to benefit from the group-based intervention. In contrast, carriers of the G allele did not demonstrate significant reductions in teacher-reported externalizing behavior over the course of the intervention, suggesting that for these individuals, the beneficial effects of the intervention may be counteracted by the effects of aggregating high-risk children into groups. During the intervention, children carrying the G allele may be more sensitive to social rewards from deviant peers and thus more distracted by them, and may be more susceptible to peer pressure. Furthermore, although such increasing ease in interacting with other at-risk children may have begun during the intervention year, for G allele carriers, the differential effects of intervention format on externalizing behavior become more pronounced the year after the intervention. One possibility is that carriers of the G allele may have been more likely to form new affiliations with deviant peers, a form of homophily (Hanish, Martin, Fabes, Leonard, & Herzog, 2005), that was enhanced after the intervention was over when interactions were not supervised by group leaders. In our sample, 70% of the youth remained in or continued on to the same school as the majority of participants in their intervention group at the one-year follow-up. The
