Our reactions to facial self–resemblance could reflect either specialized responses to cues of kinship or by–products of the general perceptual mechanisms of face encoding and mere exposure. The adaptive hypothesis predicts differences in reactions to self–resemblance in mating and prosocial contexts, while the by–product hypothesis does not. Using face images that were digitally transformed to resemble participants, I showed that the effects of resemblance on attractiveness judgements depended on both the sex of the judge and the sex of the face being judged: facial resemblance increased attractiveness judgements of same–sex faces more than other–sex faces, despite the use of identical procedures to manipulate resemblance. A control experiment indicated these effects were caused neither by lower resemblance of other–sex faces than same–sex faces, nor by an increased perception of averageness or familiarity of same–sex faces due to prototyping or mere exposure affecting only same–sex faces. The differential impact of self–resemblance on our perception of same–sex and other–sex faces supports the hypothesis that humans use facial resemblance as a cue of kinship.