Oxytocin has been dubbed the 'social hormone' for its ability to facilitate the formation of social bonds. However, it seems that its role is more subtle and complex than that. Here, researchers gave men doses of oxytocin before introducing them to attractive females. Men who were in a monogamous relationship increased the distance between themselves and the women if they had received oxytocin compared with similar men who had not received the hormone, whereas single men did not. However, the hormone had no effect on the men's interactions with other men or on their judgements of attractiveness. Oxytocin might therefore promote faithfulness in monogamous relationships by preventing men from signalling romantic interest by approaching attractive women.