A second substantial advance in the past decade has been a growing recognition of the many different peers—individuals and groups—that may exert powerful socialization effects on adolescents’ behaviors and attitudes. For many years, adolescent peer influence was measured by asking adolescents themselves to report the extent to which their “peers” engaged in specific behaviors. Advances in the broader peer relations literature clearly have since delineated a multitude of distinct, salient peer relationships (Rubin et al., 2006); past work that did not specifically define who these “peers” were had a limited picture of the specific relationships that are most influential. Given the restricted focus of prior work, it is especially exciting that research in the past decade has begun to explore more thoroughly both the best friend relationships and other social contexts in which peer socialization may occur. For instance, recent work has examined peer selection and socialization within romantic relationships (Simon, Aikins, & Prinstein, 2008) and sibling relationships (Stormshak, Comeau, & Shepard, 2004; van der Vorst, Engels, Meeus, Dekovic, & Van Leeuwe, 2007). In addition, research has begun to reflect
