1. Gender-based preferences for skin colour
In their classic doll study, Kenneth and Mamie Clark found that children almost equally preferred light- and dark-skinned dolls at 3 years of age. This was true for both black and white children. At 4 years of age, preference sharply increased for the lighter doll (76% of black children).
This light-skin preference has since been reported in children from Germany, France, Italy, Japan, and Israel (Best et al. 1975; Best et al. 1976; Iwawaki et al. 1978; Munitz et al. 1987). It does not seem limited to the U.S. and its pattern of race relations.
I myself replicated the Clark study with French-Canadian children, using two dolls that differed slightly in complexion (both would have been considered ethnically "white"). Like the Clarks, I found that preferences were more or less evenly split at age 3. Like the Clarks, I found a strong preference for the lighter doll after 3 years of age.
Unlike the Clarks, I also tested 2-year-olds. Most preferred the darker doll. In addition, the shift in preference from 2 to 4 years of age strongly coincided with loss of baby fat. Children who chose the darker doll were significantly fatter (BMI and triceps skinfold) than those who chose the lighter doll (Frost 1989).
These results mystified me until I learned that fatty tissue is the body's main source of estrogen before puberty. I tested the "estrogen hypothesis" by investigating whether skin colour preference varies with the menstrual cycle in adult women. Subjects in the estrogen-dominant phase of the menstrual cycle (first two-thirds) were likelier to choose a darker male face than were subjects in the progesterone-dominant phase (last third). This cyclical effect was absent in subjects who were on oral contraceptives or who viewed female faces (Frost 1994). These findings have recently been confirmed by Jones et al. (2005). The lead author was unaware of my own study and was testing a completely different hypothesis (apparent health of faces).
Complexion may be one of many visual cues that we use, often unconsciously, to distinguish men from women. There is, in fact, a sex difference: women are paler and men ruddier and browner (Frost 2005:54-60). We seem to be prewired to pick out this visual cue. People can identify the sex of a face by complexion alone, even when the picture is blurred and offers no other details (Russell et al. 2006; Tarr et al. 2002).
Avenues for future research
- replicate my 1994 menstrual cycle study. I would like to alter the coloration of photos of male faces to see which skin pigment is triggering the cyclical changes in female preference. Is it ruddiness (hemoglobin) or brownness (melanin)?
- study physiological responses of men to differences in female complexion and physiological responses of women to differences in male complexion. Does women's lighter complexion help to initiate sexual attraction? (Symons' fecundability signal hypothesis) Or does it alter male behaviour within an existing sexual bond? (in the direction of less aggressive and more caring behaviour, Guthrie's paedomorphic mimicry hypothesis).
Best, D.L., J.T. Field, & J.E. Williams. (1976). Color bias in a sample of young German children, Psychological Reports 38:1145-1146.
Best, D.L., C.E. Naylor, & J.E. Williams. (1975). Extension of color bias research to young French and Italian children, Journal of Cross-Cultural Psychology 6:390-405.
Clark, K.B. & M.P. Clark. (1947). Racial identification and preference in negro children, in Readings in Social Psychology, T.M. Newcomb and E.L. Hartley (eds), New York: Henry Holt, pp. 167-178.
Frost, P. (1994). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship, Perceptual and Motor Skills 79: 507-514.
------ (1989). Human skin color: the sexual differentiation of its social perception, Mankind Quarterly 30:3-16.
Guthrie, R.D. (1970). Evolution of human threat display organs, Evolutionary Biology 4:257-302.
Iwawaki, S., K. Sonoo, J.E. Williams, & D.L. Best. (1978). Color bias among young Japanese children, Journal of Cross-Cultural Psychology 9:61-73.
Jones, B.C., Perrett, D.I., Little, A.C., et al. (2005). Menstrual cycle, pregnancy and oral contraceptive use alter attraction to apparent health in faces, Proc. R. Soc. B 272:347-354.
Munitz, S., B. Priel, & A. Henik. (1987). Color, skin color preferences and self color identification among Ethiopian and Israeli born children, in M. Ashkenazi and A. Weingrod (eds.), Ethiopian Jews and Israel. New Brunswick (U.S.A.): Transaction Books, pp. 74-84.
Russell, R., P. Sinha, I. Biederman, & M. Nederhouser. (2006). Is pigmentation important for face recognition? Evidence from contrast negation, Perception, (35) 749-759.
Symons, D. (1995). Beauty is in the adaptations of the beholder: The evolutionary psychology of human female sexual attractiveness, in P.R. Abramson and S.D. Pinkerton (eds.), Sexual Nature. Sexual Culture. Chicago: University of Chicago Press, pp. 80-118.
Tarr, M.J., D. Kersten, Y. Cheng, & B. Rossion. (2002). It's Pat! Sexing faces using only red and green, Journal of Vision, 1(3), 337a
2. Sexual selection and diversification of hair and eye colour in European populations
Most humans have only one hair colour and one eye colour. Europeans are a big exception: their hair is black but also brown, flaxen, golden, or red; their eyes are brown but also blue, grey, hazel, or green. This diversity reaches a maximum in an area centred on the East Baltic and covering northern and eastern Europe. If we move outward, to the south and east, we see a rapid return to the human norm: hair becomes uniformly black and eyes uniformly brown.
Why this colour diversity? And why only in Europe? Some believe it to be a side effect of natural selection for fairer skin to ensure enough vitamin D at northern latitudes. Yet skin colour is weakly influenced by the different alleles for hair colour or eye colour apart from the ones for red hair or blue eyes. Some have no effect at all on skin pigmentation (Duffy et al. 2004; Sturm and Frudakis 2004).
Others put the cause down to intermixture with Neanderthals. Yet, according to the mtDNA that has been retrieved, no genetic continuity is discernible between late Neanderthals and early modern Europeans. Perhaps there was some gene flow between the two groups, but certainly not enough to account for the large number of Europeans with neither black hair nor brown eyes.
For others still, this colour diversity arose through random factors: genetic drift, founder effects, relaxation of natural selection, etc. But these factors could not have produced such a wide variety of hair and eye hues in the 35,000 years that modern humans have inhabited Europe. The hair-colour gene (MC1R) has at least 7 alleles that exist only in Europe and the same is probably true for the eye-colour gene (OCA2) (Rana et al. 1999). If we take the hypothesis of a relaxation of selection, nearly a million years would be needed to accumulate this amount of diversity (Harding et al. 2000; Templeton 2002). Moreover, it is odd that the same sort of diversification has evolved at two different genes whose only point in common is to colour a facial feature (Frost 2006; Makova & Norton 2005).
Thus, some kind of non-random process seems to have targeted hair and eye colour as visible characteristics. But how? And why? For some, including the geneticist Luigi L. Cavalli-Sforza, the answer is sexual selection. This mode of selection intensifies when males outnumber females among individuals ready to mate, or vice versa. The sex in excess supply has to compete for a mate and resorts to the same strategies that advertisers use to grab attention, such as the use of bright or striking colours. In other animals, bright colours are usually due to sexual selection. Sometimes the result may be a "colour polymorphism." A potential mate will respond not simply to a bright colour but also to a rare one that stands out from the crowd. By enhancing reproductive success, however, such a colour will also become more common and less eye-catching. Sexual attraction will then shift to less common variants, the eventual result being an equilibrium that maximises colour diversity (Brooks 2002; Frost 2006; Hughes et al. 1999).
This sort of rare-colour advantage has been reported in humans. Thelen (1983) prepared three series of slides featuring attractive women: one with 6 brunettes; another with 1 brunette and 5 blondes; and a third with 1 brunette and 11 blondes. Male subjects then had to select the woman in each series they would most prefer to marry. For the same brunette, preference increased significantly from the first to the third series, i.e., in proportion to the rarity of the brunettes. This rare-colour preference may account for the wide range of human hair and eye phenotypes we see today.
But why is hair and eye colour so much more diverse in Europe than elsewhere? Perhaps because sexual selection was much stronger among ancestral Europeans than in other human populations.
Sexual selection intensifies when the "Operational Sex Ratio" (OSR) ceases to be balanced, i.e., when too many of one sex are competing for too few of the other. To understand why this may have happened in ancestral humans, we can examine present-day hunter-gatherers. An OSR imbalance usually develops for two reasons: 1) longer hunting distances have increased the death rates of young men, typically because game animals are more mobile and/or less numerous per unit of land area; and 2) higher costs of providing for a second wife have reduced the incidence of male polygamy (polygyny), typically because women are procuring less food for themselves through food gathering. As a rule, OSRs are less balanced further away from the equator. In the Temperate Zone, and even more so in the Arctic, game animals roam over larger territories and gatherable food is less available in winter.
The most extreme OSR imbalance occurs among hunting peoples of the "steppe-tundra," where almost all consumable biomass is in the form of highly mobile and spatially concentrated herbivores such as caribou, reindeer, or muskox. On the one hand, men die younger because they have to cover long distances in search of herds, with no alternate food sources. On the other, men are less polygynous because they bear almost the full cost of feeding their families in a habitat that offers women little opportunity for food gathering. With fewer men altogether and even fewer polygynous ones, women have to compete for a limited supply of potential husbands.
Steppe-tundra is now reduced to fragments along the northern fringes of Eurasia and North America. During the last ice age, however, when modern humans first arrived, the Scandinavian icecap had pushed it farther south onto the plains of Europe. The more intense sunlight, combined with fertile loess soils and the Gulf Stream's moderating and humidifying influence, created an expanse of steppe-tundra with unusually high bioproductivity, even at the peak of the ice age (Hoffecker 2002: 21-26, 32-34). Less productive was the Asian steppe-tundra east of the Urals. It was drier, farther north, and largely polar desert, especially at the glacial maximum (Ray and Adams 2001). Prospects were better for continuous and substantial human settlement on the European portion of this ecological zone.
The European steppe-tundra was distinctive in another way. It took in an area that covers almost the same area where hair and eye colour is today most diverse. Could this be an imprint left on the phenotypic landscape by sexual selection?
Avenues for future research
- look into the possibility that these hair and eye colour are mildly sex-linked, as would be expected if women were more strongly selected for such characteristics. According to an unpublished study, non-black-haired and non-brown-eyed individuals have longer second fingers in relation to their fourth fingers. This indicates that these hair and eye colours are associated with a higher ratio of estrogen to testosterone before birth. Interestingly, blond hair has arisen independently among some Aborigines of central Australia and is more frequent there in women than in men (Abbie & Adey 1953).
- study non-European populations that seem to have developed new hair and eye colours independently. There are a few such cases: blond hair among central Australian Aborigines, brown hair among the Yukaghir of eastern Siberia, and fair hair among some Inuit bands of the western Canadian Arctic. Is their hair colour less diverse than that of Europeans because their exposure to sexual selection has been less intense or shorter in duration?
- analyse DNA from skeletal remains in order to chart changes in European MC1R and OCA2 variability over the last 35,000 years. If the sexual selection hypothesis is true, MC1R and OCA2 variability should have developed almost entirely during the last ice age (c. 25,000 - 10,000 BP).
Abbie, A.A., & W.R. Adey. (1953). Pigmentation in a central Australian tribe with special reference to fair-headedness, American Journal of Physical Anthropology 11:339-359.
Brooks, R. (2002). Variation in female mate choice within guppy populations: population divergence, multiple ornaments and the maintenance of polymorphism, Genetica 116:343-358.
Duffy, D.L., N.F. Box, W. Chen, J.S. Palmer, G.W. Montgomery, M.R. James, N.K. Hayward, N.G. Martin, & R.A. Sturm. (2004). Interactive effects of MC1R and OCA2 on melanoma risk phenotypes. Human Molecular Genetics 13:447-461.
Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior 27:85-103.
Harding, R.M., E. Healy, A.J. Ray, N.S. Ellis, N. Flanagan, C. Todd, C. Dixon, A. Sajantila, I.J. Jackson, M.A. Birch-Machin, & J.L. Rees. (2000). Evidence for variable selective pressures at MC1R. American Journal of Human Genetics 66:1351-1361.
Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.
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.
Makova, K, & H. Norton. (2005). Worldwide polymorphism at the MC1R locus and normal pigmentation variation in humans. Peptides 26:1901-1908.
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.
Ray, N. & J. M. Adams. (2001). A GIS-based Vegetation Map of the World at the Last Glacial Maximum (25,000-15,000 BP). Internet Archaeology 11.
Sturm, R.A., & T.N. Frudakis. (2004). Eye colour: portals into pigmentation genes and ancestry. Trends in Genetics 20:327-332.
Templeton, A.R. (2002). Out of Africa again and again. Nature 416:45-51.
Thelen, T.H. (1983). Minority type human mate preference, Social Biology 30: 162-180.
3. Possible selective effects of the proto-industrial stage of early European capitalism
During the 18th and 19th centuries, Europe dramatically increased in population. Over most of the continent, the reason was simply a fall in death rates. Some areas, however, also saw a marked rise in birth rates, notably England, the Netherlands, parts of Germany, and Ashkenazi communities in Poland. Fertility especially rose among semi-rural artisans who produced goods on contract for urban merchants (Verlagssystem or putting-out system).
During this early stage of industrial capitalism ("proto-industrialism"), artisans were no longer producing goods for a small local market. They were now serving large regions, often national in scope. Market size was dictated not so much by geographical constraints as by the ability to produce quality goods at the lowest possible price. As a result, demand was increasingly elastic and constrained only by the artisan's capacity to exploit it. Seccombe (1992: 182) describes this new economic regime:
In a typical medieval village, there had been one ironsmith, a few carpenters, and one or two masons; now a majority of households in a village would set up as framework knitters, silk-weavers, linen-makers, or metalworkers. The proto-industrial economy was characterized by the massing of production of a given commodity in a particular region, saturating its markets and forcing producer households to rely for their subsistence on long-distance trade, and hence on merchants.
Labour markets, in contrast, were still rudimentary. Artisan workshops were typically family-run operations where the children performed most of the work. Some factories did exist but were staffed principally by debtors and convicted criminals. For successful artisans, the most effective way to increase workforce size was to have more children, largely by marrying earlier and also by abandoning birth control methods then in use. Entrepreneurial success thus became tightly correlated with reproductive success.
This economic regime may have had selective effects, especially if we keep in mind that the fertility differences within proto-industrial villages were much greater than the mean difference between such villages and other rural communities (usually about 10-30%). Unlike medieval craftsmen, whose potential for success or failure fell within a narrow continuum, proto-industrial artisans faced a much broader range of possible economic outcomes. Even slight advantages in efficiency could be critical:
Under the putting out system, cottage industry was based on the labour of women and children, as it had been traditionally. But in the latter half of the eighteenth century, there was a marked intensification of the working day and younger children were pressed into service as assistants. Merchants and their agents tightened the screws of labour discipline by implementing stricter product quotas, quality-control screening, tighter delivery schedules and elaborate subcontracting networks (Seccombe 1992: 207)
This social class contributed very disproportionately to population growth (Seccombe 1992:194-195, 213, 217). Their high fertility explains why Great Britain overtook France in absolute population during this period, despite British emigration to North America and elsewhere. The impact seems to have been even greater at the subnational level:
In comparison with major cities and agricultural districts, birth rates in industrial regions were usually high. Between 1730 and 1800, fertility in the industrial districts of Nottinghamshire was on average 29 per cent above levels in nearby agricultural villages. ... in cottage textile districts in Brittany, Flanders, Lombardy, Saxony, Basel, Piedmont and the canton of Zurich ... high birth rates were almost certainly due to earlier and more universal marriage. (Seccombe 1992: 217)
Most industrial growth in the latter half of the eighteenth century was confined to specific regions of the European countryside. Entire villages were swept up in the production of particular lines of standard consumer goods, as merchants extended putting-out (the Verlagssystem), by-passing the urban guilds. Cottage industry flourished in diverse areas: in Westphalia, Saxony, the Zurich uplands, Alsace, Flanders, Brittany, Lancashire, Yorkshire and Ulster (Seccombe 1992: 205).
There is an emerging consensus that significant evolutionary change can occur over relatively short spans of time (Harpending and Cochran 2002). Indeed, the evidence is strong that natural selection has been acting on the human genome even over the past 50,000 years (Eberle et al. 2006; Voight et al. 2006; Wang et al. 2006). It is likely, then, that the six to ten generations of European proto-industrialism may have produced significant allelic differences between proto-industrial villages and adjacent rural communities whose range of possible life outcomes and hence differences in reproductive success was much narrower.
Avenues for future research
1. Identify and map the spatial distribution of proto-industrial villages for districts that have had little in-migration since the 19th century. Look for correlations with alleles under recent selection (Eberle et al. 2006; Voight et al. 2006; Wang et al. 2006).
Eberle, M.A., M.J. Rieder, L. Kruglyak, & D.A. Nickerson. (2006). Allele frequency matching between SNPs reveals an excess of linkage disequilibrium in genic regions of the human genome, PLoS Genet 2(9): e142
Harpending, H., & G. Cochran. (2002). In our genes, Proceedings of the National Academy of Sciences 99(1):10-12.
Seccombe, W. (1992). A Millennium of Family Change. Feudalism to Capitalism in Northwestern Europe, London: Verso
Voight B.F., S. Kudaravalli, X. Wen, & J.K. Pritchard. (2006). A map of recent positive selection in the human genome, PLoS Biol 4(3): e72
Wang, E.T., G. Kodama, P. Baldi, & R.K. Moyzis. (2006). Global landscape of recent inferred Darwinian selection for Homo sapiens, Proc. Natl. Acad. Sci USA 103:135-140.
Frost, P. (2008a). Sexual selection and human geographic variation, Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society, The Journal of Social, Evolutionary & Cultural Psychology 2 (supp.): 49-65, www.jsecjournal.com/NEEPSfrost.pdf
------ (2008b). The spread of alphabetical writing may have favored the latest variant of the ASPM gene, Medical Hypotheses, 70:17-20.
------ (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology 133:779-781.
------ (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior 27: 85-103 http://www.sciencedirect.com/science/journal/10905138
------ (2005). Fair Women, Dark Men. The Forgotten Roots of Color Prejudice. Christchurch (New Zealand): Cybereditions. http://www.cybereditions.com/CYVIEWSUMMARY::10033
------ (2003). Review of Human Natures. Genes, Cultures, and the Human Prospect by P.R. Ehrlich, Reports of the National Center for Science Education 23(2): 33-34.
------ (2002). Review of Desolate Landscapes: Ice-Age Settlement in Eastern Europe by John F. Hoffecker. Human Nature Review 2:156-163. http://human-nature.com/nibbs/02/frost.html
------ (2001a). Modernity and falling birthrates, Encyclopedia of Birth Control, V.L. Bullough (ed.), Santa Barbara (Cal.): ABC-CLIO, pp. 176-180.
------ (2001b). Polygyny and sex ratios, Encyclopedia of Birth Control, V.L. Bullough (ed.), Santa Barbara (Cal.): ABC-CLIO, pp. 218-223.
------ (2001c). Exogenous estrogen and possible psychosexual effects, The Human Behavior and Evolution Society - 13th Annual Conference, p. 73, London (United Kingdom). http://www.hbes.com/HBES/ABST2001.PDF
------ (2000). La Disparition des Dorsétiens : assimilation ou extinction ? Colloque Nord-Laval en Sciences Humaines 2000, p. 19.
------ (1998a). Commentary on "Sexual selection and sex differences in mathematical abilities" by D.C. Geary, Behavioral and Brain Sciences 21: 443-448.
------ (1998b). Le corridor côtier et le peuplement des Amériques : le cas des proto-Algonquiens, Colloque Nord-Laval en Sciences Humaines 1998, p. 35.
------ (1997). La couleur de la peau aux premiers regards, Aspects sociologiques 6(1): 12-19.
------ (1996a). L'écart pigmentaire hommes-femmes: perspectives biosociales, Altérités. Revue québécoise des étudiants en anthropologie 1:1-18. http://www.anthro.umontreal.ca/varia/alterites
------ (1996b). Towards an understanding of human skin color in the EEA: Age-related and sex-related meanings, The Human Behavior and Evolution Society - 8th Annual Conference, Northwestern University, Chicago. http://www.hbes.com/HBES/abst96.htm
------ (1995). Un regard néodarwinien sur l'écart pigmentaire hommes-femmes, doctoral thesis, Université Laval, Department of Anthropology, 265 pp.
------ (1994a). Geographic distribution of human skin colour: A selective compromise between natural selection and sexual selection? Human Evolution, 9:141-153.
------ (1994b). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship, Perceptual and Motor Skills 79: 507-514.
------ (1992a). À fleur de peau: Gender and complexion on Ile-aux-coudres, Canadian Psychology 33(2a): 363.
------ (1992b). À fleur de peau : le teint et son évaluation dans la psyché humaine, Écologie, éthologie humaines 1: 7-17 (new series).
------ (1991a). Attitudes towards Blacks in the early Christian era, The Second Century 8(1):1-11.
------ (1991b). Changes in skin color preference during the menstrual cycle, The Human Behavior and Evolution Society - 3rd Annual Meeting, p. 57, McMaster University, Hamilton, Ontario.
------ (1991c). Review of Romantic Motives Essays on Anthropological Sensibility G.W. Stocking Jr.(ed.), History of European Ideas 13:453-454.
------ (1991d). Review of Society Against the State by Pierre Clastres, History of European Ideas 13:857-858.
------ (1990). Fair women, dark men: the forgotten roots of colour prejudice, History of European Ideas 12: 669-679.
------ (1989). Human skin color: the sexual differentiation of its social perception, Mankind Quarterly 30:3-16.
------ (1988a). Human skin color: a possible relationship between its sexual dimorphism and its social perception, Perspectives in Biology and Medicine 32: 38-58.
------ (1988b). Correlation between adiposity and preference for different colors in dolls, Evolutionary Psychology and Psychiatry Conference, The University of Michigan, Ann Arbor, Michigan.
------ (1987a). Femmes claires, hommes foncés : les racines oubliées du préjugé de couleur, Anthropologie et sociétés 11: 135-149. http://www.erudit.org/revue/as/1987/v11/n2/006424ar.pdf .
------ (1987b). La couleur de la peau humaine: Les rapports entre son dimorphisme sexuel et sa perception sociale, master's thesis, Université Laval, Department of Anthropology, 116 pp.
------ (1986). Éditorial. Épidiascope 1(1):1-3.
------ (1985a). Human sexuality and the origins of colour prejudice, Canadian Ethnology Society - 12th Annual Congress, pp. 50-51, University of Toronto, Ontario.
------ (1985b). Sexual dimorphism in human skin colour, Canadian Association for Physical Anthropology - 13th Annual Meeting, p. 12, Thunder Bay, Ontario.
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van den Berghe, P. L. & P. Frost. (1986). Skin color preference, sexual dimorphism, and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies 9:87-113.