Systematic differences in decision making between genders have been discovered in both competitive and pro-social environments. These contexts, however, have been previously studied in isolation while in naturally occurring settings pro-social and competitive pressures often overlap in economically meaningful ways. Here we report data from an experiment involving German schoolchildren where dictators are in one town and receivers in another. Our experiment informs decision making in social environments that include differing levels of competitive pressure. We find that competitive pressure significantly mitigates pro-sociality in boys, while it does not affect girls’ propensities to make fair decisions. This finding is robust to controlling for social and cognitive factors, and it may shed additional light on the evolutionary roots of human social preferences.