Hair color and blondism

Last year, I answered a number of questions from two journalists at Le Monde for an article on blonde hair. This is the full text of my answers:

1. D'où cette couleur vient-elle ?

La couleur des cheveux est déterminée principalement par un seul gène, MC1R, qui régule les proportions d'eumélanine (pigment brun-noir) et de phéomelanine (pigment jaune-rouge). Ce gène existe sous la forme de plusieurs allèles (du moins 7 chez les Européens) qui, en variant les proportions des deux pigments, produisent une gamme de couleurs, soit le noir, le brun, le roux et le blond.

2. Pourquoi s'est-elle localisée principalement en Europe ?

Les cheveux blonds s'inscrivent dans un ensemble de traits pigmentaires qui se retrouvent presque seulement chez les Européens, dont une gamme large de couleurs des cheveux, une gamme également large de couleurs des yeux, ainsi que le blanchissement maximal de la peau. Ces particularités européennes constituent une énigme, d'autant plus qu'elles ne proviennent pas d'une même cause génétique. La couleur des cheveux s'est diversifiée avec la multiplication des allèles du gène MC1R et la couleur des yeux avec la multiplication des allèles d'un autre gène, OCA2. Quant à la peau, elle s'est blanchie par l'entremise d'autres gènes encore, comme AIM1. Ces particularités ne partagent pas non plus une fonction commune, sauf le fait de colorer, visiblement, un aspect du corps humain, surtout le visage.

3. Quels ont été les mécanismes qui l'ont fait perdurer à certains endroits ?

Probablement la sélection sexuelle. C'est ce qui produit souvent les traits à couleurs vives, surtout les «polymorphismes de couleurs». Ces derniers se produisent à une intensité de sélection sexuelle très élevée, c'est-à-dire lorsqu'il faut choisir entre plusieurs candidats répondant déjà à tous les critères désirés. À ce moment-là, la valeur accrocheuse y devient déterminante. La sélection sexuelle y privilégie non seulement les couleurs vives mais aussi les couleurs rares qui sortent de l'ordinaire. Or, par le fait même de favoriser le succès reproducteur, une couleur rare se multipliera au sein de la population ; l'attirance sexuelle s'oriente alors vers les variantes moins fréquentes et plus accrocheuses. Ainsi s'installe un équilibre dynamique qui mène à une diversité de plus en plus grande. Cette préférence pour les couleurs de cheveux rares a été démontrée par le biologiste américain Thomas Thelen. Ce dernier avait préparé trois séries de diapos montrant des femmes attirantes : la première présentait 6 brunes; la deuxième 1 brune et 5 blondes; et la troisième 1 brune et 11 blondes. Il demandait ensuite à des hommes de choisir, pour chaque série, la femme qu'ils voudraient épouser. Résultat : plus la même brune était rare dans une série, plus ils la choisissaient

Chez les autres espèces, la sélection sexuelle vise surtout le mâle. Trop de mâles doivent se concurrencer entre eux pour peu de femelles, car ces dernières sont moins disponibles pour l'accouplement, étant accaparées par la grossesse ou les soins des petits. Mais cette «loi biologique» s'applique moins uniformément à homo sapiens, dont l'investissement paternel et les conditions démographiques varient énormément d'une population à l'autre. Ainsi, c'est parfois l'homme qui doit vaincre une concurrence acharnée pour trouver une femme, parfois l'inverse.

Avant l'agriculture, qui remonte à moins de 10 000 ans, les humains vivaient de la chasse et de la cueillette. Ce mode d'existence permet à la sélection sexuelle de s'intensifier en fonction de deux facteurs principaux : la surmortalité masculine et les coûts de la polygamie. Ces deux facteurs varient, à leur tour, en fonction de la latitude. À mesure qu'on s'éloigne de l'équateur, il y a moins de gibier par kilomètre carré et plus de territoire à parcourir, ce qui augmente la mortalité des hommes-chasseurs. Parallèlement, à mesure que la saison froide s'allonge, les femmes trouvent moins de nourriture à cueillir, de sorte que seuls les chasseurs très capables peuvent subvenir à une deuxième épouse. Donc, en s'éloignant de l'équateur, il y a non seulement moins d'hommes, mais aussi moins d'hommes polygames. Résultat : les femmes doivent rivaliser davantage pour trouver un homme, et cette concurrence atteint un maximum dans des environnements arctiques de type steppe-toundra, où presque toute la nourriture prend la forme de grands troupeaux migrateurs (ex. rennes). Ce type d'environnement maximise à la fois la surmortalité masculine (l'homme doit parcourir de grandes distances) et les coûts de la polygamie (l'autonomie alimentaire de la femme est réduite à zéro).

Aujourd'hui, la steppe-toundra n'occupe que les franges septentrionales de l'Eurasie et de l'Amérique du Nord. Or, pendant la dernière époque glaciaire, elle couvrait les grandes plaines de l'Europe, ayant été déplacée vers le sud par la calotte glaciaire scandinave. À l'est de l'Oural, elle s'étendait trop vers le nord et vers l'intérieur aride pour permettre un peuplement important et continu, surtout lors de l'avancée maximale des glaciers. Si un tel peuplement était possible en Europe, c'était toutefois au prix d'une pénurie chronique d'hommes et d'une intensification sauvage de la sélection sexuelle à l'égard des femmes. Vraisemblablement, c'est cette intensification qui aurait peinturé et sculpté une bonne partie de la physionomie européenne, dont deux témoins seraient aujourd'hui la palette de couleurs capillaires et celle de couleurs oculaires.

Il reste à expliquer pourquoi le troisième témoin, la couleur de la peau, ne s'est pas diversifié. D'abord, ce trait pigmentaire se distingue des deux autres dans le sens que la préférence pour une couleur rare y aurait été opposée par une autre tendance de la sélection sexuelle : celle d'accentuer un dimorphisme sexuel existant. Car la peau masculine contient plus de mélanine et d'hémoglobine que la peau féminine ; autrement dit, les femmes paraissent plutôt pâles et les hommes bruns et rougeauds. La pâleur s'associe ainsi à la féminité et c'est peut-être pour cette raison que les cultures prémodernes tendent à la favoriser chez la femme, tendance qui aurait empêché la sélection sexuelle de créer un troisième polymorphisme de couleurs. Il y aurait eu tout simplement un blanchissement progressif de la peau. C'est effectivement ce qui s'est produit chez les Européens, et ce à un degré beaucoup plus marqué que chez les indigènes des mêmes latitudes en Asie et en Amérique du Nord.

4. Comment voyez-vous cette population évoluer à 200 ans ?

Je n'ai aucune idée là-dessus. Je ne suis pas prophète.

5. Pourquoi ce mythe risque-t-il de perdurer dans des sociétés sans "blonds et blondes" naturels ?

Par des moyens artificiels. On crée non seulement des blonds peroxydés mais aussi des cheveux pourpres et verts, des couleurs tout à fait novatrices dont le but demeure le même : accrocher, attirer l'attention.

6. Par ailleurs, avez-vous des chiffres statistiques ( nous avons vu certaines courbes qui vous sont attribuées) sur la proportion de blonds naturels dans le monde, et par grandes régions du monde.

Tout dépend de la définition de «blond». Certaines études incluent les cheveux châtains alors que d'autres sont moins généreuses. Chez les personnes de souche anglaise, la proportion semble tourner autour de 20-25%. Chez les Français, elle est moindre. En dehors de l'Europe, les blonds sont rares, mais on en constate de façon sporadique en Afrique du Nord et au Moyen-Orient, ainsi que chez certains Aborigènes de l'Australie centrale et chez certains Inuit de l'Ouest de l'Arctique canadienne. Dans ces deux derniers cas, il s'agit peut-être des débuts d'une évolution par sélection sexuelle semblable à celle des Européens.

Reference

Belot, L. and V. Lorelle. 2007. Enquête. La fin des vraies blondes ? Le Monde, July 15, 2007

Are albinos smarter?

Steve Sailer asks whether albinos are smarter. He points to a 1997 New York Times article on albinism in southern Africa:

John M. Makumbe is a professor of political science. Richard Nyathi is chief librarian for a Government ministry. Stanley S. Gunda is a senior financial officer in another ministry. Not so long ago, they might have been killed at birth.

Messrs. Makumbe, Nyathi and Gunda are albinos. In Africa, far more than on any other continent, that is a lifelong curse. They lack the gene that codes the skin pigment melanin, and they are very nearsighted. As white-skinned men in a black society, they are shunned and feared as the products of witchcraft, taunted by children and drunks as ''peeled potatoes,'' ''monkeys'' and ''ghosts.''

There is a stereotype that all albinos are intelligent and accomplished, as these three men are. But Professor Makumbe said successful albinos were ''a teeny-weeny minute number.'' Most, he added, languish at home without education because they cannot see the blackboard at school or because their parents, told such children die young, will not pay for their schooling. …

All three men did well in school, despite vision problems. The genetic differences that cause albinism also change the connections between the optic nerves and the brain. Many albinos have nystagmus -- ''dancing eyeballs'' -- and myopia that, even with thick glasses, can only be corrected to about 20-200.

''Albinos seem more intelligent because they try harder,'' Mr. Nyathi said. ''You have to get out of your seat, go up to the board, squint, write two sentences, go back, and still finish the test in the same time as the others.''

Interestingly, the medical literature used to assume that albinism produces mental retardation. Not only are albinos not retarded, they actually seem to do better than average. Manganyl et al. (1974) found they had higher levels of intellectual maturity than participants in a control group. Fulcher et al. (1995) similarly observed significantly higher achievement among albinos in reading, spelling, and arithmetic skills.

Nonetheless, standard IQ tests show no difference between albinos and controls (Beckham 1946; Estrada-Hernández & Harper 2007; Fulcher et al. 1995; Kutzbach et al. 2008). It looks like albinos have the same intellectual potential but make better use of it.

This seems to be Dr. Kromberg’s explanation: ''Albinos have normal I.Q.'s but a higher capability. I think it's because they don't play in the sun all day. They stay inside and do their homework.''

On the other hand, IQ tests do not measure all aspects of intellectual performance. This is notably the case with ‘executive function,’ i.e., the ability to resist habit, the speed at which you can change the focus of your attention or the contents of your working memory, and the speed at which you can change your goals and respond appropriately. If we look at differences in these functions between identical twins, about 86% to 100% of the variability seems to be heritable (Friedman et al., in press).

Looking through the anthropological literature, I came across two references for higher intellectual attainment among albinos, and one for lower attainment:

Albinos are smart, clean, nice, and pretty. There is nothing wrong with them. [comments from traditional Hopis] (Woolf & Dukepoo 1969)

Some people think that we are useless because of our poor eyesight and skin condition. Others think we are idiots [Comments from Dominican Republic albinos] (Westhoff 1993)

... The mentality of Moon-children [albino Cuna Indians] appears to be normal, and no less than 14 of them have competed in formal school classes. It is a common belief among the Cuna that since they are unable to compete physically, they strive that much harder to succeed intellectually. (Keeler 1953)

For what it’s worth, the last reference also mentioned an apparent difference in head shape:

Head shape of Moon-children appears to be brachycephalic in a higher percentage than normal. No statistical data on this subject were collected, but in addition to my own observations the Indians themselves have remarked about this apparent correlation. (Keeler 1953)

References

Beckham, A.S. (1946). Albinism in Negro children. Journal of Genetic Psychology, 69, 199-215.

Estrada-Hernández, N., & Harper, D.C. (2007). Research on psychological and personal aspects of albinism: A critical review. Rehabilitation Psychology, 52(3), 263-271.

Friedman, N. P., Miyake, A., Young, S. E., DeFries, J. C., Corley, R. P., & Hewitt, J. K. (in press). Individual differences in executive functions are almost entirely genetic in origin. Journal of Experimental Psychology: General.

Fulcher, T., O'Keefe, M., Bowell, R., Lanigan, B., Burke, T., Carr, A., O'Rourke, M., Bolger, M. (1995). Intellectual and educational attainment in albinism. J. Pediatr. Ophthalmol. Strabismus, 32(6), 368-72.

Keeler, C. (1953). The Caribe Cuna Moon-Child and its heredity, Journal of Heredity, 44, 162-171.

Kutzbach, B.R., Summers, C.G., Holleschau, A.M., & MacDonald, J.T. (2008). Neurodevelopment in Children with Albinism. Ophthalmology, in press

Manganyl, N.C., Kromberg, J.G., Jenkins, T. (1974). Studies on albinism in the South African Negro. I. Intellectual maturity and body image differentiation. J Biosoc Sci, 6(1), 107-12.

McNeil Jr., D.G. (1997). Black, Yet White: A Hated Color in Zimbabwe, The New York Times, February 9, 1997.

Westhoff, W. (1993). A psychosocial study of albinism in a predominantly mulatto Caribbean community. Psychological Reports, 73, 1007-1010.

Woolf, C.M. and F.C. Dukepoo. (1969). Hopi Indians, inbreeding, and albinism, Science, 164, 30-37.

Intelligence and mental illness

In my last post, I suggested that intelligence, or more precisely neural plasticity, was originally an infant trait in ancestral humans. In modern humans, it seems to have progressively been extended into older age groups.

This isn’t a novel idea. It has been put forward by such people as Ashley Montagu (1989), Stephen J. Gould (1977), and Konrad Lorenz (1971). Interestingly, one author has argued that this persistent brain plasticity has made the human mind more susceptible to certain neuroses and other personality disorders.

Therefore, psychotic disorders may be the ‘price’ for the delay of maturation and for the rapid increase of brain size during the past 150,000 years. For example, normal development of the human nervous system is particularly dependent on sensory input. Social deprivation leads to stereotyped and self-injurious behaviour, resulting from an alteration of dopamine receptor sensitivity. The assumption of an enhanced susceptibility to psychotic disorders by delayed maturation may be underlined by the fact that most psychiatric disorders originate in early childhood, when the growth rate of the brain and acquisition of learned material is particularly high. (Brüne, 2000)

This point is interesting because it links up with Dr. Kawecki’s suggestion that “some diseases are a byproduct of intelligence” (see previous post).

References

Brüne, M. (2000). Neoteny, psychiatric disorders and the social brain: hypotheses on heterochrony and the modularity of the mind. Anthropology & Medicine, 7(3), 301-318.

Gould, S.J. (1977). Ontogeny and Phylogeny. Cambridge: Harvard University Press.

Lorenz, K. (1971). Studies in Animal and Human Behaviour, Vol 2. London: Methuen & Co.

Montagu, M.F.A. (1989). Growing Young. 2nd ed. New York: Bergin & Garvey.

IQ: interaction between race and age

In discussions of race and IQ, debate often focuses on a study of children fathered by soldiers stationed in Germany and then raised by German mothers (Eyferth 1961). The study found no significant difference in IQ between children with white fathers (83 subjects) and those with black fathers (98 subjects), the mean IQ being about 97 for both groups.

These findings are criticized by Rushton and Jensen (2005) on three grounds:

1). The children were still young when tested. One third were between 5 and 10 years old and two thirds between 10 and 13 years old. Since family socialization effects are stronger before puberty, a much larger sample would be needed to find a significant difference between the two groups.

2) Between 20 and 25% of the ‘black’ fathers were actually North African.

3) At the time, the US Army screened out low IQ applicants with its preinduction Army General Classification Test. The rejection rate was about 30% for African Americans and 3% for European Americans.

I have doubts about the first criticism. Throwing in more environmental differences would make an innate difference less significant and harder to detect. But why would it disappear? It should do so only if the family environment were, on average, more conducive to learning in the biracial group. Both groups, however, had the same kind of family environment, i.e., a single German mother brought up the children. Am I missing something? I guess this is where the other two criticisms come in, i.e., the fathers were either North Africans or above-average African Americans.

But why, then, do we see similar IQ scores in children whose biological parents are clearly African-American and probably below-average ones at that? I am talking here of African-American children given up for adoption. At 7 years of age, Moore (1986) found a mean IQ of 117 among those placed in middle-class white families and a mean IQ of 104 among those placed in middle-class black families. These findings are criticized by Rushton and Jensen (2005), who argue that the children were raised in enriched environments and that the sample sizes were small (23 children in each case).

We see comparable findings, however, when IQ is measured with robust standardization samples of African-American and European-American children. When Dickens and Flynn (2006) analyzed the results of the 2002 standardization sample for the WISC (Wechsler Intelligence Scale for Children), they found that IQ starts off high in African American children and then declines with age:

African-American WISC scores
Age -- IQ
4 --- 95.5
12 -- 90.5
15 -- 88.8
24 -- 84.5

Dickens and Flynn (2006) also note that these scores show a gain of 5-6 points over the scores of black children thirty years earlier. But the decline of their IQ with age has remained stable. This decline also shows up in the Minnesota Transracial Adoption Study —a longitudinal study of black, biracial (black/white), and white children adopted into white middle-class Minnesota families (Scarr and Weinberg, 1976; Weinberg, Scarr, and Waldman, 1992). The children’s IQs were measured at 7 years of age and again ten years later:

------------------- Age 7 Age 17
Black children –---- 97 --- 89
Biracial children – 109 --- 99
White children –-- 112 -- 106

These findings parallel those of the Eyferth study. Young biracial children have IQ scores that are close to those of young white children. Even young black children have relatively high IQ. In this age group, interventions to raise IQ are successful. But the effects seem to wash out with age.

Rushton and Jensen (2005) are aware of this age trend, but they attribute it to the declining influence of family environments, i.e., at younger ages the diversity of learning environments within and between families accounts for a larger proportion of total IQ variability. This phenomenon would tend to obscure other sources of variability, thus making any black-white difference less significant. But why would it raise the IQ scores of black children while leaving those of white children unchanged? I suppose one might argue that family environments are more conducive to learning among African Americans than among European Americans. Is there any basis for such a belief?

In saying that this age decline is real, I’m not excluding an innate causation. Indeed, a socially induced causation seems less consistent with the data. It would have to affect even biracial children conceived by white mothers and raised by white parents in an overwhelmingly white cultural environment.

Among humans in general, intellectual capacity seems to decline with age. Indeed, there are statements in the literature that IQ declines from one’s twenties onwards (presumably among European Americans). Is this decline due to natural aging processes? Or is it prewired into the human organism?

Perhaps the ability to acquire new information becomes less useful with age and perhaps this was even truer in ancestral humans. What we call ‘intelligence’ may have originally been an infant trait that humans lost as they grew up. With the expansion of our cultural environment, natural selection would have progressively extended this infant trait into older age groups, and more so in some populations than in others.

By way of analogy, lactose tolerance was originally an infant trait and is still so in most human populations. It has become an adult trait in those populations that have long practiced dairy farming and adult consumption of milk.

References

Dickens, W.T., and J.R. Flynn. (2006). Black Americans reduce the racial IQ gap. Evidence from standardization samples. Psychological Science, 17, 913-920.

Eyferth, K. (1961). Leistungen verscheidener Gruppen von Besatzungskindern in Hamburg-Wechsler Intelligenztest für Kinder (HAWIK). Archiv für die gesamte Psychologie, 113, 222-241.

Moore, E.G.J. (1986). Family socialization and the IQ test performance of traditionally and transracially adopted Black children. Developmental Psychology, 22, 317-326.

Rushton, P. and A.R. Jensen. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law, 11, 235-294.

Scarr, S., and Weinberg, R.A. (1976). IQ test performance of Black children adopted by White families, American Psychologist, 31, 726-739.

Weinberg, R.A., Scar, S., and Waldman, I.D. (1992). The Minnesota Transracial Adoption Study: A follow-up of IQ test performance at adolescence. Intelligence, 16, 117-135.

The down side of smart

When I encounter scientific creationists, I often hear the following argument: “If all living things have arisen through evolution, why have so few evolved to be big, strong, and smart? Evolution predicts that adaptive qualities will always be favored over non-adaptive ones. So why are so many species small, weak, and stupid?”

Perhaps because size, strength, and intelligence aren’t all they’re cracked up to be. Everything has a cost. This is true for even the most desirable of qualities. At the very least, a ‘good’ quality will drain metabolic resources away from other uses, which may be more necessary for survival.

This point has been recently made in a New York Times article that featured interviews with two biologists: Reuven Dukas at McMaster University and Tadeusz Kawecki at the University of Fribourg:

"We use computers with memory that's almost free, but biological information is costly," Dr. Dukas said. … Dr. Dukas argues that learning evolves to higher levels only when it is a better way to respond to the environment than relying on automatic responses

.… Dr. Kawecki suspects that each species evolves until it reaches an equilibrium between the costs and benefits of learning. His experiments demonstrate that flies have the genetic potential to become significantly smarter in the wild. But only under his lab conditions does evolution actually move in that direction. In nature, any improvement in learning would cost too much.

"Humans have gone to the extreme," said Dr. Dukas, both in the ability of our species to learn and in the cost for that ability.

Humans' oversize brains require 20 percent of all the calories burned at rest. A newborn's brain is so big that it can create serious risks for mother and child at birth. … Dr. Kawecki says it is worth investigating whether humans also pay hidden costs for extreme learning. "We could speculate that some diseases are a byproduct of intelligence," he said.

Reference

Zimmer, C. (2008). Lots of Animals Learn, but Smarter Isn't Better. New York Times, May 6, 2008
http://www.nytimes.com/2008/05/06/science/06dumb.html

Why are 'we' longhaired?

A reader writes:

It may be that very long hair was selected against in the tropics due to either pests or possibly the greater ill effects of dampness during the rainy season. (In the tropics during the rainy season being too covered up tends to lead to rashes and/or poor healing of blisters and other small wounds.)

This explanation makes sense if the short hair of tropical peoples is the puzzle to be explained. Why are they not like us? How did they become different from us? (Evidently, us means longhaired, melanin-challenged folk).

Well, they didn’t become different from us. We became different from them. We are the evolutionary oddity and we are the ones who need explaining.

In scientific jargon, long head hair is the ‘derived’ characteristic. All ancestral humans started off being shorthaired. Then, somehow, for some reason, and in some populations, their head hair began to grow to remarkable lengths. Why? Isn’t that the evolutionary conundrum?