Women can judge sexual unfaithfulness from unfamiliar men's faces.

We routinely form impressions of people from their faces, and these impressions sometimes contain a kernel of truth. Impressions of trustworthiness are central to interpersonal relationships, but their accuracy remains contentious. Here, we investigated whether sexual trustworthiness (faithfulness) can be accurately judged from opposite-sex strangers' faces. Women's ratings of men's unfaithfulness showed small-moderate correlations with men's past unfaithfulness (cheating, poaching). Women used masculinity as a valid cue to unfaithfulness. Men's unfaithfulness ratings showed small, non-significant correlations with unfaithfulness, although formal tests for sex differences yielded equivocal results. Women were less likely than men to erroneously classify unfaithful individuals as faithful. We conclude that impressions of sexual faithfulness from faces have a kernel of truth, at least for women, and that they may help people assess the quality of potential mates about whom they have minimal behavioural information.