This study provides a comprehensive analysis of gender differences in performance caused by two different dimensions of competition –rivalry for resources and status ranking. It also examines two mechanisms behind such differences: (1) gendered beliefs about performance differences in competitiveness; and (2) prescriptive stereotypes about women having to show warmth towards others. The results indicate that in the absence of any competitive dimension men and women perform equally well. Any competitive dimension, however, leads to women doing worse than men. These results are explained by men’s beliefs that they are better than women, and by women’s adherence to a prescribed stereotype of not harming others. Gender differences under competition seem to be endogenous to situational contexts, just like they are without competition.