Friendships with members of our own group (ingroup) and other groups (outgroups) shape our attitudes toward outgroups. Research on intergroup contact has shown that the numbers of outgroup and ingroup friends we have influence our outgroup attitudes, whereas research on socialization has shown that the attitudes held by our friends influence our outgroup attitudes. Past research, however, examined these processes in isolation, which precludes discerning whether having friends, or the attitudes held by our friends, are both important in shaping our outgroup attitudes, and, if so, which is more important. To disentangle these effects, we conducted a 5-wave social network study in 2 ethnically diverse schools (N = 1,170 students). By applying a novel longitudinal coevolution model, we were able to separate the effects of having ingroup and outgroup friends (contact effects), and the effects of those friends’ attitudes (socialization effects), on individuals’ outgroup attitudes, while controlling for friendship selection processes. In so doing, we found that it is principally the attitudes of ingroup friends—not outgroup friends’ attitudes or having ingroup and outgroup friends alone—that predict individuals’ outgroup attitudes. Our findings have important theoretical implications, as we demonstrate that combining the divergent approaches of intergroup contact and socialization enables us to better understand outgroup attitude development. Our findings also have practical implications, as we show that, even in diverse environments, individuals rely primarily on friends from their own group to inform their attitudes toward other groups. (PsycInfo Database Record (c) 2022 APA, all rights reserved)