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

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.