Women and eye color

Inheritance of eye color doesn’t follow a simple Mendelian model. Although the blue-eye allele (C) is less dominant than the brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes blue-eyed. There is also a sex difference, with women having a more diverse palette of eye colors. (source)

Most humans have black hair and brown eyes. Europeans display a much more diverse range of hues, their hair being also brown, flaxen, golden, or red and their eyes being also blue, gray, hazel, or green.

This diversification has gone farther in European women than in European men. According to a twin study, women have a more diverse palette of hair colors, with a greater prevalence of lighter shades, particularly red hair (Shekar et al., 2008).

Women also have a more diverse palette of eye colors, according to a recent study of six SNPs associated with eye color. When both copies of the so-called blue-eye allele are present, the resulting phenotype is more variable in women than in men (Martinez-Cadenas et al., 2013). This translates into a greater range of female eye colors in regions, like northern and eastern Europe, where blue eyes are the single most common phenotype (Walsh et al.,2012). As the study’s authors observe, “in populations with very high blue-eye frequency, such as Iceland or Holland, females show greater proportion of green eyes at the expense of blue eyes” (Martinez-Cadenas et al., 2013). The authors also confirmed earlier findings that eye color doesn’t follow a simple Mendelian model. Although the blue-eye allele (C) is less dominant than the brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes blue-eyed. 

Thus, both hair color and eye color tend to be more diverse in women. There is, however, one difference. With hair color, the “derived” alleles are more fully expressed in women than in men. With eye color, they are less fully expressed. This seems to argue against the theory that hair and eye color diversified through a process of sexual selection that acted more strongly on women than on men. Since blue eyes are the derived phenotype, they should be more fully expressed in women because the female sex is the main target of this selection pressure. Yet the reverse is actually true.

The reason may be physiological. It seems easier to produce new eye colors by modifying the way the blue-eye allele is expressed than by simply creating new alleles. Thus, to produce a range of hues that extends beyond brown and blue, the so-called blue-eye genotype must be more common than the actual incidence of blue eyes. As a result, “more females bearing the ‘blue eye genotype’ (HERC2/OCA2 CC homozygous genotype) end up having green or intermediate eyes” (Martinez-Cadenas et al., 2013).

Other recent studies

A recent paper has confirmed that European eye color diversified through some kind of selection pressure, and not random factors like genetic drift or founder effects. Blue-eye alleles show a very strong signal of selection (Donnelly et al., 2012). Another study, however, has failed to find any preference for blue eyes over other colors, an indication that all eye colors are at selective equilibrium, at least for the German population under study. This finding may be related to the already high frequency of blue eyes in that population:

Perhaps the frequency of eye colors plays a role. In most countries, blue eyes are less prevalent than other eye colors and may have the image of something special and more valuable. If this assumption is true, brown eyes should be preferred in countries where the majority of the population has blue eyes. (Gründl et al, 2012).

In this case, sexual selection is frequency-dependent, shifting to whichever eye color is least frequent. Eventually, an equilibrium is reached where color novelty is in balance with other characteristics, such as color brightness, that may increase sexual attractiveness.

This last finding shows the opportunistic nature of sexual selection. When too many of one sex have to compete for mating opportunities with too few of the other sex, there will be selection for any traits that increase mating success. In many cases, these traits will hyperstimulate a mental algorithm that is used for sex recognition. In other cases, hyperstimulation will simply involve use of bright or novel colors that can better engage visual attention and remain longer in memory.  

References

Donnelly, M.P., P. Paschou,  E. Grigorenko, D. Gurwitz, C. Barta, R-B. Lu, O.V. Zhukova, J.-J. Kim, M. Siniscalco, M. New, H. Li, S.L.B. Kajuna, V.G. Manolopoulos, W.C. Speed, A.J. Pakstis, J.R. Kidd, and K.K. Kidd. (2012). A global view of the OCA2-HERC2 region and pigmentation, Human Genetics, 131, 683–696.

http://europepmc.org/articles/PMC3325407

Gründl, M., S. Knoll, M. Eisenmann-Klein, and L. Prantl. (2012). The blue-eyes stereotype: Do eye color, pupil diameter, and scleral color affect attractiveness? Aesthetic Plastic Surgery, 36, 234–240.

http://link.springer.com/article/10.1007/s00266-011-9793-x#page-1

Martinez-Cadenas, C., M. Pena-Chilet, M. Ibarrola-Villava, and G. Ribas. (2013). Gender is a major factor explaining discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the IrisPlex model, Forensic Science International: Genetics, 7, 453–460.

http://europepmc.org/abstract/MED/23601698

Shekar, S.N., D.L. Duffy, T. Frudakis, G.W. Montgomery, M.R. James, R.A. Sturm, & N.G. Martin. (2008). Spectrophotometric methods for quantifying pigmentation in human hair—Influence of MC1R genotype and environment, Photochemistry and Photobiology, 84, 719–726.

https://genepi.qimr.edu.au/contents/p/staff/shekaretal2008b1.pdf

Walsh, S., A. Wollstein, F. Liu, U. Chakravarthy, M. Rahu, J.H. Seland, G. Soubrane, L. Tomazzoli, F. Topouzis, J.R. Vingerling, J. Vioque, A.E. Fletcher, K.N. Ballantyne, and M. Kayser. (2012). DNA-based eye colour prediction across Europe with the IrisPlex system, Forensic Science International: Genetics, 6, 330–340.

http://www.fsigenetics.com/article/S1872-4973(11)00144-X/abstract

Eye color, face shape, and perceived personality traits

Averaged face of blue-eyed male subjects (left). Averaged face of brown-eyed male subjects (right). Czech population. (Kleisner et al., 2010)

Karel Kleisner’s team is continuing its work on eye color, face shape, and perceived personality traits:

We tested whether eye color influences perception of trustworthiness. Facial photographs of 40 female and 40 male students were rated for perceived trustworthiness. Eye color had a significant effect, the brown-eyed faces being perceived as more trustworthy than the blue-eyed ones. Geometric morphometrics, however, revealed significant correlations between eye color and face shape. Thus, face shape likewise had a significant effect on perceived trustworthiness but only for male faces, the effect for female faces not being significant. To determine whether perception of trustworthiness was being influenced primarily by eye color or by face shape, we recolored the eyes on the same male facial photos and repeated the test procedure. Eye color now had no effect on perceived trustworthiness. We concluded that although the brown-eyed faces were perceived as more trustworthy than the blue-eyed ones, it was not brown eye color per se that caused the stronger perception of trustworthiness but rather the facial features associated with brown eyes. (Kleisner et al., 2013)

Eye color is thus associated in males with a specific face shape, even when the men are of the same ethnic background, i.e., Czechs in this study. Face shape is more robust if eye color is brown and less so if eye color is blue. Furthermore, a robust male face seems to evoke an image of dominance and trustworthiness, whereas a less robust one is perceived as being less dominant and less trustworthy.

When the first study came out, in 2010, I thought this association between eye color and face shape was due to ethnic admixture, i.e., some of the brown-eyed participants had Jewish or Roma ancestry. If this were so, however, face shape would vary to a greater degree among the brown-eyed participants than among the blue-eyed ones, yet the current study found no such difference. The “ethnic admixture” explanation also fails to explain why blue eyes are associated with a less robust face in men but not in women.

Are European facial features actually female facial features?

There thus seems to be a linkage between eye color and face shape. This finding is consistent with findings for other European physical traits, especially bright or colorful facial features. These European traits—white skin, multi-colored hair, multi-colored eyes, and a less robust face shape—are actually female traits. They seem to be due to a selection pressure that first acted on early European women and then spilled over on to early European men. This phenotypic change affected both sexes because the traits in question are at most mildly sex-linked.

The direction of sex linkage is consistent with the above model of selection. Hair color became polymorphic in Europeans through the emergence of brighter hues, but this evolutionary change is most pronounced in European women. According to a twin study, hair is lighter in women than in men, with red hair being especially more common. Women also show more variation in hair color (Shekar et al., 2008). Again, this evolutionary trend seems to have been driven by European women with European men tagging along.

There is also unpublished evidence that “European” hair and eye colors (i.e., non-black hair and non-brown eyes) are associated with a higher degree of estrogenization before birth, as indicated by digit ratio. This prenatal estrogenization might also favor a more feminine face shape. Face shape would thus correlate with eye color because of a shared determining factor: the degree to which the fetus is estrogenized or androgenized in the womb. Such a correlation would have been stronger in the male participants than in the female participants because estrogenization is overdetermined in females, i.e., almost all girls are exposed to enough estrogen in the womb to feminize their face shape, whereas this level of estrogenization would be reached only in blue-eyed boys.

In one respect, however, the blue-eyed face looks less feminized, the chin being relatively narrower and longer. It might be that this is what happens when the trajectory of facial development is initially programmed for a feminine appearance before birth but is not supported by circulating estrogen later in life.

References

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4),169-191. http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103 http://www.sciencedirect.com/science/journal/10905138

Kleisner, K., L. Priplatova, P. Frost, & J. Flegr. (2013). Trustworthy-looking face meets brown eyes, PLoS One,  8(1): e53285. doi:10.1371/journal.pone.0053285

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053285

Kleisner, K., T. Kočnar, A. Rubešova, and J. Flegr. (2010). Eye color predicts but does not directly influence perceived dominance in men, Personality and Individual Differences, 49, 59–64.

Shekar, S.N., D.L. Duffy, T. Frudakis, G.W. Montgomery, M.R. James, R.A. Sturm, & N.G. Martin. (2008). Spectrophotometric methods for quantifying pigmentation in human hair—Influence of MC1R genotype and environment. Photochemistry and Photobiology, 84, 719–726.