Recent evolution of complex behavior

I once knew an African student who told me that his language had no words for “good” or “evil”. When the missionaries translated their materials into his language, they had to write “Jesus is beautiful” instead of “Jesus is good.”

This sort of semantic evolution has occurred in all human languages. People have expressed new concepts by recycling older ones. Typically, these older concepts refer to primary sensations that are largely hardwired, i.e., ‘beautiful’, ‘cold’, ‘hot’, and so on.

Interestingly, it looks as if some of this hardwiring is being reused when we imagine evolutionarily recent concepts. For instance, when we recognize someone as friendly, we use neural pathways that are associated with the recognition of warmth.

During the autumn of 2006, a series of volunteers arrived at Yale University's psychology building. Each was greeted in the lobby by a researcher, who accompanied them up to the fourth floor. In the elevator, the researcher casually asked the volunteer to hold the drink she was carrying while she noted down their name. The subjects did not know it, but the experiment began the moment they took the cup.Once in the lab, the 40 or so volunteers read a description of a fictitious person and then answered questions about the character. Those who had held an iced coffee, rather than a hot one, rated the imaginary figure as less warm and friendly, even though eachvolunteer had read the same description. Answers to other questions about the figure, such as whether the character appeared honest, were unaffected by the type of drink. (Williams & Bargh, 2008).

Similarly, we have developed the ability to form moral judgments by building upon a mental algorithm that serves to judge cleanliness:

In one recent study, Simone Schnall at the University of Plymouth, UK, and colleagues showed half their volunteers a neutral film and the other half the toilet scene from the film Trainspotting. (The uninitiated need only use their imagination here: the clip features what is described as the "worst toilet in Scotland".) Those who viewed the Trainspotting clip subsequently made more severe judgements about unethical acts such as cannibalism than volunteers who had viewed the neutral scene. Exposing subjects to a fart smell and placing them in a filthy room had a similar effect (Schnall et al., 2008).

Chen-Bo Zhong of the University of Toronto in Canada and Katie Liljenquist, now at Brigham Young University in Provo, Utah, asked volunteers to read a first-person account of either an ethical act or an act of sabotage. They then had to rate the desirability of various household objects, including soap, toothpaste, CD cases and chocolate bars. Those who had read the sabotage story showed a greater preference for the cleaning products than those who had not.

A self-perception of physical cleanliness thus seems linked to one of moral cleanliness. Moreover, like Pontius Pilate, the mere act of washing your hands reduces feelings of moral responsibility.

… In another part of their study, Zhong's team asked volunteers to recall an unethical deed from their past. Under the guise of a health and safety precaution, he then gave half the subjects antiseptic wipes to clean their hands. The participants were then asked if they would take part in another experiment, this time to help out a desperate graduate student. Only 40 per cent of the subjects who had cleaned their hands volunteered, compared with almost three-quarters of those who hadn't. (Zhong & Liljenquist, 2006)

These studies suggest an answer to a question that has bothered me. How is that complex and relatively recent social behaviors often have moderate to high heritabilities? Surely such hardwiring would have taken an impossibly long time to evolve? Just think of all the genes involved…

Well, the answer is surprisingly simple. Natural selection has simply altered a hardwired algorithm that already exists. There’s no need to build it all from scratch. You just jerry-rig what you already have.

Big Blogger?

Blogger has recently deleted one of my favorite blogs: Mangan’s Miscellany. Why? It’s not clear. When Dennis Mangan tries to log on, he gets this brief message:

Account temporarily disabled We apologize for the inconvenience. Accounts may be disabled because of a perceived violation of either the Google Terms of Service or product-specific Terms of Service.

Mangan’s Miscellany covered a wide range of subjects. Many of them were politically sensitive and some, like his views on vitamin D therapy and the HIV epidemic, struck me as being misinformed. On all of these issues, however, his writing had always been civil, intelligent, and open to divergent views. If his blog has no place on the Internet, we’re really no better than the Chinese — who at least are upfront about their cyber-censorship.

Interestingly, Blogger does not delete blogs that feature pornography, unless it’s really ‘hard’. They simply insert a warning page that advises the viewer of the ‘mature content.’ Of course, that’s porn. Ideas are something else…

Please make a complaint:
http://www.google.com/quality_form?q=help&hl=en&safe=off

References

Giles, J. (2009). Icy stares and dirty minds: Hitch-hiking emotions, New Scientist 2725:http://www.newscientist.com/article/mg20327252.200-icy-stares-and-dirty-minds-hitchhiking-emotions.html* 15 September 2009

Schnall, S., J. Haidt, G.L. Clore, A.H. Jordan. (2008). Disgust as embodied moral judgment, Personality and Social Psychology Bulletin, 34, 1096-1109.

Williams, L.E. & J.A. Bargh. (2008). Experiencing physical warmth promotes interpersonal warmth, Science, 322, 606-607

Zhong, C.B. & K. Liljenquist, K. (2006). Washing away your sins: Threatened moralityand physical cleansing, Science, 313, 1451-1452.

Does civilization select against intelligence?

We know the brain has been evolving in human populations quite recently," said paleoanthropologist John Hawks at the University of Wisconsin at Madison.

Surprisingly, based on skull measurements, the human brain appears to have been shrinking over the last 5,000 or so years.

"When it comes to recent evolutionary changes, we currently maybe have the least specific details with regard the brain, but we do know from archaeological data that pretty much everywhere we can measure - Europe, China, South Africa, Australia - that brains have shrunk about 150 cubic centimeters, off a mean of about 1,350. That's roughly 10 percent," Hawks said.

"As to why is it shrinking, perhaps in big societies, as opposed to hunter-gatherer lifestyles, we can rely on other people for more things, can specialize our behavior to a greater extent, and maybe not need our brains as much," he added. (source)

It’s usually assumed that humans have steadily increased in intellectual capacity. But what if this trend reversed with the advent of civilization? As societies grow more complex, perhaps the average human has not had to know so much. He or she can ‘delegate’ tasks (not that such delegation is always voluntary). Perhaps civilization has made us dumber, not smarter.

Yes, the ‘great civilizations’ have made major contributions to the arts and sciences, typically through upper-class patronage of creative individuals—who otherwise would have to worry about their next meal. The down side, however, is that this emancipation of creativity requires a much larger number of helots. The latter also specialize in their own way—in doing the grunt work that others consider beneath them.

In the ancient world, intellectual life—the debating, pondering, and creating of new ideas—was confined to a small powerless minority, too few in number to generate the critical mass that makes intellectual ferment possible. There were no conferences, no academic journals, and no scientific associations. For the most part, there were only isolated individuals who felt estranged from the world around them. The more renowned ones had disciples in their entourage. But that was it.

This situation contrasts with that of Western Europe and then North America from the 17th century to the 20th. That intellectual ferment was broad-based. It took place within a large swath of the population that could understand the ideas being generated, and that could argue the pros and cons. It was this democratization of intellectual activity that made the West so exceptional.

I’m increasingly convinced that extreme social stratification—i.e., the creation of a small class of intellectuals and a much larger helot class—is inimical to true scientific progress. The intellectuals are too few in number, and too dependent on the system, to make any real contribution.

Skin bleaching

Since the mid-20th century, ‘skin bleaching’ has become more and more common among dark-skinned populations. It involves lightening skin color by means of topical preparations that contain hydroquinone, cortisone, or mercury. These products are effective, but prolonged use may damage the skin by making the epidermis thinner and by breaking down collagen fibers. Despite being condemned by the medical profession and increasingly restricted by governments, they can easily be obtained in various places: hair-stylist salons, subway stations, African public markets, etc.

Skin bleachers seem to be used the most in South Asia and its diaspora. Next come sub-Saharan Africa and its diaspora (West Indies, Brazil, United States, Western Europe, etc.), the Philippines and elsewhere in Asia. The market is mainly young and female. Thus, rate of use is 61.4% among Surinamese women of Indian origin less than 26 years old, as compared to 13.1% among young Surinamese of other origins (Javanese, African, etc.) (Menke, 2002).

In Africa, rate of use is 25% in Bamako, Mali, up to 52% in Dakar, Senegal, up to 35% in Pretoria, South Africa, and up to 77% in Lagos, Nigeria (Ntambwe, 2004). The practice has become so widespread that it has been nicknamed maquillage – ‘make-up’ (Ondongo, 1984). According to one African specialist, men encourage it by considering light-skinned women to be more attractive, intelligent, moral, desirable, and chaste. In contrast, dark-skinned women are said to look mean, evil, stupid, and untrustworthy (Ntambwe, 2004). This opinion is consistent with the results of a survey among Ghanaian women. Most of the respondents thought that men prefer light skin in a woman: “Sometimes if you really want to marry a particular man, you have to bleach” (Fokuo, 2009)

In Jamaica, users do not seem motivated by shame of their Black identity. Surveys show them having as much racial self-esteem as non-users. The motivation is more to make one’s face ‘cool’, to imitate one’s peers, to look pretty and attract a partner, and to feel good about oneself. There is also the influence of popular culture, such as Eurocentric beauty contests and singers who glorify women with light brown skin. In the dance-hall song Browning, Buju Banton says he loves his light-skinned girlfriend, his ‘browning’, more than his car, his motorbike, and his money. In Bleach On, Captain Barkey tells girls to keep on bleaching their skin (Charles, 2009).

Strangely, these products have become increasingly popular among South Asians, Africans, and West Indians for the past half-century, yet the same period has also seen these peoples regain much of their cultural independence. In advertising, magazines, or TV serials, one sees many more women from the local population than there were before.

Actually, it’s not so strange. Back when the local media recycled images of women from Western sources, the female audience had trouble identifying with them; there was a gap between the two. Because these images are now adapted to the local reality, they project a stronger normative influence on local women, who are keener to imitate them. These women, however, are still darker-skinned than the somatic norm being projected. This is especially so with Indian ‘Bollywood’ films but is also the case with serial dramas in Latin America and the Arab world.

References

Charles, C.A.D. (2009). Skin bleachers’ representations of skin color in Jamaica, Journal of Black Studies, 40, 153-170.

Charles, C.A.D. (2003). Skin bleaching, self-hate, and Black identity in Jamaica, Journal of Black Studies, 33, 711-728.

Fokuo, J. Konadu. (2009). The lighter side of marriage: Skin bleaching in post-colonial Ghana, Research Review NS, 25(1), 47-66.

Menke, J. (2002). Skin bleaching in multi-ethnic and multicolored societies. The case of Suriname, paper presented to the CSA Conference, Nassau, Bahamas, May 27 – June 1, 2002, Coping with Challenges, Contending with Change.
http://www.colorfoundation.org/pdf/skin%20bleach%20Sur%20CSA%20220502.doc

Ntambwe, M. (2004), 'Mirror mirror on the wall, who is the FAIREST of them all?' Science in Africa, March. http://www.scienceinafrica.co.za/2004/march/skinlightening.htm

Ondongo, J. (1984), Noir ou blanc ? Le vécu du double dans la pratique du « maquillage » chez les Noirs, Nouvelle Revue d’Ethnopsychiatrie, 2, 37-65.

Lévi-Strauss and gene-culture co-evolution

With the recent death of Claude Lévi-Strauss, there has been an outpouring of praise for his contributions to anthropology, notably the struggle for a more politically conscious anthropology and the shift from biological determinism to cultural determinism. This praise tells us more about the praisers than about Lévi-Strauss himself. In truth, he scarcely resembled the image presented in most of his obituaries, having denounced in his book Tristes tropiques the intrusion of a “utopian spirit” into his field.

With a shelter of legalistic and formalistic rationalism, we similarly build an image of the world and society where all problems can be settled by a courtroom approach whose logic is artful maneuvering, and we do not realize that the universe is no longer composed of what we are talking about.

Nor was he a complete cultural determinist. Like many thinkers of his generation, he felt that culture has contributed just as much as biology to differences among human populations. This is not, however, the same as believing that biology has created only skin-deep differences. He made this clear in a speech at our university in 1979:

… I would not feel truly anthropologist or structuralist if I did not accept that all questions should be discussed, and the question of the respective share of nature and nurture in human culture seems to me one of the most important ones we can and ought to ask ourselves. This issue has been made sterile for years and years by the false categorizations of physical anthropology related to the belief in the existence of human races.

However, we must not forget that, as anthropologists, the aspects of the question that will always appeal to us will be much less the genetic determination of culture or cultures than the cultural determination of genetics. By this I mean that a culture always will be made much less by its members’ gene pool than it will contribute to shaping and altering this gene pool.

The selection pressure of culture—the fact that it favors certain types of individuals rather than others through its forms of organization, its ideas of morality, and its aesthetic values—can do infinitely more to alter a gene pool than the gene pool can do to shape a culture, all the more so because a culture’s rate of change can certainly be much faster than the phenomena of genetic drift. (Lévi-Strauss, 1979, p. 24-25)

He is clearly referring here to the concept of gene-culture co-evolution. But just what are these genetic traits that cultures have shaped differently in different human populations? He doesn’t seem to mean minor physiological processes, like an improved ability to digest milk or carbohydrates. In fact, he seems to be referring to mental and behavioral traits, especially when he mentions ‘ideas of morality’. Is he saying that there has been selection for differences in moral capacity among human populations?

And if cultures have shaped different gene pools differently wouldn’t these gene pools be ‘races’? Did Lévi-Strauss think through this line of thought? Perhaps in denying the race concept he was simply making the kind of ritual denunciation that most anthropologists make … and only half-believe.

It is probably too late to find out what he really meant. This is not a line of thought that he seems to have pursued in his other publications, at least none I am aware of.

References

Lévi-Strauss, C. (1985). Claude Lévi-Strauss à l’université Laval, Québec (septembre 1979), prepared by Yvan Simonis, Documents de recherche no. 4, Laboratoire de recherches anthropologiques, Département d’anthropologie, Faculté des Sciences sociales, Université Laval.

Lévi-Strauss, C., (1955). Tristes tropiques, Paris.

Was Roman Britain multiracial?

Historians often assume that the Romans changed Britain politically but not demographically. The indigenous elites adopted Roman culture while the mass of the population remained Celtic. When the Anglo-Saxons arrived in the fifth century, much of this population fled to Wales and Cornwall, where they would retain their language and traditions. Meanwhile, those who remained behind were obliterated through a process of ethnic cleansing and coerced assimilation.

This historical account may be false. First, the Roman occupation seems to have brought profound demographic change. This has been suspected for some time on the basis of unusual burial objects and epigraphic inscriptions that record the presence of individuals from throughout the Roman Empire. Now, after analyzing remains from two burial grounds near Roman York, a research team has concluded that the buried individuals had diverse geographic origins (Leach et al., 2009). In particular, the craniometric data revealed many of sub-Saharan or Egyptian origin. At the ‘Trentholme Drive’ burial ground, 66% clustered most closely with Europeans, 23% with sub-Saharan Africans, and 11% with Egyptians. At the ‘Railway’ burial ground, the proportions were 53% European, 32% sub-Saharan, and 15% Egyptian.

York was a legionary fortress, so these individuals may have been legionnaires. There are, in fact, epigraphic references to African soldiers and even a written account about one in a history of the Emperor Septimius Severus (146-211 AD) (Scriptores Historiae Augustae, p. 425).

On another occasion, when he was returning to his nearest quarters from an inspection of the wall at Luguvallium (Carlisle) in Britain, at a time when he had not only proved victorious but had concluded a perpetual peace, just as he was wondering what omen would present itself, an Ethiopian soldier, who was famous among buffoons and always a notable jester, met him with a garland of cypress-boughs. And when Severus in a rage ordered that the man be removed from his sight, the Ethiopian by way of jest cried, it is said, “You have been all things, now, O conqueror, be a god.”

Why were these Africans so far from home? In the case of the Egyptians, Rome thought it unwise to station soldiers among people of the same ethnic background. The temptation would be strong to side with the locals if a rebellion occurred. In the case of the sub-Saharan Africans, they were recruited into the army for the same reason that Germanic barbarians were recruited: Rome could not meet its manpower requirements solely from within its empire. There was also a perception that the Romans had become soft and that barbarians made better soldiers.

Finally, Rome, like many multi-national empires, had a policy of moving people around in order to promote a common identity and to eliminate ethnic distinctiveness. The Assyrians had perfected this policy, e.g., the deportation of the Jews to Babylon and their replacement by other peoples. The Roman authorities used their army to this end. They wished to create an atomized society where regionalism or ethnicity could not mobilize resistance to imperial rule.

It is likely that these legionnaires had a major demographic impact wherever they were stationed, especially if we include the many officials, petty functionaries, traders, and others who came in their wake. Much of Roman Britain thus seems to have been Romanized in culture and multiethnic in origin.

This, in turn, calls for a few other reinterpretations. Wales and Cornwall are not Celtic-speaking today because they took in Romano-British refugees fleeing Anglo-Saxon invaders. They were simply those parts of Britain that had remained Celtic in language, culture, and population. The rest—present-day England—had long become heavily Romanized and cosmopolitan.

Nor do we have to postulate a process of ethnic cleansing and coerced assimilation to explain the extinction of Roman Britain in the 5th and 6th centuries. As Seccombe (1992) points out, the Roman Empire suffered from negative population growth. Not enough people married and had children to offset relatively high mortality among infants and young adults. In breaking down local collective identities, whether ethnic or regional, the Empire had created an atomized and increasingly anonymous society without the carrots and sticks that tightly knit societies use to push individuals down the path of family formation.

Once Rome had pulled its troops out of Britain in the early 5th century, there was no longer an inflow of people to offset the demographic deficit. The local population fell into decline, and the decline accelerated in the 6th century when plagues killed three out of every ten people. The Romano-British needed no help from the Anglo-Saxons to die out. They did it largely on their own.

References

Leach, S., M. Lewis, C. Chenery, G. Müldner, & H. Eckardt. (2009). Migration and diversity in Roman Britain: A multidisciplinary approach to the identification of immigrants in Roman York, England, American Journal of Physical Anthropology, 140, 546-561.

Scriptores Historiae Augustae – Septimius Severus 22:4-6, transl. D. Magie (1922-1932) Vol 1, London: Heinemann.

Seccombe, W. (1992). A Millennium of Family Change. London: Verso.

Are we all Middle Easterners now?

Dienekes is arguing that Middle Eastern farmers demographically replaced Europe’s original population between 8,000 and 3,000 years ago. This argument seems to be proven by two recent papers that show no genetic continuity between Europe’s late hunter-gatherers and early farmers. The continent’s current gene pool seems to owe very little to the original Upper Paleolithic and Mesolithic inhabitants. So goes his argument.

This argument raises one obvious problem. It implies that the physical characteristics of Europeans, especially northern Europeans, arose recently and over a short time.

How short? As late as 7500 years ago, hunter-fisher-gatherers still inhabited Europe above a line running from the Netherlands to the Black Sea. The line then gradually moved north, reaching northern Germany about 5500 BP and the eastern and northern agricultural areas of Scandinavia around 4300 BP. This leaves very little time for the evolution of the northern European phenotype, i.e., lightening of the skin to pinkish-white and diversification of hair and eye color into a wide range of hues. This phenotype is attested by historical records going back over two thousand years, so we’re left with a time window of less than five thousand years.

Is that enough time for so much phenotypic change? Perhaps, but the selection pressures would have to be very strong.

Let’s turn to the first of the two papers. Bramanti et al. (2009) compared mtDNA sequences from late hunter-gatherers and early farmers who had lived in northern and central Europe (Lithuania, Poland, Russia, Germany). There was no evidence of genetic continuity between the two populations.

But this paper raises several other points:

1. Modern Europeans are almost as distant genetically from the early farmers as they are from the late hunter-gatherers. To be ancestral to modern Europeans, these farmers and their descendents would need a very low female population size (less than 3,000 individuals). As the authors admit, this figure is well below current archaeological estimates (124,000 individuals).

2. The sample sizes are very small for the early farmers (25 individuals) and the late hunter-gatherers (20 individuals).

3. The sample of late hunter-gatherers covers a much longer time frame (15,400 – 4300 BP) than does the sample of early farmers (7650 - 7400 BP).

In sum, the authors have tried to describe the gene pool of late European hunter-gatherers with data from 20 individuals spread over four countries and over some 11,000 years.

Can such a sample be representative? Doubtful. Besides the smallness of the sample, the late hunter-gatherers were not a homogeneous population. By their time, Europe had completely changed ecologically. Open tundra had given way to forest and it was no longer possible to hunt wandering herds of reindeer. Hunter-gatherers now lived in smaller and more localized groups. Each group would have had its own genetic profile as a result of genetic drift and founder effects.

Even if these 20 individuals fairly represented late hunter-gatherers, the genetic continuity hypothesis is not disproved by genetic differences between them and early farmers. Undoubtedly, some hunter-gatherers adopted farming earlier than others and thus contributed more to the early farmer gene pool. Others never adopted farming and thus contributed nothing. Founder effects would have been considerable.

There are thus two serious problems with Bramanti et al. (2009):

1. The sample of late hunter-gatherers is too small and too scattered over space and time to be representative of the late hunter-gatherer gene pool;

2. The genetic continuity hypothesis does not assume that the early farmer gene pool was a representative cross-section of the late hunter-gatherer gene pool.

Let’s turn to the other paper. Malmström et al. (2009) retrieved mtDNA from 19 late hunter-gatherers and 3 early farmers who lived in southern Scandinavia. The late hunter-gatherers show no genetic continuity with the early farmers or with modern Scandinavians but they do show genetic continuity with modern Baltic populations (i.e., Latvians). This seems consistent with archaeological evidence that the eastern Baltic was a refugium for Europe’s last hunter-gatherers. Indeed, the inland boundaries of Latvia, Lithuania, and Old Prussia may hark back to a time when these people fished and sealed from coastal stations part of the year and then moved some distance inland to hunt game the rest of the year.

This study has the merit of being more narrowly focused in time and space. Like the other study, however, it suffers from very small sample sizes and the likelihood of founder effects. In fact, the early farmer sample is so small that genetic continuity with modern Scandinavians is unsure.

What now?

The challenge now will be to enlarge this sample of late hunter-gatherers. By ‘enlarge’, I don’t simply mean a larger sample. I also mean a larger number of geographic locations to be sampled. Late hunter-gatherers were a heterogeneous bunch. Some contributed a lot to the future gene pool. Others went extinct.

The ‘losers’ were small inland hunting bands with low population densities. They were less able to integrate agriculture into their nomadic way of life and also more likely to retreat in the face of much larger farming communities.

The ‘winners’ were semi-sedentary coastal groups with relatively high population densities. Because such groups depended more on fishing and sealing than on hunting and gathering, they could more readily integrate farming into their lifestyle, if only as a secondary subsistence activity. They were also more numerous and likelier to withstand encroachment by farming communities.

References

Bramanti, B., M.G. Thomas, W. Haak, M. Unterlaender, P. Jores, K. Tambets, I. Antanaitis-Jacobs, M.N. Haidle, R. Jankauskas, C.-J. Kind, F. Lueth, T. Terberger, J. Hiller, S. Matsumura, P. Forster, & J. Burger. (2009). Genetic discontinuity between local hunter-gatherers and Central Europe’s first farmers, Science, 326, 137-140

Malmström, H., M.T.P. Gilbert, M.G. Thomas, M. Brandström, J. Storå, P. Molnar, P.K. Andersen, C. Bendixen, G. Holmlund, A. Götherström, & E. Willerslev (2009). Ancient DNA Reveals Lack of Continuity between Neolithic Hunter-Gatherers and Contemporary Scandinavians, Current Biology, doi:10.1016/j.cub.2009.09.017

Face and gender recognition

It is told that an elder came to Scete with his son who was not yet weaned. The boy was raised in the monastery and did not know there were women. When he became a man, the demons represented images of women to him. He was astonished and informed his father. Now one day the two of them went to Egypt and, seeing some women, the young man told his father, “Father, those are the ones who would come and see me at Scete during the night!”

Sayings of the Fathers (Apophthegmata Patrum) 5th century (Regnault, 1966, p.73)

We seem to be born with the ability to recognize the human face. Even infants as young as 1 month old show a consistent, spontaneous preference for face-like stimuli over nonface-like patterns. Such recognition seems guided by an inborn representation of the main facial features, particularly the eyes and the mouth (Pascalis & Kelly, 2008). Brain-damaged subjects provide further evidence of a mental module that specifically processes facial images:

Associative visual agnosia does not always seem to affect the recognition of all types of stimuli equally. The selectivity in some cases of agnosia lends support to the view that there are specialized systems for recognizing particular types of stimuli. The best known example of this is prosopagnosia, the inability to recognize faces after brain damage. Prosopagnosics cannot recognize familiar people by their faces alone, and must rely on other cues for recognition such as a person’s voice, or distinctive clothing or hairstyles. The disorder can be so severe that even close friends and family members will not be recognized. Although many prosopagnosics have some degree of difficulty recognizing objects other than faces, in some cases the deficit appears strikingly selective for faces. (Farah, 1996)

If this mental representation is inborn, does it come in two forms, one for a female face and another for a male face? Or is it gender-neutral? By studying visual adaptation to facial images, Little et al. (2005) concluded that different neural populations process male and female faces. This difference seems to exist at the level of higher-level neurons that code for the entire face, rather than for specific characteristics (Bestelmeyer et al., 2008). These findings were partially replicated by Jaquet (2007), who found evidence for both common and sex-selective neurons.

Ramsey-Rennels and Langlois (2006) reviewed the literature on male and female face recognition by infants:

First, 3- to 4-month-olds have more difficulty discriminating among male faces and subsequently recognizing them than they do female faces (Quinn et al., 2002). Second, older infants are more skilled at categorizing female faces than they are at categorizing male faces: Whereas 10-month-olds easily recognize that a sex-ambiguous female face does not belong with a group of sex-typical female faces, they have more difficulty excluding a sex-ambiguous male face from a group of sex-typical male faces (data interpretation of Younger & Fearing, 1999, by Ramsey et al., 2005). In addition, there is a lag between when infants recognize that female voices are associated with female faces and when male voices are associated with male faces; infants reliably match female faces and voices at 9 months (Poulin-Dubois, Serbin, Kenyon, & Derbyshire, 1994) but do not reliably match male faces and voices until 18 months. Even at 18 months, infants are more accurate at matching female faces and voices than they are at matching male faces and voices (Poulin-Dubois, Serbin, & Derbyshire, 1998).

This evidence could be interpreted in two ways: a) infants better recognize female faces because they have more experience with mothers than with fathers; or b) female face recognition develops earlier than male face recognition because humans have evolved to recognize a female caregiver at an early age. To date, there has been no attempt to replicate the above findings with mother-absent/father-present infants. Quinn et al. (2002) found that such infants show a weak preference for male faces (59%) but there is no indication that they are better at recognizing male faces than female ones.

References

Bestelmeyer, P.E.G., B.C. Jones, L.M. DeBruine, A.C. Little, D.I. Perrett, A. Schneider, L.L.M. Welling, & C.A. Conway. (2008). Sex-contingent face aftereffects depend on perceptual category rather than structural encoding, Cognition, 107, 353-365.

Duchaine, B.C., G. Yovel, E.J. Butterworth, & K. Nakayama. (2006). Prosopagnosia as an impairment to face-specific mechanisms: Elimination of the alternative hypotheses in a developmental case, Cognitive Neuropsychology,

Farah, M.J. (1996). Is face recognition ‘special’? Evidence from neuropsychology, Behavioural Brain Research, 76, 181-189.

Jaquet, E. (2007). Perceptual aftereffects reveal dissociable adaptive coding of faces of different races and sexes, PhD thesis, School of Psychology, University of Western Australia.

Little, A.C., L.M. DeBruine, & B.C. Jones. (2005). Sex-contingent face aftereffects suggest distinct neural populations code male and female faces, Proceedings of the Royal Society of London, Series B, 272, 2283-2287.

Pascalis, O., & D.J. Kelly. (2008). Face processing, in M. Haith & J. Benson (eds.) Encyclopedia of Infant and Early Childhood Development, pp. 471-478, Elsevier.

Quinn, P.C., Yahr, J., Kuhn, A., Slater, A.M., & Pascalis, O. (2002). Representation of the gender of human faces by infants: A preference for female. Perception, 31, 1109–1121.

Ramsey-Rennels, J.L., & J.H. Langlois. (2006). Infants’ differential processing of female and male faces, Current Directions in Psychological Science, 15, 59-62.

Regnault, D.L. (1966). Les sentences des pères du désert. Les Apophtegmes des pères. Sarthe: Abbaye Saint-Pierre de Solesmes.