Facial masculinity does not appear to be a condition-dependent male ornament and does not reflect MHC heterozygosity in humans

Abstract

Recent studies have called into question the idea that facial masculinity is a condition-dependent male ornament that indicates immunocompetence in humans. We add to this growing body of research by calculating an objective measure of facial masculinity/femininity using 3D images in a large sample (n = 1,233) of people of European ancestry. We show that facial masculinity is positively correlated with adult height in both males and females. However, facial masculinity scales with growth similarly in males and females, suggesting that facial masculinity is not exclusively a male ornament, as male ornaments are typically more sensitive to growth in males compared with females. Additionally, we measured immunocompetence via heterozygosity at the major histocompatibility complex (MHC), a widely-used genetic marker of immunity. We show that, while height is positively correlated with MHC heterozygosity, facial masculinity is not. Thus, facial masculinity does not reflect immunocompetence measured by MHC heterozygosity in humans. Overall, we find no support for the idea that facial masculinity is a condition-dependent male ornament that has evolved to indicate immunocompetence.

Keywords: MHC heterozygosity; facial masculinity; human evolution; immunocompetence handicap hypothesis; sexual selection.

Fig. 1.

Variation in facial masculinity and sexual dimorphism. (A) Density plot showing a bimodal distribution of facial masculinity. (B) Heat maps showing Cohen’s D of dimorphism between average male and female faces before (Top) and after (Bottom) correction for allometry.

Fig. 2.

Facial masculinity is positively correlated with height within and between the sexes. (A) Relationship between FM_overall and height. (B) Results of linear model between FM_QL and height and other covariates. Regions in yellow are significant after Bonferroni correction for 7,150 QLs (P < 0.05/7150 = 6.93 × 10⁻⁶). SI Appendix, Fig. S11 shows results for all covariates.