest ages. Finally, Knecht et al. 2011 found drinking-related selection and a trend effect of exposure to drinking peers (ages predominantly 11–13). Note that none of these studies separated onset from subsequent drinking. In fact, conflating the two has been typical in empirical studies of peers and adolescent drinking, even though many theoretical treatments explicitly discuss onset as a distinct and salient phenomenon (Dodge et al., 2009; Petraitis, Flay, & Miller, 1995). Thus, it is possible that the selection and exposure effects reported have to do with onset only, subsequent drinking only, or both. In contrast, our analysis is limited to onset alone.
