Sunday, December 19, 2010

Review of 2010

Drinking from the wrong chalice? By his mid-40s, Michael Jackson had skin like parchment.

The end of 2010 is drawing nigh, and the time has come to review my predictions from last year.

Brain growth genes

Back in 2005, it was found that human populations vary considerably at two genes, ASPM and microcephalin, that control the growth of brain tissue. The finding seemed to be ‘huge’ in its implications. Then, it all fizzled out. No correlation could be found between variation at either gene and differences in mental ability or head circumference (Mekel-Bobrov et al., 2007; Rushton et al., 2007).

A recent study has now shown that ASPM and several other genes (MCPH1, CDK5RAP2, CENPJ) do in fact influence growth of brain tissue, specifically cortical tissue.

… In 2010, we’ll probably see further developments in this area. Stay tuned.

This year did see further developments. Interestingly, these gene loci seem to interact with sex and ethnicity in their effects:

[In a Norwegian study by Rimol et al.] for each of the 15 positive SNPs, the association was sex-specific with all significant results for CDK5RAP2 SNPs being found only in males, whilst the significant results for MCPH1 and ASPM were only found in females.

The second study, by Wang et al., only considered variation in the coding sequence of MCPH1 but found that one non-synonymous SNP is associated with male cranial volume but not female cranial volume in a Chinese population of nearly 900 individuals, supporting a role for sex in the action of microcephaly genes. Intriguingly, it also suggests that SNPs in the same locus can have opposite effects in males and females, as for MCPH1 an exonic SNP contributes to Chinese male cranial volume whilst intronic SNPs and SNPs downstream of the coding sequence are associated with Norwegian female brain size. As the authors discuss, these results strongly suggest some microcephaly variants may influence brain development dependent on hormonal background or through interactions with genes which are differentially expressed between the sexes, potentially contributing to sex specific differences in brain structure. (Montgomery & Mundy 2010)

But why did earlier studies find nothing?

First, many of the previous studies only tested for associations with the few, recently derived ASPM and MCPH1 haplotypes which were the focus of claims of recent positive selection, while both Rimol et al. and Wang et al. consider a larger number of SNPs for which there is no a priori evidence for selection. Second, despite the possibility of deriving clear hypotheses of what phenotypes these loci should affect, many previous studies examined traits that are, at best, not directly relevant (e.g. IQ or altruism) or quite distantly removed (e.g. adult head circumference). (Montgomery & Mundy 2010)

Many people had thought that all variation in mental capacity shows up on IQ tests. So they threw in the towel once it became apparent that IQ does not vary with genetic variation at these loci.

So how do these loci affect mental capacity? I’ve argued that the most recent ASPM variant seems to be associated with the spread of alphabetical writing. It may thus assist the visual cortex in recognizing, storing, and processing strings of alphabetical script (Frost 2008).

Alternatively, Dediu and Ladd (2007) have argued that ASPM and microcephalin variants correlate with use or non-use of tone languages. This hypothesis has been tested with Chinese, Korean, Hmong, and American Caucasians who had little training in tone recognition, i.e., they were not musicians and did not engage in singing or instrument playing. The Chinese and Koreans consistently outperformed the other participants when asked to identify the relative distance between two tones. The Hmong showed no such advantage, even though they shared the ASPM and microcephalin characteristics of the Chinese and Koreans (Hove et al., 2010). Thus, while some East Asian populations apparently are better at processing differences in pitch, this aptitude seems to be unrelated to ASPM or microcephalin.

Early modern human genome

Scientists have retrieved mtDNA from a 30,000 year-old hunter-gatherer from Kostenki, Russia. This seems to be part of a trend to study the genome of early modern humans.

The Kostenki mtDNA genome was entirely sequenced, despite problems that seemed intractable (difficulties in distinguishing between early modern human DNA and contamination from present-day human DNA). The authors concluded: “With this approach, it may even become possible to analyze the nuclear genomes of early modern humans” (Krause et al., 2010).

This development is indeed promising. If we can compare early modern DNA with present-day nuclear DNA, we’ll find out the exact genetic changes, especially those in neural wiring, that led to the ‘big bang’ of modern human evolution some 80,000 to 60,000 years ago. Unfortunately, this ‘big bang’ almost certainly took place in Africa, where the climate is much less conducive to DNA preservation.

Ethnic differences in vitamin D metabolism

This year will see further evidence that natural selection has caused differences in metabolism among different human populations, including vitamin D metabolism.

For instance, many populations have long been established at latitudes where vitamin-D synthesis is impossible for most of the year. Some of these populations can get vitamin D from dietary sources (e.g., fatty fish) but most cannot. In these circumstances, natural selection seems to have adjusted their metabolism to reduce their vitamin-D requirements. We know that the Inuit have compensated for lower production of vitamin D by converting more of this vitamin to its most active form (Rejnmark et al., 2004). They also seem to absorb calcium more efficiently, perhaps because of a different vitamin-D receptor genotype (Sellers et al., 2003). Even outside the Arctic zone, there seem to be differences in vitamin-D metabolism from one population to another. In particular, vitamin-D levels seem to be generally lower in darker-skinned populations (Frost, 2009).

… Unfortunately, our norms for adequate vitamin intake are based on subjects or populations of European origin. We are thus diagnosing vitamin-D deficiency in non-European individuals who are, in fact, perfectly normal. This is particularly true for African Americans, nearly half of whom are classified as vitamin-D deficient, even though few show signs of calcium deficiency—which would be a logical outcome. Indeed, this population has less osteoporosis, fewer fractures, and a higher bone mineral density than do Euro-Americans, who generally produce and ingest more vitamin D (Frost, 2009).

… What will be the outcome of raising vitamin-D levels in these populations? Keep in mind that we are really talking about a hormone, not a vitamin. This hormone interacts with the chromosomes and gradually shortens their telomeres if concentrations are either too low or too high. Tuohimaa (2009) argues that optimal levels may lie in the range of 40-60 nmol/L. In non-European populations the range is probably lower. It may also be narrower in those of tropical origin, since their bodies have not adapted to the wide seasonal variation of non-tropical humans.

If this optimal range is continually exceeded, the long-term effects may look like those of aging …

I hope that people of African or Native origin will resist the vitamin-D siren song. Otherwise, many of them will become shriveled-up husks by their mid-40s … just like
Michael Jackson.

Evidence continued to mount this year that vitamin-D metabolism differs by ethnicity. For risk of atherosclerosis, the optimal range is lower among African Americans than among European Americans. A sample of African Americans showed a positive correlation between calcified plaque formation and blood levels of vitamin D (25(OH)D), despite a negative correlation among European Americans over the same range (Freedman et al., 2010).

Another study of African Americans found that blood levels of 25(OH)D decreased linearly with increasing African ancestry, the decrease being 2.5-2.75 nmol/L per 10% increase in African ancestry. Sunlight and diet were 46% less effective in raising these levels among subjects with high African ancestry than among those with low/medium African ancestry (
Signorello et al. 2010).

The New York Times has recently covered the growing unease with vitamin D supplements:

The very high levels of vitamin D that are often recommended by doctors and testing laboratories — and can be achieved only by taking supplements — are unnecessary and could be harmful, an expert committee says.

… The 14-member expert committee was convened by the
Institute of Medicine, an independent nonprofit scientific body, at the request of the United States and Canadian governments. It was asked to examine the available data — nearly 1,000 publications — to determine how much vitamin D and calcium people were getting, how much was needed for optimal health and how much was too much.

… Some labs have started reporting levels of less than 30 nanograms of vitamin D per milliliter of blood as a deficiency. With that as a standard, 80 percent of the population would be deemed deficient of vitamin D, Dr. Rosen said. Most people need to take supplements to reach levels above 30 nanograms per milliliter, he added.

But, the committee concluded, a level of 20 to 30 nanograms [50 to 75 nmol/L] is all that is needed for bone health, and nearly everyone is in that range.

… It is not clear how or why the claims for high vitamin D levels started, medical experts say. First there were two studies, which turned out to be incorrect, that said people needed 30 nanograms of vitamin D per milliliter of blood, the upper end of what the committee says is a normal range. They were followed by articles and claims and books saying much higher levels — 40 to 50 nanograms or even higher — were needed.

After reviewing the data, the committee concluded that the evidence for the benefits of high levels of vitamin D was “inconsistent and/or conflicting and did not demonstrate causality.”

Evidence also suggests that high levels of vitamin D can increase the risks for fractures and the overall death rate and can raise the risk for other diseases. (Kolata 2010)

H/T to Tod


Dediu, D., and D.R. Ladd (2007). Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin. Proceedings of the National Academy of Sciences, 104, 10944-10949.

Freedman B.I., L.E. Wagenknecht, K.G. Hairston KG et al. (2010). Vitamin D, adiposity, and calcified atherosclerotic plaque in African-Americans. Journal of Clinical Endocrinology & Metabolism, 95, 1076-1083.

Frost, P. (2009). Black-White differences in cancer risk and the vitamin-D hypothesis, Journal of the National Medical Association, 101, 1310-1313.

Frost, P. (2008). The spread of alphabetical writing may have favored the latest variant of the ASPM gene, Medical Hypotheses, 70, 17-20.

Hove, M.J., M.E. Sutherland, and C.L. Krumhansl. (2010). Ethnicity effects in relative pitch, Psychonomic Bulletin & Review, 17, 310-316.

Kolata, G. (2010). Report Questions Need for 2 Diet Supplements, The New York Times, November 29, 2010

Krause, J., A.W. Briggs, M. Kircher, T. Maricic, N. Zwyns, A. Derevianko, and S. Pääbo. (2010). A Complete mtDNA genome of an early modern human from Kostenki, Russia, Current Biology 20, 231–236.

Mekel-Bobrov, N., Posthuma D., Gilbert S.L., et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Hum Mole Genet, 16, 600–8.

Montgomery, S.H. and N.I. Mundy. (2010). Brain Evolution : Microcephaly genes weigh in, Current Biology, 20(5), R244

Rejnmark L, Jørgensen ME, Pedersen MB, et al. (2004). Vitamin D insufficiency in Greenlanders on a Westernized fare: ethnic differences in calcitropic hormones between Greenlanders and Danes, Calcif Tissue Int, 74, 255-263.

Rimol, L.M., I. Agartz, S. Djurovic, A.A. Brown, J.C. Roddey, A.K. Kähler, M. Mattingsdal, L. Athanasiu, A.H. Joyner, N.J. Schork, et al. for the Alzheimer’s Disease Neuroimaging Initiative (2010). Sex-dependent association of common variants of microcephaly genes with brain structure. Proceedings of the National Academy of Science. USA, 107, 384–388.

Rushton, J.P., Vernon, PA.., Bons, T.A. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK, 3, 157–60.

Sellers EAC, Sharma A, Rodd C. (2003). Adaptation of Inuit children to a low-calcium diet, CMAJ, 168, 1141-1143.

Signorello, L.B., S.M. Williams, W. Zheng, J.R. Smith, J. Long, Q. Cai, M.K, Hargeaves, B.W. Hollis, and W.J. Blot. (2010). Blood vitamin D levels in relation to genetic estimation of African ancestry, Cancer Epidemiology, Biomarkers & Prevention, 19(9), 2325–2331.

Tuohimaa, P. (2009). Vitamin D and aging, Journal of Steroid Biochemistry and Molecular Biology, 114, 78-84.

Wang, J.K., Li, Y., and Su, B. (2008). A common SNP of MCPH1 is associated with cranial volume variation in Chinese population. Human Molecular Genetics, 17, 1329–1335.

Sunday, December 12, 2010

Gender/face recognition: hue and luminosity

Averaged female face (left) and averaged male face (right). The key facial regions for gender recognition, in terms of either response time or accuracy, seem to be where facial skin borders the lips or the eyes.

The human face is a special visual object. We do not learn to recognize it. Instead, it is processed by the brain via a hardwired mechanism. There seems to be an evolutionary tendency to hardwire recognition of objects that appear often enough while being significant enough to human existence.

One task of this mechanism is to tell a female face from a male face. To this end, we unconsciously focus on several visual cues, including skin tone. It is well established that skin pigmentation visibly differs between men and women. This sexual dimorphism reflects differences in both constitutive pigmentation (untanned skin) and facultative pigmentation (tanning capacity). In comparison to women, men have higher concentrations of melanin and hemoglobin in their skin and lower concentrations of carotene. Men also tan more readily than women do for equal exposure times.

The psychologist Richard Russell argues that this visual cue has two components: (1) the absolute luminosity of facial skin and (2) the contrast between this luminosity and that of the lips and the eyes. Now a study from the Université de Montréal points to a third component: differences in hue, i.e., the degree of ruddiness and yellowness. Hue assists gender recognition particularly where facial skin borders the mouth region. In contrast, luminosity is most helpful where facial skin borders the eye/eyebrow region.

This gender recognition works faster with hue than with luminosity. If the observer is too distant or the lighting too dim, the brain falls back on the “slow channel” of luminosity:

This suggests that humans do use chromatic cues for discriminating face gender: When it’s informative, they use it and respond rapidly (for evidence that color is perceived faster than shape, see Holcombe & Cavanagh, 2001; Moutoussis & Zeki, 1997a, 1997b); when it’s not, they have to rely on the more robust and more sluggish luminance cues.
(Dupuis-Roy et al., 2009)

Interestingly, the authors conclude that this mechanism may be located in the infero-temporal cortex, since this brain region is involved in both face perception and color perception.

H/T to Eugene


Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting. Journal of Vision, 9(2), 10, 1–8., doi:10.1167/9.2.10.

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

Russell, R., B. Duchaine, and K. Nakayama. (2009). Super-recognizers: People with extraordinary face recognition ability. Psychonomic Bulletin & Review, 16(2), 252-257.

Russell, R. and P. Sinha. (2007). Real-world face recognition: The importance of surface reflectance properties, Perception, 36, 1368-1374.

Russell, R., P. Sinha, I. Biederman, and M. Nederhouser. (2006). Is pigmentation important for face recognition? Evidence from contrast negation, Perception, 35, 749-759.

Sunday, December 5, 2010

Sex, ethnicity, and facial skin perception

Postgraduate students, School of Psychology, Cardiff University

A recent study from Cardiff University (Wales) has found interesting sex differences in the way men and women evaluate facial skin color, specifically for faces of white, black, or mixed-race origin. The female participants evaluated White faces the least favorably out of all male facial photos. Conversely, the male participants evaluated Black faces the least favorably out of all female facial photos. Participants of both sexes gave relatively low ratings to White faces for a wide range of characteristics (attractiveness, competence, dominance, warmth, maturity, strength, masculinity).

Previous research has suggested that perceived attractiveness and personality are affected by the race such that White faces are more attractive but less masculine than Black faces. Such studies, however, have been based on very small stimulus sets. The current study investigated perceived attractiveness and personality for 600 Black, White and mixed-race faces. Many of the investigated personality traits were correlated with race when rated by White participants. Attractiveness specifically was greater for Black male faces than White male faces and among mixed-race faces. Blackness correlated with increased attractiveness. A reverse pattern was found for female faces with Whiteness being associated with attractiveness. The results are discussed in terms of the sexual dimorphism demonstrated in skin color. (Lewis 2010)

These findings are partially consistent with previous studies:

Feinman & Gill 1978
When a thousand American students were surveyed on their physical preferences in the opposite sex, 30% of the males versus 10% of the females disliked black skin. Conversely, 56% of the males versus 82% of the females disliked very light skin.

van den Berghe & Frost 1986
According to a cross-cultural survey, lighter skin is more strongly preferred for women than for men in all culture areas.

Frost 1994
Women have varying preferences over the menstrual cycle when choosing between human faces that differ slightly in skin tone. When pairs of male faces are shown, the darker face is more strongly preferred by participants in the first two-thirds of the cycle (high ratio of estrogen to progesterone) than by those in the last third (low estrogen/progesterone ratio). In contrast, when pairs of female faces are shown, skin-tone preference remains unchanged throughout the cycle.

Nonetheless, Lewis (2010) differs from these previous studies on three points:

1). The participants were asked to evaluate major differences in human skin color that clearly have racial/ethnic significations. In contrast, the previous studies examined how men and women evaluate minor differences. Van den Berghe and Frost (1986) found a cross-cultural preference for lighter-skinned women, but only in the sense of their being lighter than average for the local population. Similarly, Frost (1994) only examined female response to minor differences in skin tone.

2). Dark skin was generally preferred. This preference was merely stronger for male faces than for female faces. In contrast, van den Berghe and Frost (1986) found that light skin was generally preferred, with this preference being stronger in response to female faces. Frost (1994) likewise found that light skin was generally preferred, with this preference being weaker with regard to male faces during the first two-thirds of the menstrual cycle. Even then, the lighter male face remained the more popular of the two.

3). There was no control for phase of menstrual cycle. The sex difference in preference would probably have been greater if the author had excluded those female participants who were in the last third of the menstrual cycle.

The first point probably explains the second one. The author examined how men and women respond to major differences in skin color, and such differences have meanings that go far beyond sexual aesthetics. Because the white British participants had to choose among very divergent skin colors, their responses were almost certainly contaminated by ‘prejudice avoidance’, i.e., they avoided giving low ratings to non-white faces for fear of seeming prejudiced. Since anti-white prejudice is not stigmatized, the tendency would be to overcompensate—to err on the safe side.

Overcompensation is suggested by the results. Black faces were given top rating on all 7 items by the female participants and on 4 of the 7 by the male participants. White faces failed to get top rating on any item. This is particularly surprising given that all of the participants were white British. They apparently wished to avoid seeming prejudiced—even to the point of systematically rejecting their own people.

This source of bias does not invalidate the overall finding, i.e., the sex difference in face ratings. All of the participants were presumably immersed in the same ideological environment, and there is no reason to believe that prejudice avoidance is weaker in men than in women.


Feinman, S., & Gill, G.W. (1978). Sex differences in physical attractiveness preferences. Journal of Social Psychology, 105, 43‑52.

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.

Lewis, M.B. (2010). Who is the fairest of them all? Race, attractiveness and skin color sexual dimorphism, Personality and Individual Differences, 50, 159-162.

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.