16. joulukuuta 2014

Ihonvärin mahdollinen yhteys sukupuoleen


Antropologi Peter Frost:in blogista tuli vastaan uusin kirjoitus, jossa jälleen kerran on aiheena ihonväri. Peter on jo 80-luvulla julkaissut tutkimuksia seksuaalivalinnasta koskien ihon, silmien ja hiusten moninaista väriä varsinkin eurooppalaisissa.

Yltä löytyy mielenkiintoinen kuva kahdesta henkilöstä, joista vasemman ihmiset ovat nähneet naisena ja oikean miehenä. Todellisuudessa kyse on samasta henkilöstä, jonka ihonväriä on tummennettu oikeassa kuvassa.

Tässä ei pidä tehdä päätelmiä, että tumma ihonväri tekisi naisista yleisesti miehiä tai miehen näköisiä, koska eihän sukupuolen määrittely ainoastaan kasvoihin perustu ja ovathan myös eurooppalaiset miehet yleisesti vaaleaihoisia, mutta se ei tee miehistä naisen näköisiä.

Tästä huolimatta tutkimukset yleisesti osoittavat miesten yleismaailmallisesti näkevän useammin vaaleamman ihonvärin kauniina naisella. Yksi selitys tähän on mahdollisesti siinä, että estrogeeni tekee naisista vaaleamman ja punertavan sukupuolen, mutta toinen selitys on myös siinä, että ikääntyessä ihonväri tummenee, joten vaaleampi ja myös tasainen ihonväri näyttää nuorekkaalta ja terveeltä.
Skin color differs by sex: women are fairer and men browner and ruddier. Women also exhibit a greater contrast in luminosity between their facial skin and their lip and eye areas. These differences arise from differing concentrations of three skin pigments: melanin (brown); hemoglobin (red); and carotene (yellow).

[...]

Women are fairer than men in all human populations. The difference is greatest in people of medium color and least in very dark- or very fair-skinned people, apparently because of "floor" or "ceiling" effects (Frost, 2007).

[...]

Skin color also differs by age. It can be used to distinguish younger from older women, since the contrast in luminosity between facial skin and the lip/eye areas decreases with age (Porcheron et al., 2013). It can also be used to recognize infants. All humans are born with very little melanin, and the resulting pinkish-white skin is often remarked upon in different cultures.

This is especially so where adults are normally dark-skinned, in striking contrast to newborns. In Kenya, the latter are often called mzungu ('European' in Swahili), and a new mother may ask her neighbors to come and see her mzungu (Walentowitz, 2008). Among the Tuareg, children are said to be whitened by the freshness and moisture of the womb (Walentowitz, 2008). The situation in other African peoples is summarized by a French anthropologist: "There is a rather widespread concept in Black Africa, according to which human beings, before 'coming' into this world, dwell in heaven, where they are white. For, heaven itself is white and all the beings dwelling there are also white. Therefore the whiter a child is at birth, the more splendid it is" (Zahan, 1974, p. 385). A Belgian anthropologist makes the same point: "black is thus the color of maturity [...] White on the other hand is a sign of the before-life and the after-life: the African newborn is light-skinned and the color of mourning is white kaolin" (Maertens, 1978, p. 41).

[...]

The sex-specific aspects of skin color have influenced the development of cosmetics in many cultures. Even in ancient times, women would use makeup to increase the natural contrast in luminosity between their facial skin and their lip/eye areas (Russell, 2009; Russell, 2010). They would also make their naturally fair complexion even fairer by avoiding the sun and applying white powders or bleaching agents.
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Europeans, particularly northern and eastern Europeans, are unusually colored. Their hair can be not only black but also brown, flaxen, golden, or red, and their eyes not only brown but also blue, gray, hazel, or green. Their skin is pale, almost like an albino’s. This color scheme is more developed in women than in men and seems to have been selected for its visual properties, particularly brightness and novelty. Sexual selection is a likely cause. It favors eye-catching colors and, if strong enough, can produce a color polymorphism, i.e., whenever a visible feature becomes differently colored through mutation, the new color will spread through the population until it loses its novelty value and becomes as frequent as the original one.

Such selection is consistent with 1) the many alleles for European hair and eye color; 2) the high ratio of nonsynonymous to synonymous variants; and 3) the relatively short time over which this color diversity developed. Sexual selection will target women if they outnumber men on the mate market. Among early modern humans, such imbalances resulted from 1) a low polygyny rate (because few men could provide for a second wife and her children) and 2) a high risk of early male death (because long hunting distances increased exposure to environmental hazards).

Sexual selection of women was stronger at latitudes farther from the equator, where men were less polygynous and more at risk of death while hunting. It was strongest on continental steppe-tundra, where men provided for almost all family food needs by pursuing herds of reindeer and other herbivores over long distances. Although this type of environment is now fragmentary, it covered until 10,000 years ago a much larger territory—the same area where, today, hair and eyes are diversely colored and skin almost milk white.
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Another dimorphic feature that may underpin part of the lower ratings for the female African faces (but increase those for male African faces) is that adult males on average have darker skin than females, and that this affects attractiveness judgments (van den Berghe and Frost, 1986; Lewis, 2011). Obviously skin color dimorphism alone is not the only factor that matters, or all of our participants would have rated European female faces as most attractive (which they did not always do), and European male faces as least attractive (which they never did), but this factor is likely to have interacted with other factors to produce the patterns we found. The ordering of the male preferences for mixed race faces tracks variation in skin color, such that the darker-skinned mixes are rated as less attractive than the lighter-skinned mixes, so perhaps this is a more important cue for males.
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Our findings confirm earlier studies insofar as facial pictures of ovulatory women were chosen significantly more often as being more attractive, healthy, sexy, sociable, trustworthy, young, and likeable than luteal faces. We could identify the shape changes that occur between the luteal and ovulatory state. The lower face is more robust in the luteal phase, the nose is broader, and the eyebrows are more pronounced. This corresponds to what have been described as masculine features in the literature (ROBERTS et al., 2004). In the ovulatory phase the lips are fuller and the whole face is less robust. Fuller lips and a fragile lower face have been previously associated with youthfulness and high levels of oestrogen (GRAMMER et al., 2003; SYMONS 1995). The increased redness of the face is probably due to higher peripheral blood circulation
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