Sexual Selection on Human Faces and Voices
Puts, David A., Jones, Benedict C., DeBruine, Lisa M., The Journal of Sex Research
In some animal species, the two sexes differ so greatly in appearance that they could be mistaken for separate species. Such was the case when Carl Linnaeus, the father of modern taxonomy, misclassified male and female mallard ducks as Anas boschas and Anas platyrhynchos, respectively (Andersson, 1994). Although men and women may not be as divergent in appearance as male and female mallards, they are not far off. By one subjective assessment, humans are the eighth most visually sexually dimorphic primates (tied with gorillas and white-faced sakis), placing humans in the 90th percentile for visual sexual dimorphism (Dixson, Dixson, & Anderson, 2005). The visual dissimilarity between men and women is partly due to men's greater height and weight, but largely attributable to sex differences in body fat and muscle distribution (Lassek & Gaulin, 2009), along with conspicuous sex differences in body hair and, especially, facial hair. Not only do men and women differ in their soft tissue distribution, but they also differ in skeletal structure (e.g., Enlow & Hans, 1996). Besides the pelvis, probably the most obvious of the many human skeletal sex differences occur in the face. Men tend to have more prominent brow ridges and a longer lower face, including a larger, more angular mandible and squarer chin.
Men and women also have markedly different vocal characteristics. Voice pitch, measured by mean speaking fundamental frequency, is approximately twice as high in women as in men--a sex difference of approximately 5 SDs (Baken, 1987; Puts, Apicella, & Cardenas, 2012). Perhaps less obvious, men speak in a more monotone voice--that is, the standard deviation in fundamental frequency across an utterance is lower in men than in women (Henton, 1995; Puts, Apicella, & Cardenas, 2012). The formant frequencies (frequencies of high energy) of men's voices are also lower and more closely spaced, producing a deeper, fuller vocal timbre in men than in women (Fitch, 1997).
For those interested in understanding the social dynamics of human sexuality, such anatomical and acoustic sexual dimorphisms are particularly relevant. As we will see, these traits affect attractiveness and perceptions of dominance, and predict mate preferences and behaviors related to competition for mates. Thus, clarifying why men and women look and sound different will elucidate how appearance mediates interpersonal relationships, in general, and romantic relationships, in particular. In this review, we focus on sex differences in faces and voices. We consider these aspects of the phenotype because they are highly conspicuous, highly sexually dimorphic, and contain abundant information about the individual. Moreover, a greater quantity of pertinent research has been conducted on faces and voices than any other conspicuous aspect of the human phenotype, and these literatures closely parallel one another.
Proximate Causes of Human Facial and Vocal Sex Differences
At proximate and developmental levels, sex differences in faces and voices largely are the consequence of exposure to gonadal sex steroids. For example, peri-pubertal craniofacial development produces pronounced sex differences in the size and shape of the mandible (Srael, 1969), and these changes appear to depend, in part, on elevated testosterone production in males (Verdonck, Gaethofs, Carels, & de Zegher, 1999). Facial sex differences are not simply a consequence of sex differences in overall size, as many facial sexual dimorphisms persist after controlling for allometry (Bastir, Godoy, & Rosas, 2011; Bastir, Rosas, & O'Higgins, 2006; Bulygina, Mitteroecker, & Aiello, 2006; Rosas & Bastir, 2002). Some soft tissue sex differences in the face may be produced and maintained by circulating sex steroids in adults. Men's testosterone levels predicted subjective masculinity ratings of their faces (Penton-Voak & Chen, 2004), and women's estrogen levels predicted subjective femininity ratings of their faces (Law Smith et al. …