Friday, May 27, 2011

Is eye color sex-linked?

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

If sexual selection of women diversified the eye color of early Europeans, the new colors should tend to be sex-linked, since the selection targeted women more than men. There is now evidence that blue eyes are linked to feminization of face shape.


Throughout most of the world, humans have brown eyes, black hair, and varying shades of dark skin. An exception is Europe, especially its northern and eastern portions. Here, eyes are not only brown but also blue, gray, hazel, or green. Hair is not only black but also brown, flaxen, golden, or red. Finally, skin has been whitened to the end point of depigmentation.

How can we explain this unusual color pattern? Is it simply due to lightening of skin color? In other words, did reduction of skin pigmentation incidentally reduce eye and hair pigmentation? The diverse palette of eye and hair colors would thus be a side effect of selection for lighter skin.

This explanation is dubious. For one thing, the gene loci are not the same. European skin has whitened mainly through allelic change at SLC45A2 (AIM1) and SLC24A5 (Soejima et al., 2005; Voight et al., 2006). European hair has diversified in color through a proliferation of new alleles at MC1R (Makova & Norton, 2005; Rana et al., 1999). European eyes have diversified in color mainly through a proliferation of new alleles at the OCA2-HERC2 gene complex (Duffy et al., 2007; Kayser et al., 2008; Sturm & Frudakis, 2004; Sturm et al., 2008).

Some of these novel hair and eye colors are also associated with lighter skin, notably the red hair and blue eye phenotypes. But why would selection for lighter skin lead to a proliferation of new alleles for hair and eye color—most of which have little or no influence on skin color? Why is it that red hair and blue eyes have not reached fixation in any human population, even those with milk-white skin?

Sexual selection?

I have argued that the cause was an intensification of sexual selection among early Europeans, specifically sexual selection of women (Frost, 2006; Frost, 2008). When too many women have to compete for too few men, there is selection for visible female traits that hyperstimulate certain algorithms in the male brain, particularly those for gender recognition or fertility assessment.

Among hunter-gatherers—and all humans were hunter-gatherers until 10,000 years ago, sexual selection of women tends to intensify farther away from the equator. On the one hand, hunting distances lengthen, thus increasing male mortality. On the other, polygyny becomes less common because women are less able to provide for themselves and their children in winter, thus making it costlier for a man to provide for a second wife. Result: a growing imbalance between the numbers of women and men on the mate market.

Among early modern humans, these two equator-to-arctic trends reached a common end-point in the steppe-tundra of northern Eurasia, such as existed during the last ice age (25,000 – 10,000 years ago). Hunting distances were very long because almost all food came from highly mobile herds of mammals, notably reindeer. Male provisioning was similarly at a maximum, women having little or no food autonomy.

The Eurasian steppe-tundra was continuously inhabited only in portions of its European end, where it ran farther south because of the large icecap over Scandinavia and where the moderating influence of the Atlantic Ocean and the Gulf Stream made the climate moister and milder. Here, the effects of intense sexual selection could accumulate and be passed on from generation to generation. Conversely, northern Asia appears to have suffered episodes of complete depopulation, particularly at the height of the last ice age.

Sexual selection: color novelty and color diversification

When sexual selection is weak, the adaptive equilibrium is dominated by selection for a dull, cryptic appearance that reduces detection by predators (Kirkpatrick, 1987). As sexual selection grows stronger, the equilibrium shifts toward a more noticeable appearance that retains the attention of potential mates, typically by means of vivid and/or novel colors.

One outcome may be a polymorphism of brightly colored phenotypes, due to selection shifting to scarcer and more novel hues whenever a color variant becomes too common (Endler, 1980; Frost, 2006; Hughes et al., 1999; Hughes et al., 2005; Olendorf et al., 2006). This frequency dependence has been shown in humans. Thelen (1983) presented male participants with slides showing attractive brunettes and blondes and asked them to choose, for each series, the woman they would most like to marry. One series had equal numbers of brunettes and blondes, a second 1 brunette for every 5 blondes, and a third 1 brunette for every 11 blondes. Result: the rarer the brunettes were in a series, the likelier any one of them would be chosen.

But why would this stronger selection diversify hair and eye color while simply whitening skin color? The answer seems to be that sexual selection acts on skin color not only through rare-color preference but also by hyperstimulating a gender-recognition algorithm, i.e., by accentuating a visible female-specific trait. In our species, female skin has less melanin and hemoglobin than does male skin, i.e., women look paler, men browner and ruddier (Edwards & Duntley, 1939; Hulse, 1967; Jablonski & Chaplin, 2000).

Sex linkage?

If European skin had whitened through selection for lighter-skinned women, it should have whitened more in women than in men, thus becoming more sexually dimorphic. Yet skin color actually seems to be less dimorphic in Europeans than in other humans (Madrigal & Kelly, 2006). This finding does not necessarily invalidate the sexual selection hypothesis. It may be that the pigmentary sex difference cannot fully express itself in light-skinned populations. Skin color is dimorphic because girls progressively lighten in color during adolescence, and such lightening may be less easily expressed if melanin production is already low. Indeed, this dimorphism seems to be almost absent in people whose skin starts off with very little pigment, such as Dutch and Belgian subjects (Frost, 2007).

And hair and eye color? Sexual selection of women should favor novel hair and eye colors more so in women than in men. Granted, both polymorphisms arose over a short and relatively recent span of time, so sexual selection would have worked with those alleles that were initially available and, for the most part, not sex-linked. But surely a few of these mutant alleles would have been sex-linked and, as such, favored over non-sex-linked ones.

This does seem to be the case with hair color. Blond hair darkens with age more slowly in women than in men (Olivier, 1960, p. 74). A ‘digit ratio’ study indicates that prenatal exposure to estrogen is higher in individuals with blond hair or non-brown eyes (Mather et al., unpublished). The same study, however, found no evidence of sexual dimorphism. Women and men had roughly the same proportions of hair and eye colors among the 18-to-38 year olds under study. Such a dimorphism, if it does exist, may be a transient one limited to younger age groups.

Moreover, Kleisner et al. (2010) have found an apparent sex linkage between blue eyes and feminization of face shape. They initially wished to determine whether eye color influences perception of male dominance, using facial pictures of Czech men. The results showed that brown-eyed men were rated as more dominant than blue-eyed men. As a control, the authors repeated the experiment after altering the facial photos of the brown-eyed men to make them blue-eyed. These altered photos were still rated as more dominant.

Careful study of the photos revealed that the brown-eyed men had more masculine facial features:



In contrast with blue-eyed males, brown-eyed males have statistically broader and rather massive chins, broader (laterally prolonged) mouths, larger noses, and eyes that are closer together with larger eyebrows. In contrast, blue-eyed males show smaller and sharper chins, mouths that are laterally narrower, noses smaller, and a greater span between the eyes.

The perception of brown-eyed men as more dominant is thus due to their more masculine facial appearance—and not to their brown eyes. The authors suggest that some kind of sex linkage may be responsible, while adding: “Repeating this study in other populations with polymorphism in eye color can test this hypothesis.”

If true, this finding would be consistent with Liberton et al. (2009) who found that European face shape has differentiated from West African face shape through a selective force that has acted primarily on women. Both findings, in turn, would support my argument that many of the differences we see among human populations are not due to differences in natural selection, and hence differing natural environments. Instead, the cause lies in differing intensities of sexual selection, and whether this selection has primarily targeted men or women (Frost, 2008).

When men are the prime targets of sexual selection, as in tropical ‘female farming’ societies, the result is accentuation of certain male features. Meanwhile, there is a slackening of female-targeted sexual selection, which leads to women having a more functional and less ornamental appearance. This pattern is reversed when women are the prime targets of sexual selection (Frost, 2008).

Ethnic substructure?

There is admittedly an alternate explanation for the above finding: ethnic substructure among the Czech subjects. The Czech Republic has historically been home to ethnic minorities who statistically differ from ethnic Czechs in facial appearance, i.e., Jews, Germans, and Roma. Jews and Roma, like other populations of Mediterranean or southwest Asian origin, tend to have brown eyes and a more robust face shape. Conversely, Germans are likelier to have blue eyes and more gracile faces. Although the Jewish and German communities were severely decimated during World War II and its aftermath, with many survivors later emigrating, there still remain significant numbers of Czech citizens who are wholly or partly of Jewish or German origin. Since WWII, there has also been an influx of Roma into the Czech Republic.

It would be difficult to rule out this kind of explanation, even if one questioned the subjects of the study. After the last war, many Czech citizens of Jewish or German origin felt it best to conceal their ancestry, for fear of discrimination or even expulsion from the country. Today, their grandchildren may be completely unaware of their origins.

This is all the more reason to replicate the results with subjects from another population, preferably one with as little ethnic substructure as possible.

References

Duffy, D.L., G.W. Montgomery, W. Chen, Z.Z. Zhao, L. Le, M.R. James, N.K. Hayward, N.G. Martin, & R.A. Sturm. (2007). A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation. American Journal of Human Genetics, 80, 241-52.

Edwards, E.A., & S.Q. Duntley. (1939). The pigments and color of living human skin. American Journal of Anatomy, 65, 1-33.

Endler, J.A. (1980). Natural selection on color patterns in Poecilia reticulata. Evolution, 34, 76‑91.

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://www.jsecjournal.com/articles/volume2/issue4/NEEPSfrost.pdf

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

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

Hughes, K.A., L. Du, F.H. Rodd, & D.N. Reznick. (1999). Familiarity leads to female mate preference for novel males in the guppy, Poecilia reticulata. Animal Behaviour, 58, 907-916.

Hughes, K.A., F.H. Rodd, & D.N. Reznick. (2005). Genetic and environmental effects on secondary sex traits in guppies (Poecilia reticulata). Journal of Evolutionary Biology, 18, 35-45.

Hulse, F.S. (1967). Selection for skin color among the Japanese. American Journal of Physical Anthropology, 27, 143-156.

Jablonski, N.G., & G. Chaplin. (2000). The evolution of human skin coloration. Journal of Human Evolution, 39, 57-106.

Kayser, M., F. Liu, A.C.J.W. Janssens, F. Rivadeneira, O. Lao, K. van Duijn, M. Vermeulen, P. Arp, M.M. Jhamai, W.F.J. van Ijcken, J.T. den Dunnen, S. Heath, D. Zelenika, D.D.G. Despriet, C.C.W. Klaver, J.R. Vingerling, P.T.V.M. de Jong, A. Hofman, Y.S. Aulchenko, A.G. Uitterlinden, B.A. Oostra, & C.M. van Duijn. (2008). Three genome-wide association studies and a linkage analysis identify HERC2 as a human iris color gene. The American Journal of Human Genetics doi:10.1016/j.ajhg.2007.10.003

Kirkpatrick, M. (1987). Sexual selection by female choice in polygynous animals. Annual Review of Ecology and Systematics, 18, 43-70.

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.

Liberton, D.K., K.A. Matthes, R. Pereira, T. Frudakis, D.A. Puts, & M.D. Shriver. (2009).
Patterns of correlation between genetic ancestry and facial features suggest selection on females is driving differentiation. Poster #326, The American Society of Human Genetics, 59th annual meeting, October 20-24, 2009. Honolulu, Hawaii.

Mather, F., J.T. Manning, & P.E. Bundred. (unpublished). 2nd to 4th digit ratio, hair and eye colour in Caucasians: Evidence for blond hair as a correlate of high prenatal oestrogen.

Makova, K, & H. Norton. (2005). Worldwide polymorphism at the MC1R locus and normal pigmentation variation in humans. Peptides, 26, 1901-1908.

Olendorf, R., F.H. Rodd, D. Punzalan, A.E. Houde, C. Hurt, D.N. Reznick, & K.A. Hughes. (2006). Frequency-dependent survival in natural guppy populations. Nature, 44, 633-636.

Olivier, G. (1960). Pratique anthropologique. Paris: Vigot Frères.

Rana, B.K., D. Hewett‑Emmett, L. Jin, B.H.-J. Chang, N. Sambuughin, M. Lin, S. Watkins, M. Bamshad, L.B. Jorde, M. Ramsay, T. Jenkins, & W-H. Li. (1999). High polymorphism at the human melanocortin 1 receptor locus. Genetics, 151, 1547‑1557.

Soejima, M., H. Tachida, T. Ishida, A. Sano, & Y. Koda. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.

Sturm, R.A., D.L. Duffy, Z.Z. Zhao, F.P.N. Leite, M.S. Stark, N.K. Hayward, N.G. Martin, & G.W. Montgomery. (2008). A single SNP in an evolutionary conserved region within intron 86 of the HERC2 gene determines human blue-brown eye color. The American Journal of Human Genetics, 82, 424-431.

Sturm, R.A., & T.N. Frudakis. (2004). Eye colour: portals into pigmentation genes and ancestry. Trends in Genetics, 20, 327-332.

Thelen, T.H. (1983). Minority type human mate preference. Social Biology, 30, 162-180.

Voight, B.F., S. Kudaravalli, X. Wen, & J.K. Pritchard. (2006). A map of recent positive selection in the human genome. PLoS Biology, 4(3), e72
doi:10.1371/journal.pbio.0040072

25 comments:

Anonymous said...

"Jews and Roma, like other populations of Mediterranean or southwest Asian origin, tend to have brown eyes and a more robust face shape."

Are you sure about this? My understanding was the Mediterraneans had higher craniofacial gracility relative to Northern Europeans who supposedly have more of a robust "Paleolithic survivor" element in the classical anthropology.

e.g. http://dienekes.blogspot.com/2009/06/relationship-of-cranial-robusticity-to.html

"Whereas South Europe is among the lowest scoring groups on both the shape and robusticity vectors... The more gracile groups (e.g., South European)"

Anonymous said...

Addendum: When I have read in the past that Northern Europeans are more gracile, the people who I have read seem to me to have confused gracefulness in proportion (e.g. nasal bones of correct proportions and so on) with gracility in the sense of being lightly built. I find it plausible N.E. might be more so in the case of the former (or equally as much that they would not be), but it is important to distinguish between the two things, as they are quite distinct.

Nanonymous said...

First thought seeing the pic: I know these guys! I went to school with both of them. One was Pole, another Ashkenazi Jew. Seriously, the resemblance is remarkable. So yeah, I think these two faces simply represent different populations.

Peter Frost said...

Anon,

I see your point. Is there another pair of terms I should be using? I suppose I could use "pedomorphic" for one face. But what term would describe the other?

Nano,

The problem with the ethnic substructure explanation is that the difference in facial structure existed only among the male subjects, i.e, the blue-eyed women and the brown-eyed women had the same facial structure. This sex difference suggests that blue eyes somehow interfere with the masculinization of facial features.

Tod said...

"This sex difference suggests that blue eyes somehow interfere with the masculinization of facial features"

Wow !

Jeffery said...

Conversely, northern Asia appears to have suffered episodes of complete depopulation, particularly at the height of the last ice age.

The height of the last ice age was around 20,000 years or so ago.

I had heard that Mongoloids evolved in northern Asia. Did they not exist ~20,000 years ago? If not, who moved into northern Asia after the height of the last ice age to evolve into Mongoloids? Or had Mongoloids already evolved somewhere else and then moved into northern Asia after the height of the last ice age?

Ben10 said...

Peter, I agree globally with your sexual selection mechanism, however, why would a robust hunter from the end of the last ice age, select for pretty pedomorphic (or neotenic) but fragile characters in its offsprings, if conditions are so rough as you describe. If toughness is what's needed to survive, the male hunter 'should' choose women that will give him tough boys, not pretty boys.
And at the end of the ice age, ~12000 years ago, conditions were certainly rough. That is, IF these genes are really that old.

But if these genes are not so old, in the range 6000-3000 years old, then living conditions in terms of food shortage, might have inmproved at this time, and the excess of women to men that your hypothesis needs to work might have been just a result of wars or male dominance. A dominant male with sixty brides in his harem and 59 males left outside of it just produces the same result. And since survival conditions were not so tough anymore, maybe here the dominant male could have choosen to engender pretty neotenic boys instead of tough roughnecks.

Ben10 said...

...so, selection for neoteny in a rough environment doesn't make much sense, unless of course there are other advantages associated with neotenic characters. It can't be brute testosterone-loaded physical force, by definition, but it could be intellectual. Ice age hunters with blue eyes could have been smarter, or it could have been a developmental advantage: at the age where the brown eyed hunters started to suffer a decline in testosterone, which marked the beginning of the end for them, blue eyed hunters would enjoyed a relatively steadyer supply of testosterone and would have a longer active life, and a longer life as well, such as at the end, all the oldest active hunters would be blue-eyed.
Smart versus longevity or both ? Latin writters insisted very much that among the early german tribes, courage was the most appreciated quality in a man, not longevity. But courage and relative stupidity does more harm than good, so for this virtue to develop, being smart is a pre-requisite.

Peter Frost said...

Tod,

Yes, this is a significant finding. The more I think about the ethnic substructure explanation, the less plausible it seems. But, like any finding, this result needs to be replicated in another population.

Jeffery,

Skeletal remains are limited from northern Asia at that time depth. They are described in the literature as "archaic Mongoloids." I imagine the soft-tissue traits would put them somewhere between an Amerindian and an Ainu.

I suspect that early Europeans looked pretty much like Ainu 25,000 years ago. The changes to skin, hair, and eye color were later developments.

Ben10,

I have trouble following your reasoning. Most people don't mate for the purpose of providing their offspring with certain heritable characteristics. This is true even nowadays, when people are (or should be) aware of genetics.

People experience sexual attraction for reasons that are largely unconscious. Do you stop to think why a particular woman appeals to you physically?

Sister Y said...

This makes me wonder if gayness in males correlates with eye/hair/skin pigmentation! Especially given that study that in general people can tell by someone's face if he's gay.

Ben10 said...

Peter, thanks for your answer.

"I have trouble following your reasoning"

Well, your theory needs an excess of men to women to work, which is well explained by the harsh conditions in postglacial europe.
But then the men are attracted and choose women with pedomorphic features, but who therefore will give them kids with pedomorphic features, that is, less robust, more delicate. If nothing else, the boys with delicate features are now at a disadvantage in the harsh environment.
That's why I say you need something else in your theory to counterbalance for this decrease in fitness.
Think about it. I am not the only one who will se a problem here.

"...Most people don't mate for the purpose of providing their offspring with certain heritable characteristics. This is true even nowadays, when people are (or should be) aware of genetics..."

I am not sure of that. Is this a proven fact?. I don't even think that people are more aware of genetic 'today' than they were in the past. Modern tabloids and your own theory suggest quite the opposite. Anyway, inheritance of characteristics must have been an obvious fact even for a neolithic man and I would say that the harshest the environment, the more aware of genetic inheritance the hunter must have been.

'People experience sexual attraction for reasons that are largely unconscious'
That I agree. The unconscious reason might be that people find attractive the characteristics they want to see in their offspring, and these characteristics might not even be their own.

Ben10 said...

Sorry, I meant an excess of women to men...we know that.

Erik Holland said...

Peter Frost: I left the following comment earlier. For some reason it never posted… trying one more time or I will just post it at my own site. This time I will leave out the links just in case they are the reason for the comment not showing up.

This study shows no support for an association between eye color and masculinization of the face or of blue eyes somehow interfering with masculinization of the face.

To start with, the image presented on top shows the differences exaggerated three-fold, for clarity. Second, the crucial data are in the deformation of the thin plate splines. We can contrast what is known about how masculinization and feminization affect face shape with how the brown-eyed composite differs from the blue-eyed one (brown vs. blue difference) and what is the expected direction if the brown-eyed composite is a more masculine version of the blue-eyed composite (expected direction).

feature | brown vs. blue difference | expected direction

lip width | wider | wider

lip thickness | thicker | thinner

philtrum length | shorter | longer

chin width | wider | wider

chin rounding | rounder | more angular

eye width | wider | narrower

distance between inner eyes | narrower | narrower

distance between outer eye and zygomatic arches | narrower | wider

superior medial orbital margin | displaced superiorly | displaced inferiorly

The list above is not exhaustive, but it is clear that the brown-eyed composite is not a more masculine version of the blue-eyed composite because the correlation structure underlying the shape differences does not coincide with the masculine-feminine discriminant.

So what accounts for the difference? Sampling involving different ethnicities and/or other sampling issues. Why was a similar result not seen in females? Sampling issues. Just by looking at differences such as thicker nasal bones in the nasion region and a shrunk columella (fleshy divider of the nostrils) in the blue-eyed composite, you know that the study was confounded by sampling from different ethnicities.

On a different note…

The blue-eyed composite disproportionately comprises of samples from a population with finer facial features (not more feminine, as the comparison above shows). One commentator had an issue with the comment on finer facial features being more frequent in northern populations, and he or she cited data on cranial robusticity. However, the cranium is different from the face, and no rule says that robusticity must be concordant across different parts of the skeleton.

Anonymous said...

This makes me wonder if gayness in males correlates with eye/hair/skin pigmentation!

Yea pretty much...

Peter Frost said...

Erik,

Can you elaborate please? Which facial features on the left are inconsistent with reduced masculinization?

The sample sizes are fairly large: 40 males and 40 females. No, I don't see how random variability could explain the absence of this effect in the female subjects. You're being a bit too dismissive on this point ...

Erik Holland said...

Peter Frost: The differences chart I listed above already mentions some of the features of the blue-eyed composite that are not only inconsistent with reduced masculinization but consistent with increased masculinization. Look at the column labeled “expected direction”; it lists how the brown-eyed composite must differ from the blue-eyed one to be more masculine with respect to the shape considered. Obviously, the blue-eyed composite has a more masculine shape on some counts and less masculine on others, which means that the differences between them are not one of masculinization but something else, which is clearly ethnicity in part.

Regarding a null finding in the female subjects, random variability is not the only sampling issue. Use your imagination.

Let us make some assumptions. Assume that the population with finer facial features comprises of half the general population and has an IQ advantage of 0.3 SD; that the male IQ advantage is 0.3 SD; Charles University, which is the top university of the Czech Republic, has an IQ advantage of 1 SD (the male student IQ is +0.83 SD compared to the male population, the female student IQ is + 1.17 SD compared to the female population); that the male IQ variance is 1.06 SD-squared; and that the female IQ variance is 0.87 SD-squared. These are reasonable assumptions, well within the published literature of other populations. From these figures we can calculate that roughly 64% of the male students and 75% of the female students will be from the population with finer facial features. Assuming that the prevalence of brown eyes is 40% in the population with finer facial features and 60% in the others, in a random sample of brown-eyed students from the university, close to half the males will comprise of individuals from the population with finer facial features, whereas two thirds of the females will comprise of individuals from the population with finer facial features. So what would you see in the female results? This is mere speculation, but you get the idea that you do not have to limit yourself to random variability.

Anonymous said...

Holland:

However, the cranium is different from the face, and no rule says that robusticity must be concordant across different parts of the skeleton.

The paper in question deals with cranial, facial and masticatory landmarks - http://tinyurl.com/63yju9a.

There seems to no finding of a component indicating Europeans are particularly specially less robust on the facial or masticatory landmarks in a way removed from their general craniofacial robustness, however you may read it yourself and come to your own conclusions.

I have no doubt that Europeans have finer features than other people insofar as they are relatively narrow faced and have relatively pinched vertexes to their facial features. However, this is (at least in theory) different to robustness to some degree - a non-robust feature can come to a blunt vertex, while a robust feature can taper to a narrow vertex. To use an analogy, a larger and more robust "pyramid" can have a finer point than a smaller and less robust "hill".

Peter Frost said...

Erik,

There might be a slight misunderstandng between the two of us. There are two possible hypotheses:

1. The association between blue eyes and reduced robustness is direct. The same allele that favors bue eyes also interferes with the masculinization of the face during development. Since female faces follow a different trajectory, they would be largely unaffected.

2. The associaton between eye color and face shape is simply an artefact of ethnic mixture, i.e., some of the blue-eyed males are partly of German origin or some of the brown-eyed males are partly of Jewish or Roma origin. These ethnic outliers would be absent from the female participants.

I still see the first hypothesis as more plausible. Reduced masculinization of face shape does not necessarily mean feminization. (Do infants have thicker lips than adults in both sexes?)

It may be that some of the participants were of partly Jewish or partly German origin. But why would such individuals be preponderantly male. I have trouble following your reasoning.

Erik Holland said...

To the person commenting on cranial robusticity as “anonymous”: Please use another alias as it is difficult to follow exchanges with this one.

Yes, the study takes into account numerous facial landmarks but for the purpose of measuring overall craniofacial size (minus lower jaw) and overall shape (not of the discrete features that constituted the assessment of robusticity). Your pyramid analogy refers to the need to separate shape from size, but the authors controlled for overall size when they addressed the discrete features that comprise their assessment of robusticity, which does not capture facial robusticity.

Initially I skimmed the abstract of the study on cranial robusticity, and now after looking at it more carefully, it still turns out that the study was about cranial robusticity, not facial robusticity. Masticatory landmarks do not mean that the face has been significantly accounted for as some of the muscle attachments are cranial. The only parts of the face that were taken into account for assessing robusticity were a region of muscle attachment on the cheekbones and a part of the orbit (not relevant to visual assessment in a live human), and this was assessed to investigate whether cranial robusticity has something to do with mastication. There is no way to interpret this study in terms of robusticity of the face.

And just consider some of the populations sampled. Some of the populations sampled from Germany and the Czech Republic population were over 10,000 years old (could be up to 40,000 years old), and a sub-group of American Indians and the Belgian sample are presumably a few thousand years old, too. The northernmost parts of Europe sampled comprised of collections from central Germany, central Belgium and southern Poland.

Erik Holland said...

Peter Frost: You have brought up robusticity and masculinization in the same vein in relation to blue eyes. The masculinization part is easy to address as it is clear that one composite is not a more masculine version of the other (e.g., features a and d more masculine in one, features c and f more masculine in the other).

Yes, reduced masculinization is not the same as feminization, but in the comparisons I mentioned, I referenced mostly linear measurements, which vary along a single dimension: they can increase or decrease. Thus, feminization will shift these linear measurements in the same direction as reduced masculinization. So if I substitute feminization with “reduced masculinization,” the argument remains that the eye colors are not associated with greater or lesser masculinization.

Now, robusticity that is not related to masculinization is a different matter. If there is a direct relationship between blue eyes and facial robusticity, this is a strange relationship since the blue-eyed composite has a face that is overall less robust but has some parts that are more robust, such as the upper nasal region that joins the forehead. And then there are differences that are not plausibly related to robusticity, such as the nasal columella (fleshy divider that separates the nostrils).

The more plausible explanation is that the relationship is indirect, involving different eye color frequencies in ethnic groups that differ in face shape, but then why was the find not observed in women? The scenario I gave you, a hypothetical one that does not invoke random sampling issues, is that given reasonable assumptions about differences in IQ distributions between men, women and different ethnic groups, one could easily end up with the male and female samples having different ethnic compositions, which could also result from cultural differences involving more or less males from one group going to college.

You did bring in ethnic groups by mentioning jews and roma, which the brown-eyed composite does not look like. The roma are an educationally backward minority that are unlikely to be present in sufficient numbers in a top university. There are very few jews in the Czech Republic. Even if one assumes that they enjoy a 0.5 SD IQ advantage compared to the Europeans that they live among, as in the U.S., you would not expect enough of them among the participants to significantly affect the results. I do not suppose that Charles University is a leading study destination for international jewish students, but even if it were, why would the authors recruit international students?

If you want to look up ethnic groups, browse the old anthropological literature to read about a group in the region described as the Dinaric type: eyes set relatively close and open more widely, brown eyes and dark hair most frequent, large nose that is narrow on top and convex in profile, long face with a prominent chin, etc.... something approaching the brown-eyed composite.

P.S.: I hope you remove the spam above (discount viagra). Leaving such comments encourages spammers.

Erik Holland said...

Peter Frost: You have brought up robusticity and masculinization in the same vein in relation to blue eyes. The masculinization part is easy to address as it is clear that one composite is not a more masculine version of the other (e.g., features a and d more masculine in one, features c and f more masculine in the other).

Yes, reduced masculinization is not the same as feminization, but in the comparisons I mentioned, I referenced mostly linear measurements, which vary along a single dimension: they can increase or decrease. Thus, feminization will shift these linear measurements in the same direction as reduced masculinization. So if I substitute feminization with “reduced masculinization,” the argument remains that the eye colors are not associated with greater or lesser masculinization.

Now, robusticity that is not related to masculinization is a different matter. If there is a direct relationship between blue eyes and facial robusticity, this is a strange relationship since the blue-eyed composite has a face that is overall less robust but has some parts that are more robust, such as the upper nasal region that joins the forehead. And then there are differences that are not plausibly related to robusticity, such as the nasal columella (fleshy divider that separates the nostrils).

The more plausible explanation is that the relationship is indirect, involving different eye color frequencies in ethnic groups that differ in face shape, but then why was the find not observed in women? The scenario I gave you, a hypothetical one that does not invoke random sampling issues, is that given reasonable assumptions about differences in IQ distributions between men, women and different ethnic groups, one could easily end up with the male and female samples having different ethnic compositions, which could also result from cultural differences involving more or less males from one group going to college.

You did bring in ethnic groups by mentioning jews and roma, which the brown-eyed composite does not look like. The roma are an educationally backward minority that are unlikely to be present in sufficient numbers in a top university. There are very few jews in the Czech Republic. Even if one assumes that they enjoy a 0.5 SD IQ advantage compared to the Europeans that they live among, as in the U.S., you would not expect enough of them among the participants to significantly affect the results. I do not suppose that Charles University is a leading study destination for international jewish students, but even if it were, why would the authors recruit international students?

If you want to look up ethnic groups, browse the old anthropological literature to read about a group in the region described as the Dinaric type: eyes set relatively close and open more widely, brown eyes and dark hair most frequent, large nose that is narrow on top and convex in profile, long face with a prominent chin, etc…. something approaching the brown-eyed composite.

P.S.: I hope you remove the spam above (discount viagra). Leaving such comments encourages spammers.

Erik Holland said...

Peter Frost: You have brought up robusticity and masculinization in the same vein in relation to blue eyes. The masculinization part is easy to address as it is clear that one composite is not a more masculine version of the other (e.g., features a and d more masculine in one, features c and f more masculine in the other).

Yes, reduced masculinization is not the same as feminization, but in the comparisons I mentioned, I referenced mostly linear measurements, which vary along a single dimension: they can increase or decrease. Thus, feminization will shift these linear measurements in the same direction as reduced masculinization. So if I substitute feminization with “reduced masculinization,” the argument remains that the eye colors are not associated with greater or lesser masculinization.

Now, robusticity that is not related to masculinization is a different matter. If there is a direct relationship between blue eyes and facial robusticity, this is a strange relationship since the blue-eyed composite has a face that is overall less robust but has some parts that are more robust, such as the upper nasal region that joins the forehead. And then there are differences that are not plausibly related to robusticity, such as the nasal columella (fleshy divider that separates the nostrils).

The more plausible explanation is that the relationship is indirect, involving different eye color frequencies in ethnic groups that differ in face shape, but then why was the find not observed in women? The scenario I gave you, a hypothetical one that does not invoke random sampling issues, is that given reasonable assumptions about differences in IQ distributions between men, women and different ethnic groups, one could easily end up with the male and female samples having different ethnic compositions, which could also result from cultural differences involving more or less males from one group going to college.

You did bring in ethnic groups by mentioning jews and roma, which the brown-eyed composite does not look like. The roma are an educationally backward minority that are unlikely to be present in sufficient numbers in a top university. There are very few jews in the Czech Republic. Even if one assumes that they enjoy a 0.5 SD IQ advantage compared to the Europeans that they live among, as in the U.S., you would not expect enough of them among the participants to significantly affect the results. I do not suppose that Charles University is a leading study destination for international jewish students, but even if it were, why would the authors recruit international students?

If you want to look up ethnic groups, browse the old anthropological literature to read about a group in the region described as the Dinaric type: eyes set relatively close and open more widely, brown eyes and dark hair most frequent, large nose that is narrow on top and convex in profile, long face with a prominent chin, etc…. something approaching the brown-eyed composite.

P.S.: I hope you remove the spam above. Leaving such comments encourages spammers.

Peter Frost said...

Erik,

Ah, finally I understand. You believe that population substructure exists among ethnic Czechs, i.e., Czechs can be subdivided into different endogamous groups.

There are few if any geographic or social barriers in the Czech Republic to maintain such groups. Even if such groups existed in the past, they would have long ago melted into the general population.

You didn't answer my previous question. Are you arguing that lips are thicker in infants than in adults?

Tod said...

Look at the averaged face of brown-eyed male subjects, notice anything about the lips ? By my way of thinking the expression is a half smile. The blue eyed face has no trace of a similar expression.

Second to fourth digit ratio and face shape. Look at figure 2.

Notice anything about lips on the male low 2D:4D face?

The expression is a wolfish smirk, a considerable contrast to the high digit ratio face.

Erik Holland said...

Peter Frost: Regarding population substructure in the Czech Republic, you contend that even if such groups existed in the past, they would have long ago melted into the general population. As you very well know, humans do not mate randomly. Assortative mating tends to maintain clusters notwithstanding extensive overlap. Extrapolation that given increasing overlap the clusters will disappear may not pan out—the overlap may stabilize or reverse; such is the power of assortative mating.

20th century anthropological texts describe different physical types in the Czech Republic and neighborhood. Aside from the Dinarics, you will find mention of the Norics, a group that represents a transition between the Dinarics and more northern people that started living in the Czech Republic, and then the northern types themselves. These labels have not been standardized and anthropology has generally shifted away from describing such clusters and overlap with more rigor, but indications are that the blending of some populations of different geographic origins that settled in the Czech Republic has not been complete.

I do not see the point of the question of the lip thickness of infants vs. adults. I did not say that lips are thicker in infants than adults.