Is sexual dimorphism under present selection in humans?

Abstract

Like most mammalian species, humans show sexual dimorphism: the two sexes exhibit sextypical morphological traits. Humans show dimorphism in the facial structure, body size/shape, physical strength, the amount of and distribution of muscle and fat mass, voice pitch, finger ratios, and the growth of facial and body hair. Such dimorphisms are argued to be sexually selected. This thesis focussed on testing biological fitness benefits as a function of exaggerated sexual dimorphism (masculinity in men; femininity in women) in humans. In Chapter 2, a comprehensive meta-analysis showed that men’s body masculinity (increased strength/muscularity) significantly predicted greater mating/reproductive success. A more masculine voice pitch, increased height, and higher testosterone levels positively predicted mating, but not reproduction. Facial masculinity and finger ratios did not significantly predict either. Chapter 3 focussed on women’s traits. Novel analyses of two archival datasets indicated that women with more appealing faces reported fewer births, but greater offspring survival. Furthermore, a systematic literature review showed no consistent associations between women’s traits and reproductive outcomes, and meta-analyses of men’s and women’s fitness as a function of facial attractiveness showed no relationships. In Chapter 4, we tested whether more dimorphic parents had offspring who showed better health and/or earlier sexual onset in two samples. We detected very few significant associations, whereby fathers with greater facial dominance, strength, and height had offspring in better health, and more facially attractive and healthy-looking fathers had offspring who showed earlier sexual onset. Meta-analysing the relationships between parental facial traits and offspring health across the two samples revealed effects close to zero. Lastly, in Chapter 5 we tested whether more masculine men had partners who were more feminine/attractive, healthier, and/or had higher mate value in two U.K. samples. Men with more masculine bodies had more facially appealing partners, and more facially appealing men had higher partner-rated mate value but self-reported somewhat worse health. Except for the meta-analytic effects in Chapter 2, none of the significant effects here remained after correcting for multiple comparisons. The only robust associations we detected were thus with respect to masculinity in men’s bodies increasing fitness outcomes. Overall, the results from this thesis give very limited support to the notion that other dimorphic traits confer men and women fitness benefits. The implications for human sexual selection models are discussed.