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.

1 comment:

Sean said...

Tanning beds are getting popular among young women and from Googling it armpits seem to be a area that gets special attention