Symmetry Is Related to Sexual Dimorphism in Faces: Data Across Culture and Species

Abstract

BackgroundMany animals both display and assess multiple signals. Two prominently studied traits are symmetry and sexual dimorphism, which, for many animals, are proposed cues to heritable fitness benefits. These traits are associated with other potential benefits, such as fertility. In humans, the face has been extensively studied in terms of attractiveness. Faces have the potential to be advertisements of mate quality and both symmetry and sexual dimorphism have been linked to the attractiveness of human face shape.Methodology/Principal FindingsHere we show that measurements of symmetry and sexual dimorphism from faces are related in humans, both in Europeans and African hunter-gatherers, and in a non-human primate. Using human judges, symmetry measurements were also related to perceived sexual dimorphism. In all samples, symmetric males had more masculine facial proportions and symmetric females had more feminine facial proportions.Conclusions/SignificanceOur findings support the claim that sexual dimorphism and symmetry in faces are signals advertising quality by providing evidence that there must be a biological mechanism linking the two traits during development. Such data also suggests that the signalling properties of faces are universal across human populations and are potentially phylogenetically old in primates.

Facial asymmetry and genetic ancestry inLatinAmerican admixed populations

“…On the other hand, FA patterns were widely assumed as indicators of development instahility (Palmer and Stroheck, 1986;Livshits and Kohyliansky, 1991;M0ller and Swaddle, 1997;Milne et al, 2003 h;DeLeon, 2007.;Little et al, 2008;De~eon and Richtsmeier, 2009;Ozener, 2010aOzener, , h, 2011Ozener and Fink, 2010;Weisensee, 2013 proportionality among individual and asymmetry covariance structures, thus suggesting that the same developmental processes underlie the expression of shape variation at hoth leveis (Klingenberg and Mclntyre, 1998). Another important aspect when dealing with comparisons among DA and FA is that, usually, it is assumed that the "normal" expectance, or optimal phenotype for a population is FA = O or perfect symmetry, which is not necessarily a strong null hypothesis as David (2001) argues andFarrera et al (2014) suggest for a sample of Mexicans.…”

“…Mardia et al (2000) and Schaefer et al (2006) defined the individual asymmetry (IA) component as the additive decomposition of DA, FA, and measurement error. Among the different asymmetries defined by Van Valen (1962), FA can be seen as the most relevant concerning studies of facial variation because it is considered a common indicator of development instability that can be measured directly on bilateral human phenotypic attributes (Palmer and Strobeck, 1986;M~ller and Swaddle, 1997;Milne et al, 200~;Little et al, 2008;peLeon and Richtsmeier, 2009;Ozener and Fink, 2010;Ozener, 2010;Weisensee, 2013), and it is observable in other animais as well (e.g., Leary and Allendorf, 1989;Clarke, 1993;MJ~Jller, 1996;Allenbach et al, 1999;Lens and Van Dongen, 2008;Little et al, 2012). Some previous analyses, however, challenged the existence of such straightforward relationship between FA and developmental instability (McKenzie and Clarke, 1988;Graham et al, 1993;Bjorksten et al, 2000;Lens et al, 2002).…”

Body Odor Quality Predicts Behavioral Attractiveness in Humans

“…For example, correlations are seen between face and body attractiveness (Feinberg et al, 2005;, between facial and body odor attractiveness (Rikowski & Grammer, 1999;Thornhill et al, 2003), and between facial attractiveness and the second-to-fourth digit ratio (2D:4D, where low ratios are related to masculinity) (Ferdenzi, Lemaître, Leongómez, & Roberts, 2011). Beyond attractiveness, facial masculinity is related to facial symmetry Little et al, 2008), and is also related to the second-to-fourth digit ratio (2D:4D, where low ratios are related to masculinity) (Burriss, Little, & Nelson, 2007;Fink, Manning, Neave, & Grammer, 2004;Fink, Seydel, Manning, & Kappeler, 2007;Manning, 2002;but see Ferdenzi et al, 2011).…”

Asymmetry in Facial Expressions as a Function of Social Skills

“…Previous studies have reported that males and females differ in facial shape [33] and facial muscle reactivity [34]. If males and females differed in social skills, the gender differences in social skills could be a potential confounding factor in the analysis of the relationships between social skills and facial expressions.…”