The sex ratio of the local population (i.e., the ratio of males to females) influences mating-related behaviors in many species. Recent experiments show that male-biased sex ratios increase the amount of financial resources men will invest in potential mates, suggesting that sex ratios influence allocation of mating effort in humans. To investigate this issue further, we tested for effects of cues to the sex ratio of the local population on the motivational salience of attractiveness in own-sex and opposite-sex faces. We did this using an effort based key-press task, similar to those that have been used in previous studies to assess the motivational salience of attractive faces. The motivational salience of facial attractiveness was assessed in samples of faces in which the ratio of male to female images was manipulated (i.e., an own-sex biased condition, an opposite-sex biased condition, and an unbiased control condition). Participants were 292 heterosexual men and 292 heterosexual women (mean age = 24.3 years, SD=5.9 years). Participants were randomly allocated to one of the three versions of the key-press task. The motivational salience of attractive opposite-sex, but not own-sex, faces was greater in the own-sex biased (i.e., high competition for mates) than opposite-sex biased (i.e., low competition for mates) condition. Moreover, this effect was not modulated by participant sex. Our data demonstrate that individuals allocate effort to attractive and unattractive individuals facultatively, changing response patterns according to perceived characteristics of the local population. Thus, these results present new experimental evidence that the perceived sex ratio of the local population modulates allocation of mating effort in men and women in similar ways.
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