Light is not always right: peri-iridial lightness reduces attractiveness via perceived sex-typicality across human populations

Abstract

Abstract: Evolutionary psychology views the human eye as special. In particular, it claims that the light peri-iridial tissues surrounding a relatively darker iris form a combination that sets us apart from other primates. From this perspective, much less attention has been paid to how eye colouration varies between humans, although evidence indicates that variations in peri-iridial and iridial colouration influence both perceived facial attractiveness and sex-typicality. To determine what aspects of eye colouration influence the perception of faces, we have measured the colour of peri-iridial eye tissues (‘the white of the eye’) and the iris in nine samples from seven distant cultures (N = 1033) across three continents. The faces were rated on facial attractiveness and sex-typicality by raters from the corresponding populations. Accounting for the effects of skin lightness, age, and facial shape, we ran a Bayesian multilevel model to estimate global and sample-specific effects of colouration of the iris and peri-iridial tissues on perceived sex-typicality and facial attractiveness. This exploratory, cross-sectional study revealed an overall preference for slightly darker peri-iridial tissues in women, whereby this association was mediated by perceived sex-typicality. Our findings challenge the notion that the light-eyed phenotype is universally preferred by human raters. Instead, they suggest a preference for a moderate phenotype, perhaps because very light peri-iridial tissues are typical of faces which are generally perceived as less feminine. Women with bluer irises were generally perceived as more attractive but findings related to other colour channels and iris features were inconsistent and varied across samples. Significance statement: The morphological variation of human eyes is an understudied phenomenon. While attention has been paid to the alleged uniqueness of human eyes (compared to other primates), little is known about how variations in eye colouration influence human perception of faces. Our study included over 1000 individuals from seven culturally distinct regions, mapped human eye variation, and tested how eye colouration influences perceived attractiveness and sex-typicality. In humans, variation in eye colouration is relatively large and differs across populations. Our findings suggest that it affects the perception of faces. Paradoxically, darker peri-iridial regions (scleras) slightly enhance the perception of femininity and female attractiveness, which challenges the idea that lighter eyes are universally preferred. Moreover, blue/light irises were in some contexts linked to a higher attractiveness. These results further refine our understanding of the role of eye colouration in perceived attractiveness.