In non-human species, increasing the proportion of potential mates in the local population often increases preferences for high quality mates, while increasing the proportion of potential competitors for mates intensifies within-sex competition. In two experiments, we tested for analogous effects in humans by manipulating pictorial cues to the sex ratio of the local population and assessing women's preferences for facial symmetry, a putative cue of mate quality in humans. In both experiments, viewing slideshows with varied sex ratios tended to increase preferences for symmetry in the sex that was depicted as being in the majority and tended to decrease preferences for symmetry in the sex that was depicted as being in the minority. In other words, increasing the apparent proportion of a given sex in the local population increased the salience of facial cues of quality in that sex, which may support adaptive appraisals of both potential mates' and competitors' quality. This effect of sex ratio was independent of (i.e. did not interact with) an effect of cues to the degree of variation in the attractiveness of individuals in the local population, whereby the degree of variation in men's, but not women's, attractiveness modulated symmetry preferences. These findings demonstrate that symmetry preferences in humans are influenced by cues to the sex ratio of the local population in ways that complement both the facultative responses that have been observed in many other species and theories of both inter-sexual and intra-sexual selection.
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