Monday, January 25, 2021

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.




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.


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)


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


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


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.



jb said...

Peter, are you familiar with The Evolution of Beauty by Richard Prum? If not I'd like to strongly recommend it to you. Prum sees himself as a champion of Darwin's original "forgotten" theory of sexual selection, which acts as a second driver of evolution, entirely independent from and sometimes antagonistic to natural selection (i.e., survival of the prettiest vs survival of the fittest). It's mostly about birds, but it does touch on humans towards the end.

If you've read it I'm curious about your opinion. I thought the basic thesis was quite convincing, but I found many of his supporting arguments to be questionable, and I felt he spent too much time on personal score settling. What did you think?

LOADED said...

Eradicate the entirety of the Caucasoid race! NOW!

frost follower said...

The eye color isn't exclusively European; there's gray, green and blue in Afghanistan as well as red hair. Kipling used it as the basis of a story, and there's a National Geographic cover about 15 (?) years ago that they followed up on a decade or so later. It doesn't detract from your article, but found the existence interesting and slightly puzzling.

Peter Frost said...


I haven't read Plum's book. It's now on my list of "must-reads."


Don't worry. Just keep your mouth shut and keep smiling.

frost follower,

The European phenotype used to extend into Central Asia and even into northern Asia. At sites in south-central Siberia, dating from the third millennium BC to the fourth century AD, analysis of ancient DNA has shown that most of the buried individuals had blue or green eyes, light hair (blond, red, light brown), and light skin. Old Chinese records mention south Siberian peoples with “green eyes” and “red hair.”

Bouakaze, C., C. Keyser, E. Crubézy, D. Montagnon, and B. Ludes. (2009). Pigment phenotype and biogeographical ancestry from ancient skeletal remains: inferences from multiplexed autosomal SNP analysis. International Journal of Legal Medicine 123(4): 315-325.

Keane, A.H. (1886). Asia with Ethnological Appendix. London: Edward Stanford, p. 703.

LOADED said...

How can I keep my mouth shut and smile at the same time? White people dont have lips!

Bruce said...

It seems that northern dog breeds frequently have blue eyes. Siberian huskies are an example. I wonder how this developed?

Peter Frost said...


Eye color has probably diversified in dogs through human selection. In general, eye color is more diverse in domestic animals than in wild animals:

Human populations and breeds of domestic animals are composed of individuals with a multiplicity of eye (= iris) colorations. Some wild birds and mammals may have intraspecific eye color variability, but this variation seems to be due to the developmental stage of the individual, its breeding status, and/or sexual dimorphism. In other words, eye colour tends to be a species-specific trait in wild animals, and the exceptions are species in which individuals of the same age group or gender all develop the same eye colour. Domestic animals, by definition, include bird and mammal species artificially selected by humans in the last few thousand years. Humans themselves may have acquired a diverse palette of eye colors, likewise in recent evolutionary time, in the Mesolithic or in the Upper Paleolithic.

Bruce said...

Thanks Peter.

When we take our Husky out, we invariably get comments on the beauty of her eyes. I can believe humans selected for that.

Anonymous said...

I have wondered about blue eyes in dogs. While they do show up in various breeds, they seem most common in certain northern breeds like Huskies. It seems like they were deliberately selected for in those breeds like Huskies, while when they show up in other breeds where blue eyes are rare, they seem more like a fluke.

IF blue eyes in Europeans and certain northern breeds like Huskies aren't due to some kind of convergent evolution from being in a similar environment, do you suppose an ancient blue eyed European or proto-European population which had a preference for blue eyes selected for blue eyes in their dogs?