Humans do not perceive conspecifics with a greater exposed sclera as more trustworthy: a preliminary cross-ethnic study of the function of the overexposed human sclera

Abstract

Understanding the adaptive function of the unique morphology of the human eye, in particular its overexposed white sclera, may have profound implications for the fields of evolutionary behavioural science, and specifically the areas of human interaction and social cognition. Existing hypotheses, such as the cooperative eye hypothesis, have attracted a lot of attention but remain untested. Here, we: (i) analysed variation in the visible sclera size in humans from different ethnic backgrounds and (ii) examined whether intraspecific variation of exposed sclera size is related to trust. We used 596 facial photographs of men and women, assessed for perceived trustworthiness, from four different self-declared racial backgrounds. The size of the exposed sclera was measured as the ratio between the width of the exposed eyeball and the diameter of the iris (sclera size index, SSI). The SSI did not differ in the four examined races and was sexually monomorphic except for Whites, where males had a larger SSI than females. In general, the association between the SSI and trustworthiness was statistically insignificant. An inverted U-shaped link was found only in White women, yet the strength of the effect of interaction between sex and race was very small. Our results did not provide evidence for the link between exposed sclera size and trustworthiness. We conclude that further investigation is necessary in order to properly assess the hypotheses relating to the socially relevant functions of overexposed sclera.

Sex differences in ocular morphology in Caucasian people: a dubious role of sexual selection in the evolution of sexual dimorphism of the human eye

“…Our results on SSI are in line with the previous findings reported by Danel et al (2018b), who found that SSI is sexually dimorphic in Caucasians, using a large multi-ethnic dataset with self-identified racial categories. The current analysis performed on data from a homogenous population confirms that the horizontal dimension of the white sclera is sexually dimorphic in Caucasians.…”

“…A potential specificity to Caucasians of the sex differences in SSI (see Danel et al (2018b)) and a high magnitude of sexual dimorphism in WHR observed particularly in Caucasians (Hajnis et al 1994; and discussed above) may suggest that some geographically restricted ecological conditions contributed to the observed sexual dimorphism. However, the nonsignificant correlations between the examined eye features and facial markers of attractiveness point out that mechanisms other than sexual selection may have contributed to the development of these sex differences.…”

“…A potential link between scleral morphology and human sexual selection was first noted in our previous study (Danel et al 2018b). Using a large sample of almost 600 men and women from four self-identified racial backgrounds (Ma et al 2015), we analysed variation in sclera size as measured with the sclera size index (SSI; Kobayashi and Kohshima 1997).…”

“…Using a large sample of almost 600 men and women from four self-identified racial backgrounds (Ma et al 2015), we analysed variation in sclera size as measured with the sclera size index (SSI; Kobayashi and Kohshima 1997). The study found that the size of the exposed sclera was similar in men and women self-identifying as Asian, Black and Latino but not in those self-identifying as White, where SSI was sexually dimorphic, and men had significantly more exposed sclera than women (Danel et al 2018b). We discussed these findings with reference to the theory that specific ecological conditions intensified sexual selection in ancestral Caucasian populations of north-eastern and central Europe (see Frost (2006Frost ( , 2014).…”

“…In the current work, we further explored sex differences in ocular morphology in north-eastern and central European Caucasians, represented by Czechs, using a different sample than Danel et al (2018b). Our main aim was to confirm sexual dimorphism in eye morphology and carry out exploratory analyses with a view to identifying its adaptive nature (or lack thereof).…”