Sunday, December 9, 2018

Inuit and vitamin D



Inuit mothers (Wikicommons - Ansgar Walk) - Inuit have low levels of vitamin D. Does this mean they're not getting enough? Or have their bodies adapted to an environment where it cannot easily be made in the skin or obtained from the diet?



Inuit people have "insufficient" vitamin D, even among those who eat a traditional diet and live a traditional lifestyle. There are consequently moves afoot to remedy this insufficiency by providing vitamin D supplements. In my opinion, this is a response to a largely nonexistent problem and will probably have adverse consequences.

My arguments are explained in an article I have just published in Inuit Studies. Here is the abstract:


Inuit have vitamin D blood levels that generally fall within the range of insufficiency, even when they live on a traditional diet of fish and game meat. Without this vitamin, bones soften and become deformed, a condition called rickets in children and osteomalacia in adults. Until recent times, however, this condition was much rarer among Inuit than among non-Inuit, even when the latter included people living near Inuit communities under similar conditions of climate and housing. This rarity was attributed to extended breastfeeding and a high-meat/low-cereal diet. The situation subsequently reversed, with Inuit becoming more at risk of developing rickets, first in Labrador during the 1920s and later elsewhere. To reduce this excess risk, researchers have recommended vitamin D supplementation, arguing that breast milk has too little vitamin D and that even a traditional diet cannot provide the recommended daily intake. We should ask, however, whether the problem is definitional. Inuit may have lower levels of vitamin D because they need less, having adapted culturally and physiologically to an environment where this vitamin is less easily synthesized in the skin. These adaptations include a diet that enhances calcium bioavailability (by means of ß-casein in breast milk, certain unknown substances in meat, and absence of phytic acid), as well as genetic changes that enable vitamin D to be used more efficiently. Although Inuit are today more at risk of developing rickets than are non-Inuit, this excess risk is nonetheless small and seems to have a dietary cause-namely, early weaning and abandonment of a high-meat/low-cereal diet.


Please feel free to offer comments or criticisms.


Reference

Frost, P. (2018). To supplement or not to supplement: are Inuit getting enough vitamin D? Études Inuit Studies 40(2): 271-291.

Sunday, December 2, 2018

More unintended consequences




Fond memories, by Raimundo de Madrazo y Garreta (1841-1920). The hormonal state of pregnancy causes women to have a lower capacity for multitasking and remembering future activities. What happens when oral contraceptives maintain this hormonal state for years and years?



In my last post I reviewed the literature on oral contraceptives and behavior. Women invest more in sexual attractiveness near the time of ovulation, putting on more makeup and sending out other visual, behavioral, and olfactory cues. Oral contraceptives seem to suppress this desire to be attractive.

Parallel to these attitudinal and behavioral changes over the menstrual cycle, we also find cyclical changes to certain brain regions:

[...] a large sample of 55 women was scanned three times along their menstrual cycle in concisely defined time windows of hormonal changes. Accordingly this is the first study using a large enough sample size to assess menstrual cycle dependent changes in human brain structure with sufficient power. Results confirm a significant estradiol-dependent pre-ovulatory increase in gray matter volumes of the bilateral hippocampus, but also show a significant, progesterone-dependent increase in gray matter volumes of the right basal ganglia after ovulation. No other areas were affect by hormonal changes along the menstrual cycle. These hormone driven menstrual cycle changes in human brain structure are small, but may be the underlying cause of menstrual cycle dependent changes in cognition and emotion. (Pletzer et al. 2018).

The same research team earlier reported differences in brain structure between oral contraceptive (OC) users and non-users. OC users were closer to men in their brain structure:

Men had larger hippocampi, parahippocampal and fusiform gyri, amygdalae and basal ganglia than women. Women showed larger gray matter volumes in the prefrontal cortex, pre- and postcentral gyri. These sex-dependent effects were modulated by menstrual cycle phases and hormonal contraceptives. We found larger volumes in the right fusiform/parahippocampal gyrus during early follicular compared to mid-luteal cycle phase. Women using hormonal contraceptives showed significantly larger prefrontal cortices, pre- and postcentral gyri, parahippocampal and fusiform gyri and temporal regions, compared to women not using contraceptives. (Pletzer et al. 2010).

This study was criticized because it made no distinction between progestin-only OCs and combined progestin/estradiol OCs. The results were quite different when another research team repeated this study with participants who used only the second type of pill. OC users now had less, not more, brain volume, particularly in certain regions of the cerebral cortex: 

In 90 women, (44 OC users, 46 naturally-cycling women), we compared the cortical thickness of brain regions that participate in the salience network and the default mode network, as well as the volume of subcortical regions in these networks. We found that OC use was associated with significantly lower cortical thickness measurements in the lateral orbitofrontal cortex and the posterior cingulate cortex. These regions are believed to be important for responding to rewards and evaluating internal states/incoming stimuli, respectively. (Petersen et al. 2015 - h/t to Wanda!)

These differing results may reflect the different types of OCs in use. Because progestin, like progesterone, has anti-estrogenic effects, long-term use would tend to masculinize a woman's brain; there is consequently more gray matter in the parahippocampal and fusiform gyri, which are likewise bigger in men than in women. In contrast, when women prevent conception by taking a mix of progestin and estradiol, which more closely mimics the hormonal state of pregnancy, certain regions of their cerebral cortex will tend to atrophy.

Momnesia?

This atrophy may have an evolutionary cause. Keep in mind that a pregnant woman has to cope with a different pattern of cognitive demands: 

Pregnant women often have difficulty with multi-tasking and remembering future activities; however, they show improvement in memory for faces and recognition of emotional changes, particularly in men. They tend to have an increased sensitivity to odors, many of which are perceived as unpleasant. Perceptions of taste alter throughout pregnancy, with cravings for sweet foods in the second trimester and for salt in the third; sour tends to be preferred throughout the pregnancy. (Stadtlander 2013)

In general, the overall cognitive load is lower during pregnancy, so it makes sense that a pregnant woman’s body would allocate more resources to her developing child and fewer to her brain. The brain is, after all, the costliest organ of the human body, and it can probably cope with being a lower priority over the short term. Problems develop only if the hormonal state of pregnancy is artificially maintained for years and years.


References

Petersen, N., A. Touroutoglou, J.M. Andreano, and L. Cahill. (2015).Oral contraceptive pill use is associated with localized decreases in cortical thickness. Human Brain Mapping 36(7): 2644-2654. 

Pletzer, B., T. Harris, and E. Hidalgo-Lopez. (2018). Subcortical structural changes along the menstrual cycle: beyond the hippocampus. Scientific Reports 8: 16042 https://dx.doi.org/10.1038%2Fs41598-018-34247-4

Pletzer, B., M. Kronbichler, M. Aichhorn, J. Bergmann, G. Ladurner, and H.H. Kerschbaum. (2010). Menstrual cycle and hormonal contraceptive use modulate human brain structure. Brain Research 1348: 55-62.

Stadtlander, L. (2013).  Memory and perceptual changes during pregnancy. International Journal of Childbirth Education 28(2): 49-53.




Sunday, November 25, 2018

Unintended consequences


Oral contraceptives suppress not only ovulation but also concurrent behavioral and attitudinal changes, including the desire to look more attractive. Has "the pill" changed our culture?



The oral contraceptive pill prevents pregnancy by suppressing ovulation—in short, by fooling a woman's body into thinking it is already pregnant. In addition to medical side effects, "the pill" also seems to modify behavior and attitude, often subtly. Whereas women are more likely to initiate sexual behavior at ovulation, this effect is suppressed in women on the pill (Adams et al. 1978). The latter also prefer men whose faces are less masculine (Little et al. 2002) and lighter-skinned (Frost 1994).

Among non-pill-users, ovulation coincides with greater use of cosmetics and increased time spent putting on cosmetics (Guéguen 2012). This behavioral effect seems to be likewise suppressed by pill use:

We photographed a sample of women (N = 36) who self-reported whether or not they use the contraceptive pill, as well as their cosmetic habits. A separate sample of participants (N = 143) rated how much makeup these target women appeared to be wearing. We found that women not using the contraceptive pill (i.e., naturally cycling women) reported spending more time applying cosmetics for an outing than did women who use the contraceptive pill. We also found that the faces of these naturally cycling women were rated as wearing more cosmetics than the faces of the women using the contraceptive pill. (Batres et al. 2018)

These findings are consistent with the results of a study on professional lap dancers. The participants made $335 in tips per 5 hour shift during ovulation, but only $260 per shift during the luteal phase and $185 per shift during menstruation. Lap dancers on contraceptive pills showed no change in earnings over the menstrual cycle (Miller et al. 2007). It's unclear what sort of visual or behavioral cues were suppressed by pill use:

[...] our study did not identify the precise proximal mechanisms that influence tip earnings. These might include the previously documented shifts in body scent, facial attractiveness, soft-tissue body symmetry, waist-to-hip ratio, and verbal creativity and fluency—or they might include shifts in other phenotypic cues that have not yet been studied. We can, however, exclude some possible mediators based on previous exotic dancer research. Tip earnings are unlikely to be influenced by cycle shifts in stage-dance moves, clothing, or initial conversational content because these cues just do not vary much for professional dancers (Miller et al. 2007)

Do women naturally have a more attractive physical appearance at ovulation?


Discussion

The past half-century has seen a trend toward androgyny among women. This trend is usually attributed to feminism and pop culture, but perhaps the latter have in turn been influenced by something else.

Today, "the pill" is used by approximately 100 million women worldwide, particularly in developed countries. Has this widespread use played a role in changing our cultural and ideological environment? 

I'm not suggesting that women look less feminine today simply because more of them are on the pill and thus are hormonally altered. Rather, if a larger proportion of women are in that kind of hormonal state, it will be easier for anti-feminine fashions and ideologies to achieve a critical mass and take off among all women, including those not on the pill.


References

Adams, D.B., A.R. Gold, and A.D. Burt. (1978). Rise in female-initiated sexual activity at ovulation and its suppression by oral contraceptives. New England Journal of Medicine 299: 1145-1150.

Batres, C., A. Porcheron, G. Kaminski, S. Courrèges, F. Morizot, and R. Russell. (2018). Evidence that the hormonal contraceptive pill is associated with cosmetic habits? Frontiers in Psychology 9: 1459
https://doi.org/10.3389/fpsyg.2018.01459

Frost, P. (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(1): 507-14.
https://doi.org/10.2466/pms.1994.79.1.507.

Guéguen, N. (2012). Makeup and menstrual cycle: near ovulation, women use more cosmetics. The Psychological Record 62: 541-548. 

Little, A.C., B.C. Jones, I.S. Penton-Voak, D.M. Burt, and D.I. Perrett. (2002). Partnership status and the temporal context of relationships influence human female preferences for sexual dimorphism in male face shape. Proceedings of the Royal Society B. 269: 1095-1100. 

Miller, G., J.M. Tybur, and B.D. Jordan. (2007). Ovulatory cycle effects on tip earnings by lap dancers: economic evidence for human estrus? Evolution and Human Behavior 28: 375-381. https://doi.org/10.1016/j.evolhumbehav.2007.06.002

Sunday, November 18, 2018

Looking through a lens



The Babylonian goddess Ishtar (Louvre). The cult of feminine whiteness reached its height in a zone stretching from the Mediterranean through the Middle East and into South and East Asia



Skin color differentiates between the sexes at puberty, with complexions becoming paler in girls and ruddier and browner in boys. The cause seems to be hormonal. This is most convincingly shown by recent studies on digit ratio in men and women before and after puberty and by earlier studies on normal, castrated, and ovariectomized subjects (Edwards and Duntley 1939; Edwards and Duntley 1949; Edwards et al. 1941; Manning et al. 2004; Sitek et al. 2018).

How noticeable is this sex difference? If we take the best controlled studies, where this sexual differentiation is measured at the upper inner arm, we find that boys and girls differ after puberty by about one and a half percentage points of skin reflectance (Kalla 1983; Mesa 1983). By comparison, northern Europeans and West Africans differ by 25 to 30 percentage points (Robins 1991, Tables 7.1, 7.2).

Keep in mind that these measurements are from the upper inner arm and that measurements from other body sites show a much larger sex difference. On the buttocks and the breasts, skin reflectance differs by 6 to 15 percentage points between men and women (Edwards et al. 1939; Garn et al. 1956). The literature has always ascribed this finding to differences in dress and, hence, to differences in sun exposure, yet female skin is lighter at these sites not only because it has less melanin but also because it has less blood in its outer layers, a fact hard to reconcile with a simple tanning effect. 

There is reason to believe that women are lighter-skinned where their subcutaneous fat layer is thicker, possibly because body fat contains an enzyme (aromatase) that converts an androgen (androstenedione) into an estrogen (estrone), thus feminizing the skin (Siiteri and MacDonald 1973). Indeed, lightness of skin color correlates with thickness of subcutaneous fat in adult women (Mazess 1967).


Visual processing of this sex difference in the human mind

Whatever its actual magnitude, we seem innately predisposed to notice this small difference in skin color, particularly for face recognition. Research is ongoing, but there is a growing consensus that "color is not merely an accessory of faces, but is rather a complex and crucial feature in facial processing. [...] Further, recent work has revealed consistent patterns of connected face and color selective cortical areas, possibly reflecting a shared overlap of visual processing between faces and color" (Thorstenson 2018).

Faces are recognized by means of many neurons, some of which specialize in recognizing male faces, others in recognizing female faces, and others in recognizing both indifferently (Baudouin and Brochard 2011; Bestelmeyer et al. 2008; Jacquet and Rhodes 2008; Little et al. 2005). To recognize gender, one of the key visual criteria is skin color (Bruce and Langton 1994; Hill, Bruce, and Akamatsu 1995; Russell and Sinha 2007; Russell et al. 2006; Tarr et al. 2001; Tarr, Rossion, and Doerschner 2002). In particular, these neurons use two aspects of facial color: hue (degree of brownness and ruddiness) and luminosity (degree of contrast between lightness of facial skin versus darkness of lip/eye area). Hue is the fast channel for gender identification. If the face is too far away or the lighting too dim, this mental mechanism will switch to the slower but more accurate channel of luminosity (Dupuis-Roy et al. 2009; Nestor and Tarr 2008a; Nestor and Tarr 2008b; Tarr et al. 2001; Tarr, Rossion, and Doerschner 2002). 

It has been shown that an observer can identify the gender of a face even if the image is blurred and differs only in color (Tarr et al. 2001). Indeed, facial color seems especially crucial if face shape is not clearly visible (Yip and Sinha 2002).

Even when not observing a human face we unconsciously associate darkness with men and lightness with women. This was shown in a series of experiments with Dutch, Portuguese, and Turkish participants. In the first one, personal names were gender-recognized faster when male names were presented in black and female names in white than when the combinations were reversed. In the second experiment, very briefly appearing black and white blobs had to be classified by gender; the former were classified predominantly as male and the latter as female. Finally, in an eye-tracking experiment, observation was longer and fixation more frequent when a black or dark object was associated with a male character and a white or light object with a female character (Semin et al. 2018). Similar results come from a word-association test with Navajo participants: the color black was perceived as more potent and masculine and the color white as more active and feminine (Osgood 1960, p. 165).

We therefore perceive skin color, and especially facial color, through the lens of a mental mechanism that initially arose for gender recognition. This may explain not only why lighter skin is subconsciously perceived as feminine but also why women have sought to accentuate this relative pallor in a wide range of cultures and time periods, most often by avoiding the sun and wearing protective clothing (Frost 2010, pp. 120-123). To the same end, and often independently in different geographic regions, powders have been created from white clay, lime, chalk, or gypsum for the purpose of lightening women’s facial color or increasing its contrast with lip/eye color (Russell 2003; Russell 2009; Russell 2010).

This cult of feminine whiteness reached its height in a zone stretching from the Mediterranean through the Middle East and into South and East Asia. Here, the sex difference in skin color could develop to its fullest, without being constrained by the ceiling of very light pigmentation or the floor of very dark pigmentation. Here too were invented the first cosmetics for women, including powders to lighten skin artificially, and accessories to keep skin untanned (parasols, long gloves, conical or wide-brimmed hats). Finally, here too were created the first works of prose, poetry, and visual art, often on the theme of female beauty, including feminine whiteness. The collective imagination thus became populated with women much fairer than their real-life counterparts.


The advent of the tanned look and the end of feminine whiteness

This cult of feminine whiteness came to an end in the Western world when women embraced the tanned look in the early 20th century. This look began as a side effect of heliotherapy, i.e., the use of sun baths and sun lamps to treat rickets, tuberculosis of the skin, and other cutaneous diseases. By 1929 it had become a fashion, to the surprise of observers like this New York Times journalist: "Idealists would like to believe that the people, investigating the medical doctrine and accepting it as sober fact, went deliberately forth to get what was good for them, and took to sun baths with an avidity they had never shown for spinach, sleep, or orthopedic shoes" (Segrave 2005, p. 35). The new fad had become especially popular with women and, as such, entered into the boyish look of the 1920s: bobbed hair, large shoulders, small bust, narrow hips, and long legs. The intent was to evoke the image of a boy on the brink of puberty, as shown by the French name of this fashion trend: la garçonne.

While the first nudist movements were organizing, la garçonne freely exhibited her athletic body and enjoyed the benefits of heliotherapy. The 1920s made tanning fashionable. Translucent skin and pale complexions were relegated to the theatre prop room of fin de siècle romanticism. Thus disappeared an element of gender differentiation. The contrast of flesh colors, brown and copper for the man, white, pink, and ivory for the woman, had been a constant since Ancient Egypt in literary and pictorial representations. (Bard 1998, p. 41)

The tanned look tapped into an erotic response that had previously been marginalized and stigmatized. In Victorian era novels, the "dark lady" appears as an "impetuous," "ardent," and "passionate" object of short-lived romances (Carpenter 1936, p. 254). Similarly, in French novels of the same era "[t]he love incarnated by brown women appears as the conceptual equivalent of a devouring femininity, thus making them similar to the mythical figure of Lilith" (Atzenhoffer 2011, p. 6). This motif goes back at least to the Middle Ages in various European cultures and points to an alternate mode of eroticism:

[...] dark girls [...] are inevitably imagined as sexually more available than their fairer sisters, with whom they are implicitly or explicitly contrasted. In addition, the change of a girl's complexion, such as being burned by the sun, is to be understood as symbolic of her having crossed a sexual threshold without the benefit of marriage. (Vasvari 1999)

European folklore has sayings along the lines of "the darker the berry, the sweeter the juice," such as the following from a Venetian folk-poem:

"My lady mother always told me that I should never be enamoured of white roses," says a sententious young man; "she told me that I should love the little mulberries, which are sweeter than honey." "Cara mora," mora, or mulberry, meaning brunette, is an ordinary caressing term. (Martinengo-Cesaresco 1886, p. 95)

This is consistent with the idea that women evolved lighter skin not as a means to stimulate male sexual arousal but rather as a means to modify such arousal by reducing aggressiveness in the male observer and inducing feelings of care. Women are thus lighter-skinned for the same reason that they have less body hair, a higher pitch of voice, and a more babyish face. These are all key aspects of Konrad Lorenz's Kindchenschema (Lorenz 1971, pp. 154-164). Another ethologist, Richard Russell, was the first to use this concept to explain women's lighter skin:

I believe the sexual differences in skin color resulted from female whiteness being selected for because it is opposite the threat coloration, although the selection pressures may have been rather mild. Light skin seems to be more paedomorphic, since individuals of all races tend to darken with age. Even in the gorilla, the most heavily pigmented of the hominoids, the young are born with very little pigment. [...] Thus, a lighter colored individual may present a less threatening, more juvenile image. (Guthrie 1970)

This explanation is supported by a two-part study where each male participant was first shown pictures of women and asked to rate their attractiveness. Women with lighter skin were not rated more attractive than those with darker skin. In the second part, eye movements were tracked, and it was found that women with lighter skin were viewed for a longer time. The longer duration could indicate a slower rise and fall in sexual arousal (Garza et al. 2016). Women's lighter skin may thus lengthen and pacify male sexual arousal via a Kindchenschema effect.


Conclusion

Today, skin color is seen as a mark of ethnic identity, yet this is not the sole meaning it has had for humans. For most of history and prehistory it was seen as a mark of male or female identity.

This older meaning has received much less interest, even in academia. It is perhaps no coincidence that scholarly interest has come disproportionately from non-Westerners. In contrast, Western academics, and Americans in particular, generally view the psychological meaning of skin color as a legacy of slavery.


References

Atzenhoffer, R. (2011). Les hommes préfèrent les blondes. Les lectrices aussi. Effet de psychologie, horizons idéologiques et valeurs morales des héroïnes dans l'œuvre romanesque de H. Courths-Mahler. Colloque national (CNRIUT), Villeneuve d'Ascq, 8-10 juin 2009, 

Bard, C. (1998). Les garçonnes. Modes et fantasmes des Années folles. Paris: Flammarion.

Baudouin, J.-Y., and R. Brochard. (2011). Gender-based prototype formation in face recognition. Journal of Experimental Psychology: Learning, Memory, and Cognition 37(4): 888-898. 

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(1): 353-365.

Bruce, V., and S. Langton. (1994). The use of pigmentation and shading information in recognising the sex and identities of faces. Perception 23(7): 803-822.

Carpenter, F. I. (1936). Puritans preferred blondes. The heroines of Melville and Hawthorne. New England Quarterly 9(2): 253-272.

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.

Edwards, E.A., and S.Q. Duntley. (1939). The pigments and color of living human skin. American Journal of Anatomy 65(1): 1-33.

Edwards, E.A., and S.Q. Duntley. (1949). Cutaneous vascular changes in women in reference to the menstrual cycle and ovariectomy. American Journal of Obstetrics & Gynecology 57(3): 501-509.

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men. Endocrinology 28(1): 119-128. https://doi.org/10.1210/endo-28-1-119  

Frost, P. (2010). Femmes claires, hommes foncés. Les racines oubliées du colorisme. Quebec City: Les Presses de l'Université Laval, 202 p.

Garn, S.M., S. Selby, and M.R. Crawford. (1956). Skin reflectance studies in children and adults. American Journal of Physical Anthropology 14:101-117.

Garza, R., R.R. Heredia, and A.B. Cieslicka. (2016). Male and female perception of physical attractiveness. An eye movement study. Evolutionary Psychology 14(1): 1-16. https://doi.org/10.1177/1474704916631614 .

Guthrie, R.D. (1970). Evolution of human threat display organs. In T. Dobzhansky, M.K. Hecht, and W.C. Steere (Eds.) Evolutionary Biology 4: 257-302. New York: Appleton-Century Crofts.

Jaquet, E., and G. Rhodes. (2008). Face aftereffects indicate dissociable, but not distinct, coding of male and female faces. Journal of Experimental Psychology: Human Perception and Performance 34(1): 101-112. 

Kalla, A.K. (1973). Ageing and sex differences in human skin pigmentation. Zeitschrift für Morphologie und Anthropologie 65(1): 29-33.

Hill, H., V. Bruce, and S. Akamatsu. (1995). Perceiving the sex and race of faces: The role of shape and colour. Proceedings of the Royal Society B: Biological Sciences 261(1362): 367-373.

Little, A.C., L.M. DeBruine, and B.C. Jones. (2005). Sex-contingent face after-effects suggest distinct neural populations code male and female faces. Proceedings of the Royal Society of London, Series B 272(1578): 2283-2287.

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

Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25: 38-50.

Martinengo-Cesaresco, E. (1886). Essays in the Study of Folk-Songs. London: J.M. Dent & Sons.

Mazess, R.B. (1967). Skin color in Bahamian Negroes. Human Biology 39:145-154.

Mesa, M.S. (1983). Analyse de la variabilité de la pigmentation de la peau durant la croissance. Bulletin et mémoires de la Société d'Anthropologie de Paris, t. 10 série 13: 49-60.

Nestor, A., and M.J. Tarr. (2008a). The segmental structure of faces and its use in gender recognition. Journal of Vision 8(7): 7, 1-12, http://journalofvision.org/8/7/7/, https://doi.org/10.1167/8.7.7.

Nestor, A., and M.J. Tarr. (2008b). Gender recognition of human faces using color. Psychological Science 19(12): 1242-1246.

Osgood, C.E. (1960). The cross-cultural generality of visual-verbal synesthetic tendencies. Behavioral Science 5(2): 146-169.

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features. Perception 32(9): 1093-1107.

Russell, R. (2009). A sex difference in facial pigmentation and its exaggeration by cosmetics. Perception 38(8): 1211-1219.

Russell, R. (2010). Why cosmetics work. In R. Adams, N. Ambady, K. Nakayama, and S. Shimojo (Eds.) The science of social vision (pp. 186-203). New York: Oxford.

Russell, R. and P. Sinha. (2007). Real-world face recognition: The importance of surface reflectance properties. Perception 36(9): 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.

Segrave, K. (2005). Suntanning in 20th Century America. Jefferson (North Carolina): McFarland & Company.

Semin, G.R., T. Palma, C. Acaturk, and A. Dziuba. (2018). Gender is not simply a matter of black and white, or is it? Philosophical Transactions of The Royal Society B Biological Sciences 373(1752):20170126

Siiteri, P.K., and P.C. MacDonald. (1973).Role of extraglandular estrogen in human endocrinology, in S.R. Geiger (ed.). Handbook of Physiology, vol. 2, part 1. Washington, D.C.: American Physiology Society, Section 7, pp. 615-629.

Sitek, A., S. Koziel, A. Kasielska-Trojan, and B. Antoszewski. (2018). Do skin and hair pigmentation in prepubertal and early pubertal stages correlate with 2D:4D? American Journal of Human Biology, early view 

Tarr, M.J., D. Kersten, Y. Cheng, and B. Rossion. (2001). It's Pat! Sexing faces using only red and green. Journal of Vision 1(3): 337, 337a.

Tarr, M. J., B. Rossion, and K. Doerschner. (2002). Men are from Mars, women are from Venus: Behavioral and neural correlates of face sexing using color. Journal of Vision 2(7): 598, 598a,

Thorstenson, C.A. (2018). The social psychophysics of human face color: Review and recommendations. Social Cognition 36(2): 247-273 

Vasvari, L.O. (1999). A comparative approach to European folk poetry and the erotic wedding motif. CLCWeb. Comparative Literature and Culture 1(4)

Yip, A.W., and P. Sinha. (2002). Contribution of color to face recognition. Perception 31(8): 995-1003.

Sunday, November 11, 2018

Puberty and skin color



Skin color differentiates between boys and girls after puberty. Before puberty, girls are actually darker-skinned than boys (Kalla 1973; Mesa 1983)



Complexions differ between the sexes: women are paler and men ruddier and browner. Today, this sex difference seems hardly noticeable in Western societies, having been overwhelmed by much larger differences of race and ethnicity and further obscured since the 1920s by the tanning fad (Segrave 2005). Nonetheless, it was noticed in earlier times. A lighter hue was traditionally given to female figures and a darker hue to male figures in the visual arts of all early civilizations, including those of Italy, Greece, Egypt, China, Japan, and Mesoamerica (Capart 1905, pp. 26-27; Eaverly 1999; Pallottino 1952, pp. 34, 45, 73, 76-77, 87, 93, 95, 105, 107, 115; Siepe 2004; Soustelle 1970, p. 130; Tegner 1992; Wagatsuma 1967). This sex difference also appears in ancient Greek poetry, where women are described as “white” and men as “black” (Irwin 1974, pp. 121, 129-155). “White” skin is still key to female identity in many non-Western societies, as shown by interviews with Japanese men: "Whiteness is a symbol of women, distinguishing them from men." "One's mother-image is white" (Wagatsuma 1967, pp. 417-418).



Spectrophotometric studies

With the advent of the spectrophotometer, researchers could study skin color by measuring the percentage of light reflected by the skin, most often at the upper inner arm—where tanning is minimal. An American team thus attributed the differing complexions of men and women to differing concentrations of the three main skin pigments: melanin, hemoglobin, and carotene (Edwards and Duntley 1939). The same team showed that this sex difference was reduced by ovariectomy and even more so by castration (Edwards and Duntley 1949; Edwards et al. 1941). Later research identified puberty as the time when boys and girls diverge in skin color (van den Berghe and Frost 1986). The best controlled studies are those by Kalla (1973) and Kalla and Tiwari (1970) on South Asians and Tibetans and by Mesa (1983) on Spanish participants. The samples are large enough to measure this sexual differentiation by year and by sex. In addition to showing that girls become progressively lighter-skinned than boys during adolescence, these studies also show that girls are actually darker-skinned than boys just before puberty.

These sex and age differences thus seem to be innately programmed, specifically via the sex hormones. This hypothesis is further supported by a study of skin color in monozygotic and dizygotic twins from three age groups: 12 year olds; 13 to 15 year olds; and 16 to 18 year olds. Variance within the twin pairs differed significantly on average between monozygotic and dizygotic twins, thus “indicating a strong genetic component in the variability of skin lightness.” As in other studies, puberty had a stronger effect on female skin color than on male skin color, with girls becoming progressively lighter-skinned. For both sexes, mean within-pair variance did not differ significantly from one age group to the next, further indicating that these age changes are under genetic control (Omoto 1965).

Nonetheless, most people, including academics, have continued to ascribe the differing complexions of men and women to differences in lifestyle (see for example Eaverly 1999 and Irwin 1974). Perhaps girls become lighter-skinned after puberty because they are less free to go outside unaccompanied, as used to be the case in many cultures. The sex difference in skin color should therefore disappear as women come to resemble men in terms of lifestyle. This alternative hypothesis is supported by a recent study of young adults from Ireland, Poland, Italy, and Portugal, which found women to be darker-skinned than men on the upper inner arm, the body site most often used to measure the color of untanned skin (Candille et al. 2012). This finding contradicts findings from earlier studies on Europeans or European-descended participants (van den Berghe and Frost 1986).



Digit ratio studies (2D:4D)

Recently, the innate causation hypothesis has received support from two "digit ratio" studies. This ratio is index finger length divided by ring finger length, and it corresponds to the ratio of androgens to estrogens in the fluids of the developing fetus. A higher ratio indicates more feminization, and a lower ratio more masculinization.

A British team led by John Manning (2004) examined adults of both sexes. Lightness of skin color was found to correlate in women but not in men with digit ratio, i.e., women are lighter-skinned if their body tissues have been exposed to higher estrogen levels. This finding was true for both the left hand and the right hand, although the correlation was stronger for the left hand.

A Polish team led by Aneta Sitek (2018) looked at children just before puberty, when girls are actually darker-skinned than boys. Darkness of skin color was found to correlate in girls but not in boys with digit ratio, i.e., pre-pubertal girls are darker-skinned if their body tissues have been exposed to higher estrogen levels. This finding was true only for the right hand.

For reasons still unclear, the digit ratio of the right hand is more responsive to the sex hormones than the digit ratio of the left hand, as shown by a greater sex difference in digit ratio for the right hand than for the left (Honekopp and Watson 2010). In reviewing the literature, Honekopp and Watson (2010) argue that right-hand digit ratio is a better indicator of prenatal exposure to the sex hormones. Left-hand digit ratio seems to be more affected by hormonal exposure later in life. This is suggested by the findings of a longitudinal study: digit ratio increases in children with age, and this effect is greater for the left hand than for the right hand (Trivers et al. 2006).



Discussion

Digit ratio studies point to a hormonal cause, and not to differences in lifestyle, as the reason why skin color differentiates between boys and girls at puberty. This is consistent with earlier spectrophotometric studies on normal, castrated, and ovariectomized individuals (Edwards and Duntley 1939; Edwards and Duntley 1949; Edwards et al. 1941). Furthermore, most spectrophotometric studies have shown that women are lighter-skinned than men even at the upper inner arm—a body site normally unaffected by tanning (van den Berghe and Frost 1986).

But why were women darker-skinned than men at this body site in a recent study of young adults from Ireland, Poland, Italy, and Portugal? (Candille et al. 2012). One can only conclude that the upper inner arm is no longer a reliable site for measuring the color of untanned skin. Perhaps young Western women now make a point of tanning their underarms because they increasingly shave this part of their body and expose it to view.


References

Candille, S.I., D.M. Absher, S. Beleza, M. Bauchet, B. McEvoy, N.A. Garrison, et al. (2012). Genome-wide association studies of quantitatively measured skin, hair, and eye pigmentation in four European populations. PLoS One 7(10): e48294.

Capart, J. (1905). Primitive Art in Egypt. London: H. Grevel.

Eaverly, M.A. (1999). Color and gender in ancient painting: A pan-Mediterranean approach. In N.L. Wicker and B. Arnold (Eds). From the Ground Up: Beyond Gender Theory in Archaeology. Proceedings of the Fifth Gender and Archaeology Conference, University of Wisconsin-Milwaukee, (pp. 5-10). Oxford (England): British Archaeological Reports.

Edwards, E.A., and S.Q. Duntley. (1939). The pigments and color of living human skin. American Journal of Anatomy 65(1): 1-33.

Edwards, E.A., and S.Q. Duntley. (1949). Cutaneous vascular changes in women in reference to the menstrual cycle and ovariectomy. American Journal of Obstetrics & Gynecology 57(3): 501-509.

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men. Endocrinology 28(1): 119-128. https://doi.org/10.1210/endo-28-1-119

Honekopp J, S. Watson, (2010). Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand. American Journal of Human Biology 22(5): 619-30. https://doi.org/10.1002/ajhb.21054

Irwin, E. (1974). Colour Terms in Greek Poetry. Toronto: Hakkert.

Kalla, A.K. (1973). Ageing and sex differences in human skin pigmentation. Zeitschrift für Morphologie und Anthropologie 65(1): 29-33.

Kalla, A. K. and S.C. Tiwari. (1970). Sex differences in skin colour in man. Acta Geneticae Medicae et Gemellologiae 19(3): 472-476.

Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25: 38-50.

Mesa, M.S. (1983). Analyse de la variabilité de la pigmentation de la peau durant la croissance. Bulletin et mémoires de la Société d'Anthropologie de Paris, t. 10 série 13: 49-60.

Omoto, K. (1965). Measurements of skin reflectance in a Japanese twin sample. Journal of the Anthropological Society of Nippon (Jinruigaku Zassi) 73(4): 115-122.

Pallottino, M. (1952). Etruscan Painting. Lausanne: Skira.

Segrave, K. (2005). Suntanning in 20th Century America. Jefferson (North Carolina): McFarland & Company.

Siepe, F. (2004). Farben des Eros. Marginalien zur Kulturgeschichte der Liebes- und Schönheitswahrnehmung in Antike und christlichem Abendland. Marburg: Kline.

Sitek, A., S. Koziel, A. Kasielska-Trojan, and B. Antoszewski. (2018). Do skin and hair pigmentation in prepubertal and early pubertal stages correlate with 2D:4D? American Journal of Human Biology, early view 

Soustelle, J. (1970). The Daily Life of the Aztecs. Stanford, California: Stanford University Press.

Tegner, E. (1992). Sex differences in skin pigmentation illustrated in art. The American Journal of Dermatopathology 14(3): 283-87. 

Trivers, R., J. Manning, and A. Jacobson. (2006). A longitudinal study of digit ratio (2D:4D) and other finger ratios in Jamaican children. Hormones and Behavior 49(2): 150-156. https://doi.org/10.1016/j.yhbeh.2005.05.023

van den Berghe, P.L., and P. Frost. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies 9(1): 87-113. https://doi.org/10.1080/01419870.1986.9993516  

Wagatsuma, H. (1967). The social perception of skin color in Japan. Daedalus 96(2): 407-443.

Sunday, November 4, 2018

Getting noticed



"A rapid and effective means for getting noticed in the crowd." Lady Gaga(?) in Stylist France.



I'm back to blogging after a 3-month absence. During my hiatus the magazine Stylist France interviewed me about head hair as a form of advertising. The resulting article appeared on October 11 under the headline "Forget the slogan T-shirt. To get yourself heard, nothing is more effective than a new hairstyle."

The full interview, in French with an English translation, is provided below:

French version:

Pouvez-vous expliquer succinctement le rôle de la sélection sexuelle dans l'apparition des cheveux blonds ?

La sélection sexuelle favorise la brillance et la nouveauté. Ce qui est brillant demeure plus longtemps en mémoire; ce qui est nouveau retient plus longtemps l'attention. Si on considère les couleurs des cheveux et des yeux, on constate une évolution vers la brillance, c'est-à-dire les cheveux noirs et les yeux bruns cèdent leur place à des couleurs vives, comme les cheveux roux ou blonds et les yeux verts ou bleus.

Quant à l'évolution vers la nouveauté, celle-ci se manifeste par la diversification de la palette des cheveux et des yeux. Au début, une nouvelle couleur émerge par la mutation, puis elle se répand jusqu'à ce qu'elle perde sa nouveauté ; à ce moment-là, la pression de la sélection sexuelle se réoriente pour favoriser une couleur moins fréquente. Ainsi, un équilibre s'établit entre les diverses couleurs.

Qu'est-ce qui permet d'affirmer que la sélection sexuelle est aussi importante voire plus importante que les rayons UV dans l'apparition des cheveux blonds ?

D'abord, les gènes contrôlant la couleur de la peau et celle des cheveux ne sont pas les mêmes. On peut avoir la peau très blanche, tout en possédant les cheveux foncés. De plus, la pression de sélection exercée par les rayons UV n'explique pas la diversification des allèles contrôlant la couleur des cheveux et des yeux. Enfin, on ne voit pas cette diversification chez les peoples indigènes habitant les mêmes latitudes de l'Asie du Nord et de l'Amérique du Nord.

Vous affirmez que ces traits distinguant les Européens sur le plan visuel résultent d'une pression de sélection qui vise surtout la femme. Pourquoi pas l'homme ?

Il y a eu une pénurie d'hommes chez les premiers Européens, en partie parce que la dépendance de la viande, comme partie dominante de l'alimentation, rendait la polygamie trop coûteuse pour les hommes, sauf pour les meilleurs chasseurs. De plus, comme on le constate toujours chez les peoples chasseurs du Nord, le taux de mortalité est plus élevé chez les hommes que chez les femmes. Résultat : un surplus de femmes. Celles-ci devaient se concurrencer pour les hommes disponibles.

Finalement, il semble y avoir un parallèle fort entre la supposée attraction, aujourd'hui, des hommes pour les femmes blondes (et les stéréotypes et exemples qui en ont découlé dans la pop culture) et le phénomène d'apparition des cheveux blonds il y a 11.000 ans.

Aujourd'hui, grâce aux études de l'ADN extraits des restes humains, on sait que les cheveux blonds existaient déjà il y a 18 000 ans. Le lieu d'origine semble être chez les peoples chasseurs des plaines de l'Europe de l'est et de la Sibérie de l'ouest pendant la dernière glaciation.


English version:

Can you succinctly explain the role of sexual selection in the appearance of blond hair?

Sexual selection favors brightness and novelty. Anything bright remains longer in memory; anything novel holds attention longer. If we consider hair and eye colors, we see an evolution toward brightness, i.e., black hair and brown eyes have ceded their place to bright colors, like red or blond hair and green or blue eyes.

As for evolution toward novelty, this has manifested itself in a diversification of the palette for the hair and the eyes. Initially, a new color emerges through mutation; then it spreads until it loses its novelty; at that moment, the pressure of sexual selection reorients itself to favor a less frequent color. Thus, an equilibrium becomes established between the various colors.

What makes you think that sexual selection is as important, indeed more important, than UV radiation in the appearance of blond hair?

First, the genes controlling skin color and hair color are not the same. One can have very white skin while having dark hair. In addition, the selection pressure of UV radiation does not explain the diversification of alleles controlling hair and eye color. Finally, this diversification is not seen among indigenous peoples inhabiting the same latitudes of northern Asia and North America.

You affirm that these traits that visually distinguish Europeans result from a selection pressure that is aimed especially at women. Why not men?

There was a shortage of men among the first Europeans, partly because dependence on meat, as a dominant part of the diet, made polygamy too costly for men, except for the best hunters. In addition, as is still seen among northern hunting peoples, the mortality rate is higher among men than among women. Result: a surplus of women. Those women had to compete for the available men.

Finally, there seems to be a strong parallel between the purported attraction, today, of men for blonde women (and the resulting stereotypes and examples in pop culture) and the phenomenon of the appearance of blond hair 11,000 years ago.

Today, thanks to studies of DNA extracted from human remains, we know that blond hair already existed 18,000 years ago. The place of origin seems to be among the hunting peoples of the plains of eastern Europe and western Siberia during the last ice age.

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On rereading my answers I realize I may have misunderstood the last question. The intent seems to be:  “Given that these evolutionary processes happened thousands of years ago, how can they explain the growing popularity of blond hair today?” This intent became clearer to me when I read the article, which focuses on blondness in pop culture, and its apparent surge in popularity since the 1970s. 

This trend appears in a study of Playboy playmates from 1954 to 2007. From a low of about 35% in the mid-1960s the proportion of blonde playmates rose to a high of 60% by the year 2000 (Anon 2008). A similar trend was found by Rich and Cash (1993).

Natural blondes are actually a lot scarcer among white Americans. In a sample of undergraduates the proportions were 68% brown, 27% blond, and 5% red (Rich and Cash 1993). Similar proportions appear in a British study: 68% brown, 25% blond, 1% red, and 6% black (Takeda et al., 2006).

Natural blond hair has since become less common in the United States and the United Kingdom. Are we seeing the novelty effect in action? Are blondes becoming sexier because fewer real ones are out there?


References

Anon. (2008). Bygone brunette beauty: Fashion in hair color, Gene Expression June 29
www.gnxp.com/blog/2008/06/bygone-brunette-beauty-fashion-in-hair.php

D'Almeida, P. and M. Giuliani. (2018). Qu'elle a bien pu vouloir dire avec cette coupe ? Stylist France, October 11, pp. 2-5.

Rich, M.K., and T.F. Cash. (1993). The American image of beauty: Media representations of hair color for four decades. Sex Roles 29: 113-124.

Takeda, M.B., M.M. Helms, and N. Romanova. (2006). Hair color stereotyping and CEO selection in the United Kingdom. Journal of human behavior in the social environment 13: 85-99