An experimental test of the Westermarck effect: sex differences in inbreeding avoidance

In order to avoid inbreeding, humans and other animals develop a strong sexual aversion to individuals with whom they have lived closely in infancy and early childhood (usually biological siblings), a phenomenon called the "Westermarck effect" or negative sexual imprinting. The mechanisms underlying this phenomenon, however, remain unclear. For example, it is not known whether negative imprinting is based only on actual sexual aversion to brothers and sisters or also on generalizing the traits of their siblings to nonkin. If imprinting is more general, one could predict that people would avoid mating with all individuals that resemble their other-sex siblings. In our study, women rated morphed other-sex faces that resemble their siblings as significantly lower in sexual attractiveness than morphed faces on average, and the opposite effect was found in men—similarity to sibling increased perceived attractiveness. Interestingly, self-similarity did not influence the preferences of either men or women. These sex differences are consistent with parental investment theory, as females bear greater costs associated with inbreeding depression, perhaps explaining their deeper aversion toward engagement in sexual activities with male individuals who bear cues to relatedness. Furthermore, they indicate that faces resembling siblings are valid stimuli for investigating facial similarity preference.