Negative frequency-dependent sexual selection maintains striking polymorphisms in secondary sexual traits in several animal species. Here, we test whether frequency of beardedness modulates perceived attractiveness of men's facial hair, a secondary sexual trait subject to considerable cultural variation. We first showed participants a suite of faces, within which we manipulated the frequency of beard thicknesses and then measured preferences for four standard levels of beardedness. Women and men judged heavy stubble and full beards more attractive when presented in treatments where beards were rare than when they were common, with intermediate preferences when intermediate frequencies of beardedness were presented. Likewise, clean-shaven faces were least attractive when clean-shaven faces were most common and more attractive when rare. This pattern in preferences is consistent with negative frequency-dependent selection.

An individual's fitness depends not only on their phenotype but also on the phenotypes of their competitors and contemporaries. Sexual attractiveness may be strongly influenced by an individual's familiarity to potential mates or the rarity of the individual's phenotype. Such effects can cause negative frequency-dependent selection, maintaining striking polymorphisms in ornamentation. Here we test whether preferences for women's hair color, which is highly polymorphic between European populations, reflects patterns of positive or negative frequency-dependence. We assigned each participant to one of four experimental treatments in which we manipulated the frequency of hair colors in a set of 18 images presented consecutively. The four treatments were: blond hair rare, brown hair rare, red hair rare and an even distribution of the three hair colors. Following immediately on from this experimental phase, participants rated the same set of 9 faces, 3 with each hair color. Results showed that the experimental manipulation of hair color frequency did not significantly influence hair-color attractiveness. However, there were sex differences in ratings: men rated blond hair as most sexually attractive and women rated both blond and brown hair equally as attractive as red hair. Self-reported natural hair color had weak but significant effects on hair color preferences, but these effects disappeared when we restricted analyses to participants of European descent only. Our findings do not support the hypothesis that men's preferences for women's hair color are negative frequencydependent, favoring rare hair colors.