Fluctuating Asymmetry and Sexual Dimorphism in Human Facial Morphology: A Multi-Variate Study.

Omid Ekrami,Stefan Van Dongen,Ellen Van Assche,Susan Walsh,Mary L Marazita,Seth M Weinberg,Mark D Shriver,Peter Claes

doi:10.3390/sym13020304

Abstract

Fluctuating asymmetry is often used as an indicator of developmental instability, and is proposed as a signal of genetic quality. The display of prominent masculine phenotypic features, which are a direct result of high androgen levels, is also believed to be a sign of genetic quality, as these hormones may act as immunosuppressants. Fluctuating asymmetry and masculinity are therefore expected to covary. However, there is lack of strong evidence in the literature regarding this hypothesis. In this study, we examined a large dataset of high-density 3D facial scans of 1260 adults (630 males and 630 females). We mapped a high-density 3D facial mask onto the facial scans in order to obtain a high number of quasi-landmarks on the faces. Multi-dimensional measures of fluctuating asymmetry were extracted from the landmarks using Principal Component Analysis, and masculinity/femininity scores were obtained for each face using Partial Least Squares. The possible correlation between these two qualities was then examined using Pearson's coefficient and Canonical Correlation Analysis. We found no correlation between fluctuating asymmetry and masculinity in men. However, a weak but significant correlation was found between average fluctuating asymmetry and masculinity in women, in which feminine faces had higher levels of fluctuating asymmetry on average. This correlation could possibly point to genetic quality as an underlying mechanism for both asymmetry and masculinity; however, it might also be driven by other fitness or life history traits, such as fertility. Our results question the idea that fluctuating asymmetry and masculinity should be (more strongly) correlated in men, which is in line with the recent literature. Future studies should possibly focus more on the evolutionary relevance of the observed correlation in women.

Highlights

Sexual dimorphism and symmetry in human faces have both been proposed to signal an individual’s genetic quality [1,2,3,4]
We did not find any significant correlations between the average C-Fluctuating Asymmetry (FA) or any of the principal components (PC) of the corrected FA (C-FA) space with the masculinity scores in men, and all of the estimates were, overall, small
There was no correlation found between the absolute value of the individual measure of Directional Asymmetry (DA) (F-DA) and masculinity in men or women

Summary

Introduction

Sexual dimorphism and symmetry in human faces have both been proposed to signal an individual’s genetic quality [1,2,3,4]. Bilateral symmetry is believed to reflect an individual’s ability to block out genetic stress and environmental perturbations during their development [5]. Fluctuating Asymmetry (FA), defined as directionally-random small perturbations from perfect symmetry, is often used as an indicator of DI, and is expected to show a negative correlation with the genetic quality of an organism. Following this hypothesis, FA has been extensively studied in the human face and body [2,6,7], as well as in animals [8,9,10]. Studies that have investigated this possible link have often reported weak or non-existent correlations, even though, on average, a robust and significant correlation exists [7]

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Results

Conclusion