Theory and research on sex differences in adjustment focus largely on parental, societal, and biological influences. However, it also is important to consider how peers contribute to girls' and boys' development. This article provides a critical review of sex differences in several peer relationship processes, including behavioral and social-cognitive styles, stress and coping, and relationship provisions. The authors present a speculative peer-socialization model based on this review in which the implications of these sex differences for girls' and boys' emotional and behavioral development are considered. Central to this model is the idea that sex-linked relationship processes have costs and benefits for girls' and boys' adjustment. Finally, the authors present recent research testing certain model components and propose approaches for testing understudied aspects of the model.

Co-ruminating, or excessively discussing problems, with friends is proposed to have adjustment tradeoffs. Co-rumination is hypothesized to contribute both to positive friendship adjustment and to problematic emotional adjustment. Previous single-assessment research was consistent with this hypothesis, but whether co-rumination is an antecedent of adjustment changes was unknown. A 6-month longitudinal study with middle childhood to midadolescent youths examined whether co-rumination is simultaneously a risk factor (for depression and anxiety) and a protective factor (for friendship problems). For girls, a reciprocal relationship was found in which co-rumination predicted increased depressive and anxiety symptoms and increased positive friendship quality over time, which, in turn, contributed to greater co-rumination. For boys, having depressive and anxiety symptoms and high-quality friendships also predicted increased co-rumination. However, for boys, co-rumination predicted only increasing positive friendship quality and not increasing depression and anxiety. An implication of this research is that some girls at risk for developing internalizing problems may go undetected because they have seemingly supportive friendships.

Relations of overt and relational aggression with perceived popularity among children and early adolescents were examined in 2 studies (Ns = 607 and 1,049). Among older youths, positive concurrent relations found between overt aggression and perceived popularity became nonsignificant when relational aggression was controlled, whereas positive associations found between relational aggression and perceived popularity held when overt aggression was controlled. Aggression and perceived popularity were not positively related for the younger participants. The 2nd study also examined the temporal ordering of these relations over 6 months. For older girls, positive relations between relational aggression and perceived popularity were bidirectional. For older boys, relational aggression did not predict increased perceived popularity, but perceived popularity predicted increased relational aggression. Implications for intervention are discussed.

The current research considered the costs of caring in youths' friendships. The development of a new construct, empathetic distress, allowed for a direct test of the commonly held belief that females suffer greater vicarious distress in response to close others' stressors and problems than do males. Empathetic distress refers to one's strongly sharing a relationship partner's distress over problems to the point of taking on the partner's distress and experiencing it as one's own. This new construct was examined in an ethnically diverse sample of early adolescents who responded to a series of questionnaires in their classrooms. Results indicated that girls did experience greater empathetic distress in friendships than did boys. In addition, the current research revealed that social perspective taking in friendships (i.e., the social-cognitive ability to infer and understand the friend's perspective) had adjustment trade-offs in that it predicted greater positive friendship quality but also greater empathetic distress in the friendship. Interestingly, the associations of social perspective taking with both positive friendship quality and empathetic distress were partially mediated by co-rumination or excessive discussion of problems. Applied implications of the findings that girls' greater social perspective taking and associated co-rumination contributed both to their greater positive friendship quality but also to greater costs of caring in the form of empathetic distress are discussed.

Peer contagion of internalizing symptoms was examined within youths' friendships over 6 months. Children (Grades 3 and 5) and adolescents (Grades 7 and 9) paired in 274 reciprocal same-sex friendship dyads completed measures of depressive and anxiety symptoms, co-rumination, and self-disclosure. Depression contagion was present for all youth, and anxiety contagion was found in the sample of girls and older boys. Although normative self-disclosure did not mediate the contagion effects, co-rumination mediated the depression contagion effect for adolescents and the anxiety contagion effect in the sample of girls and older boys. Implications for interventions with youth at risk for developing internalizing symptoms are discussed.