We challenge the social categorization perspective in the team diversity literature by arguing that stereotypes and not favoritism for members of the same social category govern processes and dynamics in gender-diverse teams. We posit that team members' gender and task stereotypes generate competence attributions that shape individual team members' dominance behavior and performance in a self-fulfilling way: Team members who are attributed more competence behave more dominantly and outperform those who are attributed less competence. We further argue that pro-diversity beliefs may prevent this self-fulfilling tendency of stereotypes by inhibiting individuals' stereotype-confirming behavior. Hypotheses were tested with 97 gender-heterogeneous four-person student teams working on stereotypically masculine- or feminine-typed problems. Team members estimated each other's competence prior to collaboration. Diversity beliefs were manipulated to be either pro-diversity or pro-similarity and dominance was observed with behavioral coding. Multilevel path modeling showed that competence attributions mediated the effects of stereotypical gender-task fit on individual dominance behavior and performance under pro-similarity beliefs but not under pro-diversity beliefs. Our study thus shows that the self-fulfilling tendencies of gender stereotypes in teams can be mitigated by instituting pro-diversity beliefs.