Using a controlled experiment, we examine the role of nurture in explaining the stylized fact that women shy away from competition. We have two distinct research questions. First, does the gender composition of the group to which a student is randomly assigned affect competitive choices? Second, does the gender mix of the school a student attends affect competitive choices? Our subjects (students just under 15 years of age) attend publicly funded single-sex and coeducational schools. We find robust differences between the competitive choices of girls from single-sex and coed schools. Moreover, girls from single-sex schools behave more like boys even when randomly assigned to mixed-sex experimental groups. This suggests that it is untrue that the average female avoids competitive behavior more than the average male. It also suggests that observed gender differences might reflect social learning rather than inherent gender traits.