This study examined the hypothesis that group size can influence whether 9- to 10-year-olds display self-assertive versus self-deprecating responses to interpersonal competition, especially under stress. Findings indicated that individuals displayed more assertive behaviors during competitive game-playing in groups than in dyads, and more self-deprecating behaviors in dyads than in groups. Findings suggest that group size provides one possible mechanism for development of sex differences in self-assertive versus self-deprecating behavior. (Author/KB)