Sex differences in the whites of the eyes

 

Women have sclera (whites of the eyes) that are less yellow and less red. Although this sex difference is unknown to almost everyone, people nonetheless perceive a female face with redder and yellower sclera as less feminine.

 

 

 

A recent study has shown that men and women differ in the color of their sclera—the whites of their eyes (Kramer and Russell 2022). Sclera are less yellow and less red in women. In both sexes, they become yellower, redder, and slightly darker with age.

 

Although this sex difference is unknown to almost everyone, it does influence perceptions of masculinity and femininity. When subjects were shown pairs of androgynous young or middle-aged faces that differed only in sclera color, they perceived the one with redder and yellower sclera as being more masculine. When shown pairs of male or female faces, they perceived the female faces with redder and yellower sclera as being less feminine. Sclera color did not affect perceptions of male or elderly faces.

 

The authors concluded that “people have learned the natural image statistics associated with male and female sclera, thereby representing males as having redder and yellower sclera than females.” Such learning would presumably be unconscious. What about other factors? Could perception of sclera color be hardwired?

 

We see a similar situation with the sex difference in skin color. Today, it’s largely unknown to people in the Western world, partly because many women have reduced it through deliberate tanning and partly because ethnic differences in skin color have become more visible in daily life. Yet several studies have shown that people associate women with light colors and men with dark colors. This was the case in three experiments with Dutch, Portuguese, and Turkish subjects:

 

·         The subjects identified personal names by gender faster when male names were presented in black and female names in white than when the gender/color combinations were reversed.

·         When asked to classify briefly appearing black and white blobs by gender, they usually classified the black blobs as male and the white ones as female.

·         In an eye-tracking experiment, their eyes showed longer observation and more frequent fixation when a dark object was associated with a male character and a light object with a female character (Semin et al. 2018).

 

Parallels with eye morphology and eye colors

 

Another recent study has shown that women have rounder eyes. This sex difference exists because the exposed sclera is more horizontally exposed in men. In addition, eye fissures are narrower and less rectangular in women than in men (Danel et al. 2020, see also Danel et al. 2018).

 

The same study also showed that this sex difference is confined to Europeans: women don’t have rounder eyes in other human populations. A similar situation exists with eye colors, which have diversified only in Europeans and more so in women than in men. Specifically, the range of eye colors is more evenly distributed among women: they have the less frequent colors more often and the more frequent ones less often (Frost 2022, pp. 9, 10, 13).

 

Is the sex difference in sclera color likewise confined to Europeans? For now, we don’t know. The sclera images were taken only from German faces (Kramer and Russell 2022).

 

Parallels with facial skin color

 

The natural colors of a woman’s face seem to interact with each other. The white of the eye serves to accentuate the color of the iris, and the lighter color of the facial skin serves to accentuate the darker colors of the eye and lip areas. If we consider the natural colors of the face in both sexes, a female face has a greater contrast in luminosity between the lighter-colored facial skin and the darker-colored lip and eye area (Russell 2003; Russell 2009; Russell 2010). A rating study has shown that faces with greater facial contrast are perceived as being more feminine:

 

Facial contrast was positively correlated with rated femininity of female faces but negatively correlated with rated masculinity of male faces. After controlling for skin luminance (which is also sexually dimorphic) and ethnicity (there were both Caucasian and East Asian faces), facial contrast was still positively correlated with rated femininity of female faces and negatively correlated with rated masculinity of male faces, though the relationship was very weak for male faces. In summary, greater facial contrast was considered more feminine in female faces and less masculine in male faces. (Russell 2010, p. 10).

 

In different cultures and geographic areas, women have developed cosmetics to accentuate the sex difference in facial contrast:

 

The received style of cosmetics involves darkening the eyes and lips while leaving the rest of the face largely unchanged. This is one of two patterns of cosmetic application that could increase facial contrast (the other being to significantly lighten the entire face, except for the eyes and lips). To confirm that cosmetic application increases facial contrast, we measured the facial contrast of the set of 12 Caucasian faces that were photographed with and without cosmetics (Russell 2009). On average, facial contrast was much larger with cosmetics than without cosmetics, and greater facial contrast was found in each of the 12 faces when they were wearing cosmetics than when they were not. Both eye contrast and mouth contrast were increased by cosmetics. The large and consistent increase in facial contrast achieved with cosmetics more clearly differentiates male and female faces. (Russell 2010, p. 13)

 

Conclusion

 

Europeans, and European women in particular, have followed an unusual evolutionary trajectory. Some kind of selection made their outward features more colorful, more accentuated, and more visually striking, particularly the features on or near the face.

 

Was it sexual selection? The authors of the eye morphology study considered that possibility but rejected it because female eye morphology does not correlate with two other aspects of female attractiveness: face shape and facial averageness (Danel et al. 2020). The lack of correlation, however, may simply mean that these three aspects impose different constraints on sexual selection. Making a face more average-looking is not at all the same process as increasing its visual contrast or adding bright colors.

 

 

References

 

Danel, D.P., S. Wacewicz, Z. Lewandowski, P. Zywiczynski, and J.O. Perea-Garcia. (2018). 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. Acta Ethologica 21: 203-208. https://doi.org/10.1007/s10211-018-0296-5  

 

Danel, D.P., S. Wacewicz, K. Kleisner, Z. Lewandowski, M.E. Kret, P. Zywiczynski, and J.O. Perea-Garcia. (2020). Sex differences in ocular morphology in Caucasian people: a dubious role of sexual selection in the evolution of sexual dimorphism of the human eye. Behavioral Ecology and Sociobiology 74(115) https://doi.org/10.1007/s00265-020-02894-1

 

Frost, P. (2022). European Hair, Eye, and Skin Color: Solving the Puzzle. Washington: Academica Press. ISBN 9781680538724

 

Kramer, S.S., and R. Russell. (2022). A Novel Human Sex Difference: Male Sclera Are Redder and Yellower than Female Sclera. Archives of Sexual Behavior 51, 2733–2740. https://doi.org/10.1007/s10508-022-02304-9

 

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features. Perception 32(9): 1093-1107. http://dx.doi.org/10.1068/p5101

 

Russell, R. (2009). A sex difference in facial pigmentation and its exaggeration by cosmetics. Perception 38(8): 1211-1219. https://doi.org/10.1068/p6331

 

Russell, R. (2010). Why cosmetics work. In R.B. Adams Jr., N. Ambady, K. Nakayama, and S. Shimojo (Eds.) The Science of Social Vision, (pp. 186-203). New York: Oxford.

 

Semin, G.R., T. Palma, C. Acartürk, and A. Dziuba. (2018). Gender is not simply a matter of black and white, or is it? Philosophical Transactions of the Royal Society B Biological Sciences 373(1752):20170126. https://doi.org/10.1098/rstb.2017.0126

Sex differences in human eye morphology

 

Women have rounder-looking eyes with narrower fissures, but only in Europeans. Eyes are not sexually dimorphic in other human populations. (Petr Novak, Wikicommons)

 

 

The exposed white of the eye is larger in men than in women among Europeans but not in other human groups. This sexual dimorphism is due to the white of the eye being more horizontally exposed in men, with the result that female eyes look rounder. In addition, eye fissures are narrower and less rectangular in women (Danel et al. 2018; Danel et al. 2020).

 

This is analogous to what we see with eye color and hair color. Eyes are brown in most humans with the exception of Europeans, whose eyes may also be blue, gray, or green. Hair is black in most humans with the exception of Europeans, whose hair may also be blonde, red, or brown. In both cases, the palette of colors is more evenly balanced in women than in men. Women are less likely to have the more common hues, like blue or brown eyes and black hair. Conversely, they are more likely to have the less common hues, like green eyes and red hair.

 

There is no common genetic cause of these sex differences in eye morphology, eye color, and hair color. The genes are different in each case. The common cause seems to be some kind of selection among ancestral Europeans. Something favored the reproduction of women with rounder-looking eyes and less common eye and hair colors.

 

Was that "something" a someone? Were men selecting women through a process of sexual selection? That has been my explanation: in northern Eurasia until the end of the last ice age, women outnumbered men and had to compete for them, as a result of high male mortality and the high cost of polygyny. There was thus strong selection for women with an eye-catching appearance, and this selection ultimately changed the appearance of both sexes. The new phenotype eventually died out in northern Asia but survived in parts of Europe, which had a larger and more continuous human presence. It then spread throughout the rest of Europe almost at the dawn of history (Frost 2006; Frost 2014; Frost et al. 2017).

 

Danel et al. (2020) consider this explanation but reject it because female eye morphology does not correlate with two other aspects of female attractiveness: face shape and facial averageness. That lack of correlation, however, simply shows that each of these aspects has different constraints on the direction of sexual selection:

 

Eye morphology - the direction of sexual selection seems open-ended. Women are more attractive if they have rounder eyes.

 

Face shape - the direction of sexual selection goes into reverse beyond a certain point. Women are more attractive if they have smaller chins and smaller noses, but only up to a certain point. Excessively small chins and noses are not attractive either.

 

Facial averageness - the constraints are again different. Women become less attractive on each side of a narrow median.

 

References

 

Danel, D.P., S. Wacewicz, Z. Lewandowski, P. Zywiczynski, and J.O. Perea-Garcia. (2018). 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. Acta Ethologica 21: 203-208.

https://doi.org/10.1007/s10211-018-0296-5

 

Danel, D.P., S. Wacewicz, K. Kleisner, Z. Lewandowski, M.E. Kret, P. Zywiczynski, and J.O. Perea-Garcia. (2020). Sex differences in ocular morphology in Caucasian people: a dubious role of sexual selection in the evolution of sexual dimorphism of the human eye. Behavioral Ecology and Sociobiology 74(115)

https://doi.org/10.1007/s00265-020-02894-1

 

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior 27(2): 85-103.

https://doi.org/10.1016/j.evolhumbehav.2005.07.002

 

Frost, P. (2014). The puzzle of European hair, eye, and skin color. Advances in Anthropology 4(2): 78-88.

https://doi.org/10.4236/aa.2014.42011

 

Frost, P., K. Kleisner, and J. Flegr. (2017). Health status by gender, hair color, and eye color: Red-haired women are the most divergent. PLoS One 12(12): e0190238.

https://doi.org/10.1371/journal.pone.0190238