A lover or a fighter? Opposing sexual selection pressures on men’s vocal pitch and facial hair

The traditional assumption within the research literature on human sexually dimorphic traits has been that many sex differences have arisen from intersexual selection. More recently however, there has been a shift towards the idea that many male features, including for example male lower-pitched voices, and male beard growth, might have arisen predominantly through intrasexual selection: that is, to serve the purpose of male-male competition instead of mate attraction. In this study, using a unique set of video stimuli, we measured people’s perceptions of the dominance and attractiveness of men who differ both in terms of voice pitch (four levels from lower to higher pitched) and beard growth (four levels from clean shaven to a month’s hair growth). We found a non-linear relationship between lower pitch and increased attractiveness; men’s vocal attractiveness peaked at around 96 Hz. Beard growth had equivocal effects on attractiveness judgements. In contrast, perceptions of men’s dominance simply increased with increasing masculinity (i.e. with lower-pitched voices and greater beard growth). Together, these results suggest that the optimal level of physical masculinity might differ depending on whether the outcome is social dominance or mate attraction. These dual selection pressures might maintain some of the documented variability in male physical and behavioral masculinity that we see today.