Human romantic partners tend to have similar physical traits, but the mechanisms causing this homogamy are controversial. One potential explanation is direct matching to own characteristics. Alternatively, studies showing similarity between parent and partner support positive sexual imprinting, where individuals are more likely to choose mates with the physical characteristics of their other-sex parent. This interpretation has been strongly criticized because the same pattern could also be caused by sex-linked heritable preferences, where similarity in appearance between an individual's partner and their other-sex parent is caused by similarity in preferences between the individual and their same-sex parent. The relationships among own, parents' and same-sex partner's eye color provide an elegant test of these hypotheses, which each postulate a different best predictor of partner's eye color. While the matching hypothesis predicts this will be own eye color, the sex-linked heritable preference hypothesis predicts this will be the other-sex parent's eye color and the positive sexual imprinting hypothesis predicts this will be the partner-sex parent's eye color. Here we show that partner eye color was best predicted by the partner-sex parent's eye color. Our results provide clear evidence against matching and sex-linked heritable preference hypotheses, and support the positive sexual imprinting hypothesis of the relationship between own and partner's eye color.

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