How to hyperstimulate a sex-recognition algorithm.
Women have higher luminous contrast between their facial skin and their lip/eye
color. This contrast effect is influenced not only by degree of
lightness but also by degree of redness (source).
Women are fairer in complexion because their skin has
less melanin and less blood (Edwards and Duntley, 1939). Male castrates similarly
look pale (Edwards et al., 1941). For this reason, relative lightness or
darkness of skin seems to be key to sex recognition. People especially focus on
the luminous contrast between facial skin and the color of the lips and eyes
(Russell, 2009; Russell, 2010; see also Dupuis-Roy et al., 2009):
The luminance contrast between
the eyes and the surrounding skin and the lips and the surrounding skin has
been termed ‘facial contrast’. Female faces have greater facial contrast than
male faces, and facial contrast plays an important role in sex classification
and the perception of masculinity and femininity and also attractiveness
(Porcheron et al., 2013)
This contrast effect has influenced the development
of cosmetics. Different societies, sometimes independently of each other, have invented
ways to increase facial contrast by darkening the lips and eye area and,
conversely, by lightening facial skin (Russell, 2009; Russell, 2010).
Ancient Egypt was an early center
for the development of cosmetics (Dayagi-Mendels, 1989). Indeed, the Egyptians
“had most of the cosmetic aids which have ever been devised” (Corson, 1972,
p.8), including rouge for the lips and cheeks, eyeliner (kohl), eyeshadows, and
foundation […] In Mesopotamia, pots of colored paints for the eyes, and rouges
for the lips have been found in Sumerian tombs near Ur from 5,000 years ago.
[…] Excavations at Harappa and
Mohenjo-daro have found kohl pots and sticks for lining the eyes, as well as
red iron oxides and white lead-based compounds that have been surmised to be
rouge for the lips and cheeks and foundation for lightening the skin […]
Evidence for ancient uses of received cosmetics in the East Asia is less clear,
though there is a long history of the use of white face paint and rouge for the
lips in China and Japan (Corson, 1972). Overall, there is evidence for the idea
that the received style of cosmetics developed in multiple centers of early
technology development, and spread outward to peripheral areas such as Europe
and Southeast Asia, analogous to the spread of other technologies like agriculture
and writing. (Russell, 2010)
This contrast effect may have likewise influenced
evolution within our species. The most common human phenotype combines
relatively light skin with dark hair and dark eyes. The opposite pattern is
biologically possible but rare. In addition, the whitening of European skin might
have provided a sufficient degree of luminous contrast for hair and eye color
to diversify.
Not just
luminous contrast
Facial contrast involves differences not only in
luminosity, i.e., lightness/darkness, but also in hue, particularly the redness
of the lips versus the lack of ruddiness of adjacent facial skin. Contrast in
luminosity and hue declines with age and may provide a means for distinguishing
younger from older women:
Several aspects of facial
contrast – the luminance and color differences between the facial features and
the skin surrounding those features – were found to decrease with age. These
included the luminance contrast around the eyes and eyebrows, the red-green
(a*) contrast around the mouth and eyes, and the yellow-blue (b*) contrast
around the eyes. (Porcheron et al., 2013)
Hue contrast explains why redness—lipstick, red
clothing, etc.—seems to enhance femininity even though women are the less ruddy
sex. Red colors highlight this lack of ruddiness, thereby stimulating the
mental algorithm that people use for sex recognition:
Stephen & McKeegan [26] found
that people increase the redness (a*) of the lips to make a female face appear
more feminine and attractive. Elliot & Niesta [46] found that pictures of
women are perceived by men as more attractive and sexually desirable when they
are associated with the color red–whether by the placement of a red border
around the picture, or the presence of a red colored shirt on the woman.
(Porcheron et al., 2013)
Some questions remain unanswered. Why, for instance,
do many women redden their cheeks? Are the cheeks a secondary contrast zone?
And what about women from racial groups with dark facial skin?
This last question seems to have given the study’s
authors some trouble with the reviewers. Evidently, darker-skinned women still
have some facial contrast. They just don’t have as much, probably because they
don’t need as much. Dark skin tends to characterize highly polygynous
societies; in fact, the correlation between darkness of skin and polygyny rate
is greater than the one between lightness of skin and latitude (Manning et al., 2004). In such societies, women are almost sure to get mated and remain so as
long as they are fertile. There is thus only weak selection for those who can
better attract potential mates and more strongly stimulate sex-recognition
algorithms.
On a final note, it’s no coincidence that the
polygyny rate correlates inversely with latitude. In simple tropical societies,
especially those with year-round farming, women can more easily provide for
themselves and their children without male assistance. Since women cost so
little to support, men will tend to seize all mating opportunities. A man will
have as many wives as he can get, and not as many as he can support.
References
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. http://journalofvision.org/9/2/10/
Edwards, E.A. and S.Q. Duntley. (1939). The
pigments and color of living human skin, American
Journal of Anatomy, 65, 1-33.
http://onlinelibrary.wiley.com/doi/10.1002/aja.1000650102/abstract;jsessionid=43509EAC6110062BBF5676995DA16C6B.d01t01?deniedAccessCustomisedMessage=&userIsAuthenticated=false
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, 119-128.
http://endo.endojournals.org/content/28/1/119.short
Manning, J.T., Bundred, P.E., and Mather, F.M.
(2004). Second to fourth digit ratio, sexual selection, and skin colour, Evolution and Human Behavior, 25, 38-50.
http://www.ehbonline.org/article/S1090-5138(03)00082-5/abstract
Porcheron, A., E. Mauger, and R. Russell (2013).
Aspects of facial contrast decrease with age and are cues for age perception, PLoS ONE 8(3): e57985
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057985
Russell, R. (2010). Why cosmetics work. In Adams,
R., Ambady, N., Nakayama, K., & Shimojo, S. (eds.) The Science of Social Vision, New York: Oxford.
http://public.gettysburg.edu/~rrussell/Russell_SocialVision_cosmetics_chapter.pdf
Russell, R. (2009). A sex difference in facial
contrast and its exaggeration by cosmetics, Perception,
38, 1211-1219
http://public.gettysburg.edu/~rrussell/Russell_2009.pdf
