Thursday, 8 October 2009

Facial skin color and sexual selection

The human mind seems to use facial color to determine whether a person is male or female. A man has a relatively dark facial color that contrasts poorly with his lip and eye color. Conversely, a woman has a relatively light facial color that contrasts sharply with her lip and eye color (Russell, 2003; Russell, 2009; Russell, in press).

This kind of sex-recognition algorithm has been a channel for sexual selection in many species. When selecting a mate, an animal tends to choose the ones most easily recognizable as the opposite sex. Over many generations, such selection will cause the relevant sex-specific cues to be accentuated (Manning, 1972, pp. 47-49).

The degree of accentuation will depend on the intensity of sexual selection and on whether males have been selecting females or females selecting males. Among ancestral humans, sexual selection seems to have varied in both intensity and direction along a north-south gradient (Frost, 2006; Frost, 2008). In the tropical zone, women gathered food year-round, so a second wife would cost little in terms of food provisioning. With more men becoming polygynous, fewer women were left unmated. The pressure of sexual selection was thus on men, with women being the ones who could pick and choose mates.

This situation reversed outside the tropical zone. First, polygyny was costlier because women could not gather food in winter. Second, male mortality exceeded female mortality because men had to hunt over longer distances. Together, these two trends resulted in too few men competing for too many women. This was particularly so on continental steppe-tundra, where women had almost no opportunities for food gathering and where men had to hunt wandering herds of herbivores over long distances. The pressure of sexual selection was thus on women, with men being the ones who could pick and choose mates.

Sexual selection and lightening of skin color

If light skin is perceived as a sign of femininity, sexual selection of women should tend to lighten female skin. This kind of selection became possible once ancestral humans had left the tropical zone. On the one hand, there was less natural selection for dark skin as a barrier to intense UV radiation. On the other, as explained above, there was stronger sexual selection of women because they outnumbered men on the mate market. Women should thus be increasingly lighter-skinned than men with increasing distance from the tropical zone, this sex difference being greatest among those humans that once inhabited the large expanses of continental steppe-tundra in northern and eastern Europe. Since most skin pigmentation genes are not sex-linked, selection for lighter-skinned women would also lighten mean skin color (i.e., both males and females). Thus, mean skin color should likewise lighten along the same north-south gradient.

How do these predictions stack up against reality? They accurately describe geographic variation in mean skin color (Frost, 2008). But they poorly describe geographic variation in female depigmentation relative to male skin color. In fact, female skin reflectance exceeds male skin reflectance the most among humans at medium latitudes with medium skin color (Frost, 2007; Madrigal & Kelly, 2006). This may be a ‘ceiling effect’. Northern and eastern Europeans are close to the physiological limit of skin depigmentation. Their women cannot be much whiter than the mean skin color because they have, so to speak, very little headroom left—the mean skin color is already scrunched up against the ceiling of maximum skin whiteness.

Sexual selection and increase in facial color contrast

There seems to be similar geographic variation in the contrast between facial color and lip/eye color. This contrast is weakest among tropical humans. It is strongest, however, not among northern/eastern Europeans but among East Asians (Russell, in press). This is largely because East Asians have dark eyes and relatively light facial skin. The contrast effect is even stronger if we factor in their jet-black hair, which further sets off the lightness of the female face. Nonetheless, facial color contrast is no more sexually dimorphic among East Asians than it is among Europeans (Russell, in press).

Why would Europeans score lower than East Asians on facial color contrast? It may be that sexual selection for dark eyes and dark hair relaxed among ancestral Europeans once their facial skin had lightened to the point of becoming pinkish-white. At that point, the color contrast was more than sufficient. This, in turn, may have allowed rare color preference to generate sexual selection for diverse hair and eye colors. This process may have then acquired a dynamic of its own that competed with the older dynamic of facial color contrast. Alternately, rare color preference may have always been a weak selection pressure that manifests itself only under conditions of intense sexual selection (Frost, 2006; Frost, 2008).

Conclusion

In sum, if we examine geographic variation in skin color and in facial color contrast, the pattern is largely consistent with increasingly intense sexual selection of women along a north-south gradient. This selection would have been minimal among tropical humans and maximal among arctic humans, particularly those that once lived on continental steppe-tundra—where polygyny was constrained the most and where male mortality exceeded female mortality the most. There are, however, deviations from the expected pattern that may be due to ceiling effects and release of sexual selection for rare hair and eye colors.

Among ancestral Europeans, this process of sexual selection seems to have been a multi-stage process. It likely began c. 30,000 BP with the first penetration by modern humans of the steppe-tundra belt (southwestern France). This initial stage would correspond to certain physical changes that are common to Europeans and East Asians. Stage I ended with the onset of the glacial maximum (c. 20,000 BP), which blocked East-West gene flow by merging the Fenno-Scandian and Ural icecaps and by forming large glacial lakes along the Ob (Rogers, 1986; Crawford et al, 1997). Stages II and III would correspond to later physical changes that are specific to Europeans.

Stage I – head hair lengthens, face shape feminizes, skin lightens (30,000–20,000 BP ?)
Stage II – skin lightens to pinkish-white (20,000–15,000 BP ?)
Stage III – hair and eye color diversifies (15,000–10,000 BP ?)

References

Crawford, M.H., Williams, J.T., & Duggirala, R. (1997). Genetic structure of the indigenous populations of Siberia. American Journal of Physical Anthropology, 104, 177-192.

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), pp. 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.

Madrigal, L., & W. Kelly. (2006). Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 132, 470-482.

Manning, A. (1972). An Introduction to Animal Behaviour, 2nd edition, London: Edward Arnold.

Rogers, R.A. (1986). Language, human subspeciation, and Ice Age barriers in Northern Siberia. Canadian Journal of Anthropology, 5, 11‑22.

Russell, R. (in press). Why cosmetics work. In Adams, R., Ambady, N., Nakayama, K., & Shimojo, S. (Eds.) The Science of Social Vision. New York: Oxford University Press.

Russell, R. (2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219.

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.

25 comments:

Tod said...

Sexual selection on the steppe-tundra could be self reinforcing. If strong selection for feminine characteristics in women resulted in the men having physiques that were shifted towards feminine norms that could increase their mortality when hunting.

Being feminized the hunters of the highly mobile herds would presumably have somewhat less strength and stamina. Hence they would be more likely to be incapacitated by lower body injuries or exhaustion and die from exposure. Feminization could also negatively impact on the hunters ability to use dead reckoning rather than navigate by landmarks as women tend to do (and which would be less effective). Getting lost while searching for elusive and mobile prey in a vast featureless landscape would be another way for feminized men to die of exhaustion.

As the reduced masculine characteristics of the male hunters was a side effect of intensive selection of women with feminine characteristics, (which was dependant on male mortality) in the above scenario sexual selection would be self reinforcing. The men died on hunting trips; women had to be more feminine to get a scarce provider/husband; the men in that population became more feminized; they died even more often as a result .

Peter Frost said...

Tod,

This is similar to the point that Greg Cochran has made. If a selection pressure is strong enough and recent enough, it will probably cause negative side-effects. For example, selection for higher intelligence among Ashkenazi Jews seems to have come about through improvements in brain lipid metabolism. The same metabolic pathway, however, is associated with a number of heritable diseases that are much more common among Ashkenazi Jews.

This being said, the time frame is much longer for sexual selection among ancestral Europeans. There should have been time for selection to "debug" any negative side-effects. Long beards and mustaches, for instance, may have evolved among ancestral Europeans as a way to offset the feminization of male faces.

Tod said...

I've already linked to a couple of studies casting doubt on the 'Folic Acid is destroyed by UV-A' hypothesis including an in vivo study that concluded "neural tube defects claimed to occur after periconceptual UVA exposure are probably not due to UVA induced folate deficiency"
Here is something that may suggest what it is about intense sunlight that requires the protective role of melanin.

Influence of human serum albumin on photodegradation of folic acid in solution.

"In solutions of folic acid (FA) and human serum albumin (HSA), the FA photoproducts cause photodamage mainly to HSA rather than to FA itself"

If I understand correctly human serum albumin is damaged by UV-A, a possible justification for the folk wisdom that recommends avoiding the sun, especially the midday sun.

Robert8 said...

Peter, you put the stage II for whitening the skin at 20-15000 bp, i thought that the european gene for white skin was 12000 years old maximum, did that changed ?

Peter Frost said...

Tod,

There are probably many reasons why intense UV is harmful. Most evolutionists dismiss skin cancer as an agent of selection because it appears too late in life. In the tropical zone, however, albinoes develop skin cancer in their early 20s.

Robert8,

No, that estimate of 12,000 BP is a mean, not a maximum. It also has a large standard deviation.

But, more to the point, European skin became depigmented through allelic changes at several gene loci. The AIM1 mutation accounts for only some of this depigmentation.

I see this as being a fairly incremental process extending from 30,000 to 10,000 BP, although I wouldn't rule out some alleles being pushed to fixation over a short period of time.

Peter Frost said...

Robert8,

On second thought, even the other skin-color mutations seem to be relatively late in time, at least according to an American research team that is trying to date these changes. I'm still waiting for their results, so anything I say now on the actual dating might be premature.

Tod said...

The red hair one lightens skin

The roots of red hair, Jonathan Rees (03).
" Red-haired variants of the MC1R gene probably arose about 20-40 000 years ago. [...]
people who carry only one different allele tend to burn easily in the sun (even though they didn't have red hair), and are more likely to have a large number of freckles".


Rees' estimate agrees rather well with 'stage 1'. If 30,000 years ago skin lightened somewhat from the tawny Khoisan like hue of original Europeans the results may have been olive skin and dark hair with a reddish tint.

Robert8 said...

Peter, is there a particular reason why it fits better your theory if it is 20000 to 15000 years instead of just 12000 ?
Well, I see at least one reason. The earlyer it is, the less populated europe is and the easyer it is to explain how the fair skin variants spread into europe.
But if it happens only 12000 years ago, some parts of europe are aleady populated, and we therefore have to assume populated by people with a darker skin, who nonetheless are the ancestors of today's white europeans, according to genetic ancestry. If this is the case, your sexual selection hypothesis which took place in the north european plaines has to reach remote areas with difficult access, such as Ireland, the Pyreneans, Scotland etc. I know you said people were mobile at that time, and not so numerous, but this is still hard to explain.
For example, this article
http://en.wikipedia.org/wiki/Ancient_DNA#Neolithic_European
says:
"Neolithic European
5500 years old aDNA preserved in 11 Neolithic remains from Granollers in Catalonia show sequence similar to present day sample. The result suggesting a long-time genetic continuity, at least since Neolithic times, in Western Europe female population.[15]"

So your theory is nice but I kind of feel it is very time-frame critical to avoid to deal with geographical and genetic issues.
For exemple, Bryan Sykes proposes seven mitochondrial lineages in europe, coming from different places and time.
http://en.wikipedia.org/wiki/Seven_Daughters_of_Eve
Wouldn't you expect to see differences in sexual selection among all those lineages ? how do you deal with these different genetic clans, are they relevant in your theory ?

Tod said...

As variants associated with blond hair are newer and also less common in Scotland and Ireland than Germany ect it is reasonable to assume that Europe's ancestral populations had not all spent the same time at each of the 'stages'.

Specificaly the skin lightening (and red hair) variants common in the Celtic fringe show these peoples spent a relatively larger proportion of the time at 'Stage I and II' than blonder peoples.

Robert8 said...

I agree with Todd on that, we can see that all europeans are white skinned, but not all are blond or have fair eyes.
If we assume that sexual selection for women for these characters was equally intense everywhere in europe, in Peter's theory it means that either the sexually attractive variants came too late to become predominant in some populations or the initial populations with brown hairs and eyes were already too large to be influenced by sexual selection.
And if celtic people have spend more time in stage I and II it is probably just because they are older than blonde people.
Todd suggests that red hairs are also older and arose in celtic populations, was this variant passed to blonder people later, or did redness occured independently in the blonde populations ?

Tod said...

"if celtic people have spent more time in stage I and II it is probably just because they are older than blonder peoples"

What I meant was that today's blondest populations had ancestors who spent just as much time at STAGE I & II as but spent more time at STAGE III, probably due to their geographic location.

It is generally agreed that the variants (of MC1R) which result in red hair in today's white Europeans are significantly older than other skin lightening gene variants, but could they have produced red hair when they first appeared?

I wonder if there was anyone with true red hair before STAGE II, red hair may be a result of STAGE II selection for skin lightening (involving various genes) on top of the STAGE I variants of the MC1R gene which lightened skin somewhat by 20,000 years ago. The 'red hair' variants 30,000 years ago may not have produced red hair, just lighter - but not white - skin.

Peter Frost said...

Everyone,

As I see it, there are four key points in time:

30,000 BP - modern humans enter the Eurasian steppe-tundra belt

25,000 BP - the last ice age begins

20,000 - 15,000 - glacial maximum, humans at the European end of the steppe-tundra belt become demographically isolated from humans at the East Asian end.

10,000 BP - the last ice age ends

Stage II could not have begun earlier than 20,000 BP. Conceivably, it could have been later.

There is evidence that the steppe-tundra had a larger and denser human population after the glacial maximum than before. If so, most of the possibilities for sexual selection (in terms of new and interesting variants for hair and eye color) would have occurred within the relatively short period between 15,000 and 10,000 BP. Stage II and Stage III would therefore be compressed into this time window.

We won't really know until we get firm dates for the appearance and spread of these mutations. Hopefully, such data will soon be forthcoming.

Robert8 said...

There are so many points that are not clear to me.

Todd said:"...sexual selection would be self reinforcing. The men died on hunting trips; women had to be more feminine to get a scarce provider/husband; the men in that population became more feminized; they died even more often as a result"...and the women of these dead hunters die of starvation and so their kids. It doesn't look to me a reinforcing mechanism when everybody dies.

Beside, does it really make sense for a hunter to choose the most feminine woman if there is a risk to have feminized male heirs ?
Choosing a masculinized woman might be more advantagous. She could hunt. Beside humans, is there any example of carnivorous animals living in the toundra or anywhere in the globe, where females don't hunt ?
And if we compare today's nordic women with latine ou african women, I would say many nordic women easily display the less feminine features of all, and many hunt. That might be a recent trend but still, it doesn't make sense if the women that were supposedly selected for feminization don't look feminine or act feminine.
To me, the modern european features are less feminine than they are neotenic and therefore it is more accurate to say that sexual selection selected for neoteny, not for feminization, even if the two overlap.
Side question: was attraction for neotenic traits observed in primates beside humans ?

Peter Frost said...

"Beside, does it really make sense for a hunter to choose the most feminine woman if there is a risk to have feminized male heirs ?"

You're assuming that sexual attraction is a conscious process where people think out all of the long-term consequences. This is, to put it kindly, doubtful.

More to the point, the selection is for traits that 'look' more feminine. To the extent that such selection has negative side-effects on male offspring (by feminizing sexual orientation, for example), there would be counter-selection to 'debug' these negative effects. Women who look more feminine are not necessarily more estrogenized.

"Choosing a masculinized woman might be more advantagous. She could hunt."

Women rarely hunt among contemporary arctic peoples. The sexual division of labor is very strict on this point; the only exceptions are widows who no longer have a man to help them.

Why did humans have a sexual division of labor? It seems to be the most efficient way of supporting a family. If both the man and the woman specialize in hunting, who would be left to do the other work? There's more to raising a family than just hunting.

"To me, the modern european features are less feminine than they are neotenic and therefore it is more accurate to say that sexual selection selected for neoteny, not for feminization, even if the two overlap."

The two overlap because a neotenous physical appearance reduces male aggressiveness and stimulates male provisioning. This is important in human populations where women are highly dependent on male provisioning and where the man is in contact with the woman over an extended period (and not just for procreation).

Anonymous said...

Peter, there's some aspects of this I genuinely don't understand. I'll let you know I'm the one who's been conversing with you via email rather recently- I first came into contact with you when I gave out my email and had you delete my comments for personal reasons. Not giving out many details here since I'm still concerned about such things.

Anyway, In reference to your previous article, I don't think the skin color difference Russell shows is very apparent. To me, it's detectable, but at times very hard to make out, and as I've said before, your theories on humans perceiving male and female faces strongly on the basis of color lack any real perceptive studies at the same baseline melanin levels. The same could come into play here. It could even be further offset by tanning, if the people in Russel's study engaged in such a thing. Even if these factors somehow don't play a role or none at all, the difference is very, very slight.

This is what really confuses me, though- while it's probably true female lips are darker than male ones, why does Russell there's a sex difference in eye color? For most of humanity, the eye color is the same in all populations. Only amongst europeans, middle easterners, central asians and the like is there any difference, due to the frequency of such alleles. From what I've seen, even in populations that do possess such alleles, the relationship between sex and eye color frequency is very minor- if such a relationship even exists.

Now, it might be true that, like skin color, there will universally be a sex difference in eye color even at comparable baseline melanin differences, but this is the first time in my life I've ever heard such an idea. I can't make completely accurate judgements because I can't access the paper, but did Russell even take into account the eye colors presented in the male and female samples?

Anonymous said...

Also, what does he define as luminance differences and the such? Does this refer to how certain colors and coloration contrasts look under certain lighting conditions? Because I've always felt that this is very important to consider on this topic, especially with skin color. For extreme examples, very dark skin looks poor in low lighting conditions (wherein it can strongly blend in with the darkness), whereas very light skin can follow the same general pattern in high lighting conditions.

And, in regards to your current post, you are quite inaccurate to say the greatest contrast is among east asians when it comes to hair color. The contrast is perhaps just as strong, if not even stronger among australo-melanesians, whom exhibit high frequencies of blonde, brown, and sometimes redhair, and numerous shades in between. See: http://thestudyofracialism.org/about1096.html

And in what sense do you mean it off sets the lightness of the female face? Are you reffering to luminence differences created by the refractions of dark hair and the like? Because under certain general lighting conditions, it seems like dark hair coupled with light skin would create a strong contrast. The same would hold true of eyebrows. I mean, there are many europeans who have very light skin, along with very light hair, who, under certain general lighting conditions, show very poor contrasts between skin color and eyebrow color, to the point where the two almost just blend together. This would obviously be less likely among australo-melanesians.

One last thing- it seems like general contrasts, overall, seem to be more important than these supposed sex differences in lip color and general skin color. Among dark skinned humans, it's not hard to come across ones wherein their lips are considerably ligher than the rest of their face- IE, blacks with red or pink lips. Yet this certainly isn't a universal phenomenon, and in such a case, it's the lighter lips compared to darker skin on women that creates a stronger contrast, unlike you and Russel's theory of darker lips compared to lighter skin on men.

And in such a regard, it's impossible to come across light skinned populations with comparatively much darker lips, like, say, naturally brown or black lips on a european in comparison to red or pinks lips on a black african.

John H said...

If selection for feminine features was strongest in northern Europe then why are Nordic women and men more masculine looking than other Europeans?

Anonymous said...

I'll also clarify that I don't believe full-scale contrasts are attractive- obviously, there doesn't seem to be an intrinsic preference for blue lips or green hair in humans. I just bespeak of ones that are natural phenotypes in humans.

"If selection for feminine features was strongest in northern Europe then why are Nordic women and men more masculine looking than other Europeans?"

Frost focuses primarily on skin color and says little of other features. I don't think it's possible to make a truly proximate judgement of the masculinity and femininity of different european populations, but with greatly differing populations it is. IE, blacks compared to whites or whites compared to east asians.

East asians are obviously more feminine on average than whites. If the reverse was true, why aren't white men, by simple laws of inheritance, feminine as well? (i'm talking purely of facial features, not below the neck ones)

Also, Frost- this was on your previous post. I'm sorry to say, but this is a really poor theory:

"Long beards and mustaches, for instance, may have evolved among ancestral Europeans as a way to offset the feminization of male faces."

This would beg asking why this also didn't occur among east asians, who are cranio-facially more feminine than whites, who have less body and facial hair even when correcting for testosterone differences, why native americans, the vast majority of the time, have very, very little body and facial hair etc. outside of arcitc populations and the fuegians, etc.

While body hair and facial hair can be a strong component of masculine beauty, that also has the side-effect of being a universally disliked trait on women.

On another note, did it also occur to you that selection for light hair amongst europeans could be another aspect of vitamin D synthesis, since light penetrates more easily through light hair?

Anonymous said...

I think I understand a few things more clearly now. When you say "set off", you are just reffering to general contrasts, right? And is Russell just reffering to the color contrasts on paper as an extreme example? It seems like contrasts related to illumination isn't the focus of his work.

Peter Frost said...

Anon,

1.Richard Russell found that subjects could distinguish between male and female faces by the sex difference in skin tone, even when the faces had been blurred and offered no other details.
R. RUSSELL. Sex, beauty, and the relative luminance of facial features, Perception 32:1093-1107, 2003.

Historically, people were very aware of this sex difference. The best evidence for this is the widespread artistic convention of giving women a lighter skin color than that of men. This convention seems to have developed independently in such culture areas as ancient Egypt, Mesoamerica, Japan, etc.

2. There is no sex difference in eye color, at least none that I know of. Hair color tends to be lighter in women, at least among Europeans. Russell's argument is that the human mind uses the lips and eyes as a benchmark for mental processing of facial skin color.

3. East Asians have the highest contrast between facial skin color and eye/lip color. This is because their skin is relatively light while their eyes are uniformly dark.

4. When I say that dark hair sets off the lightness of facial color, I mean that it makes the face look lighter than it would otherwise, i.e., through a contrast effect.

5. Yes, the contrast effect seems to be weakest in tropical populations. I suspect that these populations have had little sexual selection of women to enhance the contrast between facial color and lip/eye color.

6. I threw out that idea about beards and mustaches only as a suggestion.

I wouldn't say that East Asians are more feminine-looking than Europeans (breast size is generally smaller among East Asians). It would be more exact to say that East Asians are less sexually dimorphic.

Conversely, there seems to have been more selection to maintain sexual dimorphism among ancestral Europeans than among East Asians. Why? I really don't know.

7. I've become increasingly skeptical about the vitamin D theory of human variation in skin color. The Amerindians of northern Canada are quite dark-skinned and most of them eat very little fatty fish. So why aren't they suffering from vitamin D deficiency?

In my opinion, the current recommended daily intake of vitamin D is based on shoddy medical evidence. A lot of people are taking vitamin D supplements who should not be. The long-term consequences may prove to be as tragic as those of thalidomide.

John H,

Denise Liberton has found that face shape is more feminized in subjects of European origin than in subjects of African origin (see my earlier post on this topic). I don't know whether she has tried to compare northern and southern Europeans.

Anonymous said...

Er, did my comment not go through?

Anonymous said...

My previous major comment didn't go through. Have to retype, I guess.

And Frost, I think I came off as abit too strong in our last correspondence- if I did, sorry. Also in regards to detailing too much.

"1.Richard Russell found that subjects could distinguish between male and female faces by the sex difference in skin tone, even when the faces had been blurred and offered no other details.
R. RUSSELL. Sex, beauty, and the relative luminance of facial features, Perception 32:1093-1107, 2003."

I think I've seen the images in this study. Russell's study seems deeply flawed, since he seems to have applied too broad of a coloration palette. I mean, the differences he shows are very minor, yet I can't think of anyone who'd associate such meager differences would broad differences in femininity and masculinity. Like I said in our emails, comparisons drawn from actual population differences are likely to be much more accurate.

"Historically, people were very aware of this sex difference. The best evidence for this is the widespread artistic convention of giving women a lighter skin color than that of men. This convention seems to have developed independently in such culture areas as ancient Egypt, Mesoamerica, Japan, etc."

I've detailed why I think your reasoning in this regard is faulty. For one example, it doesn't take into the greater heterogenity of populations farther away from extremities, and how you act as if living in a multiracial society makes people unaware of this on a conscious basis, when these conditions certainly existed worldwide prior to modern times.

"2. There is no sex difference in eye color, at least none that I know of. Hair color tends to be lighter in women, at least among Europeans. Russell's argument is that the human mind uses the lips and eyes as a benchmark for mental processing of facial skin color."

So why does he say women have darker eyes?

"3. East Asians have the highest contrast between facial skin color and eye/lip color. This is because their skin is relatively light while their eyes are uniformly dark."

Yes, but I was only addressing your claim about hair color and skin color. It might be greater among australo-melanesians.

Brent said...

"5. Yes, the contrast effect seems to be weakest in tropical populations. I suspect that these populations have had little sexual selection of women to enhance the contrast between facial color and lip/eye color."

The differences seen in tropical population seem to be unrelated to sexual selection, as they're just odd consequences of high melanin levels. Like I said, many dark skinned individuals exhibit significantly lighter lips, even on males.

"I wouldn't say that East Asians are more feminine-looking than Europeans (breast size is generally smaller among East Asians). It would be more exact to say that East Asians are less sexually dimorphic."

I was only talking about above the neck differences.

"Conversely, there seems to have been more selection to maintain sexual dimorphism among ancestral Europeans than among East Asians. Why? I really don't know."

Probably their diets being high in estrogen enriching chemicals.

"7. I've become increasingly skeptical about the vitamin D theory of human variation in skin color. The Amerindians of northern Canada are quite dark-skinned and most of them eat very little fatty fish. So why aren't they suffering from vitamin D deficiency?

In my opinion, the current recommended daily intake of vitamin D is based on shoddy medical evidence. A lot of people are taking vitamin D supplements who should not be. The long-term consequences may prove to be as tragic as those of thalidomide."

I agree skin color differences account for few advantages, when taking into other genetic traits that affect sunburn suceptibility, vitamin D synthesis, etc. However, it's probably just a casual human evolutionary adaption.

But this brings up another problem with your theory. If your theories on sexual selection for light skin color are so powerful, we should expect many populations to capitalize by selecting for genetic traits that would make light skin in a hot environment relatively non-disadvantageous due to other genetic traits that make up for it. That could be the case for the Khoisan. Just why are they so light?

On another note, you've mentioned the pygmies as being lighter than other blacks. However, this could be due to them living in rainforests where little sunlight penetrates as well. And in spite of this... their skin color is still quite dark.

This also reminds me how some critics of skin color being a major adaption to climactic differences say that the ancestors of modern europeans, who would have been darked skinned, would have spent enough time outdoors to get enough vitamin D intake.

Anonymous said...

Er, could you edit that last comment to anonymous? I didn't mean to enter in with my google account. Thanks in advance. I also realized my comments didn't go through due to the character limit.

n/a said...

"Denise Liberton has found that face shape is more feminized in subjects of European origin than in subjects of African origin "

Where does she say that?

In fact, she may have simply "discovered" that populations differ in their levels of sexual dimorphism.