We examine whether social preferences are determined by hormones. We do this by investigating whether markers for the strength of prenatal testosterone exposure (finger length ratios) and current exposure to progesterone and oxytocin (the menstrual cycle) are correlated with choices in social preference games. We find that subjects with finger ratios indicating high prenatal testosterone exposure give less in the trust, ultimatum and public good games and return a smaller proportion in the trust game. The choices of female subjects vary over the menstrual cycle according to a pattern consistent with a positive impact of oxytocin on giving in the trust and ultimatum games and a positive impact of progesterone on altruism. We find no impact for subjects taking hormonal contraceptives. We conclude that both prenatal and current exposure to hormones play an important role in shaping social preferences.