Negative frequency-dependent preferences and variation in male facial hair

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, withinwhichwe 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. © 2014 The Author(s) Published by the Royal Society. All rights reserved

Morphological and population genomic evidence that human faces have evolved to signal individual identity

Facial recognition plays a key role in human interactions, and there has been great interest in understanding the evolution of human abilities for individual recognition and tracking social relationships. Individual recognition requires sufficient cognitive abilities and phenotypic diversity within a population for discrimination to be possible. Despite the importance of facial recognition in humans, the evolution of facial identity has received little attention. Here we demonstrate that faces evolved to signal individual identity under negative frequency-dependent selection. Faces show elevated phenotypic variation and lower between-trait correlations compared to other traits. Regions surrounding face-associated SNPs show elevated diversity consistent with frequency-dependent selection. Genetic variation maintained by identity signaling tends to be shared across populations and, for some loci, predates the origin of Homo sapiens. Studies of human social evolution tend to emphasize cognitive adaptations but we show that social evolution has shaped patterns of human phenotypic and genetic diversity as well