The current study examined the hypothesis that group size can influence whether children display self-assertive versus self-deprecating responses to interpersonal competition, especially under stress. Twenty same-sex play-groups (N = 120) of 9- to 10-year-old children played a competitive game in groups and in dyads. Stress was induced by causing some of the children to lose the game and watch as their opponents received psychological and material rewards. Results demonstrated that both the dynamics of the game and individual reactions to stress varied consistently as a function of the social context. Individuals displayed more assertive behaviors in groups than in dyads. In contrast, individuals exhibited more self-deprecating behaviors in dyads than in groups. Given that under naturalistic conditions males are more likely than are females to interact in groups and females are more likely than are males to interact in dyads, group size provides one possible mechanism for the development of sex differences in self-assertive versus self-deprecating behaviors.