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(D) Surface warp showing the effect (exaggerated) of the minor allele (T) on lower facial shape. (C) Surface warp showing the effect (exaggerated) of the major allele (G) on lower facial shape. Both the independent (SNP) as the dependent (facial shape) variables were corrected for these covariates.
Sexual dimorphism is strongest in digit ratios involving digit 2 with palmar measurements. In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children and further developed the idea that the index was a marker of prenatal sex hormones. This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex. By 1930, statistically significant sex differences in digit ratio were established in a sample of 201 men and 109 women, after which time the sex difference appears to have been largely forgotten or ignored. Here perhaps we have evidence that certain face traits may be more involved in sexual selection than others.
The correlations with asymmetry are equivalent to the results of the regression analysis as only a single variable persists in each analysis. Tables S3, S4, and S5 show the correlations between all of the variables for each image set and for male and female images. We randomly selected 50 images from each grouping (male/female×macaque/ European/Hadza) so that each image set was equally represented in the following calculations. These were found to be sexually dimorphic in the European sample here (see below) and in previous studies .
For the European images, male (177 individuals) and female (318 individuals) participants had their photograph taken in the laboratory with a digital camera under standardised lighting conditions. If quality was unrelated to size and symmetry we would expect the cost of ornamentation to create developmental stress for their owners leading to increased asymmetry in large ornaments. Males prefer feminised female faces and females show increased preferences for masculinity in contexts consistent with masculinity signalling some aspect of quality , .
Participants were administered a short questionnaire assessing age and sex before completing the face tests. While random these images were labelled in the same manner (symmetric/asymmetric). These were made using the same methods as above but consisted of 15 randomly selected images from the appropriate groups.
The spatial-dense configurations then underwent a generalized Procrustes superimposition to eliminate differences in orientation, position and scale (Rohlf and Bookstein, 2003). Briefly, the 3D images were imported in Matlab™ 2016b for a spatial-dense registration of the images (Claes et al., 2012a), in which a symmetrical anthropometric mask was mapped onto the images, leading to a homologous spatial-dense configuration of quasi-landmarks. The process of registration, quality control and segmentation is described in detail in Claes et al. (2018). Genome-wide data was available for 8,952 subjects of the B2261 study which is titled "Exploring distinctive facial features and their association with known candidate genes." 3566 subjects of the Avon Longitudinal Study of Parents and their Children (ALSPAC) cohort (age 14–17 years) were included in this study These subjects are of Western–European descent without a history of facial defects or craniofacial surgery, recruited in Avon, England.
Through altering testosterone levels, our implicated SNPs may impact the cell populations involved in facial growth and development. The most prominent sex differences in human facial morphology tend to involve the mandible, zygomatic region (cheeks), lips, forehead, and nose (Toma et al., 2008; Koudelová et al., 2015; Kesterke et al., 2016; Matthews et al., 2018). The genetic associations between the candidate SNPs and the ratios (Supplementary Table S3) were investigated in the entire 3DFN sample, in males/females separately, and in a post-pubertal subpopulation. This could mean that, although being in high LD, both SNPs might still have a separate effect on SHBG and testosterone levels in the body, reflected in the face.
Rating sessions took place in each of the investigated populations, and raters judged only opposite-sex faces from their own population. The photographs were subsequently post-produced to adjust the eyes horizontally at the same height. Likewise, women of the Tanzanian Hadza tribe showed no preference for large body size in potential mates52 and they were more likely to marry men shorter than themselves compared to British women53. Greater mass may also be disadvantageous for hunting as it may make one more visible to prey; as indicated by the negative correlation between body size and food returns in African hunter-gatherers51.
These results indicate that testosterone-related genetic variants affect normal-range facial morphology, and in particular, facial features known to exhibit strong sexual dimorphism in humans. Finally, it is worth noting that sexual dimorphism measured from facial morphology may not capture all visual aspects of sex differences to which humans are optically sensitive. Neither cross-cultural differences in facial shape variation, sex differences in body height, nor differing preferences for facial femininity and masculinity across countries, explain the observed patterns of facial dimorphism. In conclusion, our finding of sex specific co-variation with symmetry, femininity for females, masculinity for males, indicates then that both sexual dimorphism and symmetry likely are signals advertising quality.

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