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, 21 (3), 203-208

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


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

Dariusz P Danel et al. Acta Ethol.


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.

Keywords: Cooperative eye hypothesis; Exposed sclera size index SSI; Human eye; Trustworthiness; White sclera.

Conflict of interest statement

The authors declare that they have no conflict of interest.This article does not contain any studies with human participants performed by any of the authors.


Fig. 1
Fig. 1
Sclera size index (SSI) by individuals’ race and sex. Vertical bars denote .95 confidence intervals
Fig. 2
Fig. 2
The association between perceived trustworthiness and exposed sclera size (SSI—sclera size index) categorised by sex and self-declared race. Note that the association was statistically significant only for White women (see details in the main text)

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