Using a controlled experiment, we examine the role of nurture in explaining the stylized fact that women shy away from competition. 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. Thus it is untrue that the average female avoids competitive behaviour more than the average male. This suggests that observed gender differences might reflect social learning rather than inherent gender traits.