The role of adolescents’ peer relationships in their use of alcohol has been of long-standing scientific interest. This work is driven by a well-established and still growing list of risks of excessive alcohol use for this population, for example, increased risk of depression, suicide, and other problems during adolescence (Reifman & Windle, 1995; Windle, Miller-Tutzauer, & Domenico, 1992), as well as risk for heavier drinking and alcohol dependence (Grant et al., 2006; Guttmannova et al., 2011) and cognitive impairment reaching into adulthood (Hanson, Medina, Padula, Tapert, & Brown, 2011; Sher, 2006). Adolescents who drink alcohol typically do so with similar-age friends, leading to ubiquitously observed drinking clusters (Bauman & Ennett, 1994). A number of increasingly sophisticated study designs and analyses have addressed the dynamics that form these clusters in natural settings (Ary, Tildesley, Hops, & Andrews, 1993; Kandel, 1978; Steglich, Snijders, & West, 2006; Urberg, Luo, Pilgrim, & Degirmencioglu, 2003). However, the role of peers in “onset to drinking”—defined here as the transition from no experience with drinking alcohol to having a “first full drink” (e.g., Guttmannova et al., 2011; McGue, Iacono, Legrand, & Elkins, 2006)—has received little attention, even though early onset is considered a major risk for later alcohol-related problems and dependence. In this study, we apply a stochastic actor-based model (SABM; Snijders, van de Bunt, & Steglich, 2010) of dynamic social network evolution to link early adolescents’ drinking onset to exposure to best friends who have already begun drinking. We use the term exposure instead of the more typical influence (or socialization) terminology, because it can apply to a range of mechanisms that may put youth at greater risk of onset if they have drinking friends, including, for instance, social reinforcement of attitudes, modeling, norms, and logistical and informational facilitation (Aral, 2011), not all of which are influence in the usual sense. Our study utilizes and illustrates a novel variant of the SABM that combines the conditional Markov model for network evolution first proposed by Snijders (2001) with a proportional hazard rate model for alcohol onset (Cox, 1972; Greenan, 2013). Hazard models are commonly used to evaluate risk factors for time to an event (Yamaguchi, 1991). Network evolution and alcohol onset are modeled jointly, in order to separate selection and exposure effects on drinking clusters. This approach also differs from previous models of joint network and behavioral evolution (Steglich, Snijders, & Pearson, 2010) in its focus on predictors of relative risk for initiating drinking, rather than simply whether (or how much) drinking takes place, regardless of whether it is an onset event. Early Onset to Drinking Many studies have reported a relationship between early age of onset and risk for later alcohol dependence (Chou & Pickering, 1992; Grant & Dawson, 1997; Guttmannova et al., 2011), suicidal behavior (Cho, Hallfors, & Iritani, 2007; Swahn, Bossarte, & Sullivent, 2008), and other problems (Hingson, Heeren, Jamanka, & Howland, 2000). Although there is evidence that after drinking is initiated, risk for additional problems is fundamentally genetic (McGue et al., 2006; Pagan et al., 2006; Prescott & Kendler, 1999), one must nevertheless begin drinking before such risk becomes relevant. Genetically informative studies have also found that risk of earlier onset is largely environmentally determined (Kendler, Schmitt, Aggen, & Prescott, 2008; Pagan et al., 2006), which could include exposure to drinking peers and lack of parental and other adult supervision. Peers and Onset to Drinking The theoretical case for peer drinking affecting onset is somewhat mixed. Although early adolescence is a time when peer relationships become especially important (Gardner & Steinberg, 2005), there is also evidence that middle to older adolescents perceive more peer pressure to drink (Steinberg & Monahan, 2007), perhaps because drinking does not become widespread in adolescent populations until then (Burk, van der Vorst, Kerr, & Stattin, 2012). Very few studies have examined the issue empirically. Jessor and Jessor (1975) found an association between friends’ drinking and onset to drinking among young adolescents, and Trucco and colleagues (2011) found a distinct peer exposure effect on onset while controlling for selection. Both studies measured friends’ drinking behavior by participant reports. Peers and Amount of Drinking A much larger literature has addressed peer exposure effects for alcohol quantity or frequency. Drinking is deviant only in relation to age but it is an attractive adult-like activity for many adolescents (Jessor & Jessor, 1977); it is associated with socializing and being accepted (Veenstra, Huitsing, Dijkstra, & Lindenberg, 2010), and perhaps for this reason, is typical among high-status youth (Allen, Chango, Szwedo, Schad, & Marston, 2012). This dynamic motivates both selection and influence effects in teen drinking. Three recent studies of adolescent drinking (Burk et al., 2012; Knecht, Burk, Weesie, & Steglich, 2011; Mercken, Steglich, Knibbe, & de Vries, 2012) provide a particularly relevant empirical context for our work. Each addressed alcohol use among early adolescent youth, employed a multiwave longitudinal design with complete-network friendship (all pairwise relationships) and behavioral data, and applied SABM. Results showed some similarities but important differences as well. 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. The previous discussion suggests hypotheses concerning the role peer affiliations play in onset to drinking. Although our discussion so far has focused on exposure effects, selection is important also, as relationships must be created before they can affect subsequent drinking. H1. Same drinking onset status predicts friendship selection. H2. Exposure to already-drinking peers increases the risk for onset in nondrinking early adolescents. Gender, Grade, and Adult Monitoring Gender has not typically been found to moderate selection or exposure effects for alcohol use (cf. Burk et al., 2012), but it may for onset, as the hypothesized greater importance of close friendships to girls (Schulenberg et al., 1999) may be more important than the greater susceptibility to drinking sometimes found for boys (Anderson, Tomlinson, Robinson, & Brown, 2011; Windle, 2000). H3. Same alcohol onset status is a more important source of selection preference for girls compared to boys. H4. Exposure to already-drinking peers affects girls’ rate of onset more than boys. If, indeed, drinking becomes an increasingly important peer-related activity as youth progress through early adolescence (Burk et al., 2012), onset may play an increasing role as both a reason for friendship selection and an element of peer exposure. Even though Mercken et al. (2012) did not find an age effect in an early adolescent sample, the theoretical case is plausible for alcohol onset. Earlier onset may index environmental risks for early problem behavior; in contrast, drinking in older youth is much more normative and is associated with popularity (Allen et al., 2012; Burk et al., 2012). H5. Effects of exposure to drinking peers will be stronger at higher grades. H6. Selection preference for same drinking status will increase by grade. Besides peers, parents and other relevant adults are the most likely source of behavioral influence for early adolescent youth (Veronneau & Dishion, 2011). Previous studies have identified lack of monitoring and supervision as a key component of risk for substance use among adolescents (Dishion, Nelson, & Kavanagh, 2003; Ryan, Jorm, & Lubman, 2010), which may be a result of failure by parents to supervise peer affiliations (Dodge et al., 2009). Vitaro, Brendgen, and Tremblay (2000) found that, although parental monitoring had a direct effect on problem behavior, it did not moderate associations with exposure to problem-behaving peers. However, Veronneau and Dishion (2011) found that parental monitoring buffered risks for problem behavior associated with peer relationship difficulties. Because both opportunities to drink and perceived acceptability of drinking could be affected by supervision, more monitoring may delay onset. H7. Adult monitoring predicts delayed alcohol onset. H8. Adult monitoring interacts negatively with exposure to drinking peers in predicting alcohol onset.