Updating evidence on facial metrics: A Bayesian perspective on sexual dimorphism in facial width-to-height ratio and bizygomatic width

Facial width-to-height ratio (fWHR) is an extensively studied morphological measure, which was presumably shaped by sexual selection and has been linked to a wide range of perceptual and physiological traits. Underpinning these associations is the premise that fWHR is larger in men, which empirically exhibits a mixed and equivocal pattern in the literature due to variation in measurement, large sample sizes revealing small but significant differences, and a lack of control of body size. In Study 1, in a sample of 1949 faces, we used a Bayesian hierarchical model that incorporates prior information to simultaneously estimate sexual dimorphism in fWHR, adjusted for body size, across five measurement types. While we found larger fWHR in women, comparing this effect to variability in fWHR due to image capture settings revealed no robust evidence of sex differences in fWHR. In Study 2, we investigated sex differences in facial width specifically (also adjusted for body size), again incorporating prior information, and confirmed men have greater face width than women. Advances in this area can be made by shifting focus away from arbitrary ratios like fWHR to direct measures like facial width – as well as carefully considering prior evidence of existing associations.