“A Novel Human Sex Difference: Male Sclera Are Redder and Yellower Than Female Sclera”, 2022-05-04 ():
[cf. et al 2018/ et al 2021] In a seminal study, 1997 found that the human sclera—the white of the eye—is unique among primates for its whitish color, and subsequent work has supported the notion that this coloration underlies the human ability to gaze follow. 1997 also claimed that there is no important sex difference in sclera color, though no data were presented to support the claim.
We investigated sclera color in a standardized sample of faces varying in age and sex, presenting the first data comparing male and female sclera color.
Our data support the claim that indeed there is a sex difference in sclera color, with male sclera being yellower and redder than female sclera. We also replicated earlier findings that female sclera vary in color across the adult lifespan, with older sclera appearing yellower, redder, and slightly darker than younger sclera, and we extended these findings to male sclera.
Finally, in two experiments we found evidence that people use sclera color as a cue for making judgements of facial femininity or masculinity. When sclera were manipulated to appear redder and yellower, faces were perceived as more masculine, but were perceived as more feminine when sclera were manipulated to appear less red and yellow. Though people are typically unaware of the sex difference in sclera color, these findings suggest that people nevertheless use the difference as a visual cue when perceiving sex-related traits from the face.
…while our stimulus set was carefully controlled, it was not color-calibrated. To our knowledge, no published research on sclera color has used color-calibrated images. Color calibration will be an important step for future work investigating sclera color. It is also important to note that our study of physical sex differences was underpowered to detect small effect sizes. Thus, we can claim on the basis of our data that there is a sex difference in sclera redness and yellowness, but we cannot claim that there is no sex difference in sclera luminance. It is possible that there is in fact a small sex difference in sclera luminance that our study was statistically-underpowered to detect.
[Keywords: eye, face, sexual dimorphism, age, aging, face perception, sclera]