March 06, 2007

Debate on skin-color sexual dimorphism

A letter and a response were published online in the American Journal of Physical Anthropology (no abstracts). Skin-color sexual dimorphism is the lighter pigmentation of women (of a population) relative to the men.

Human skin-color sexual dimorphism: A test of the sexual selection hypothesis. Reply to Frost (2007) (p NA)
Lorena Madrigal, William Kelly

Human skin-color sexual dimorphism: A test of the sexual selection hypothesis (p NA)
Peter Frost
Women have lighter skin than men do across a wide range of populations, even on the unexposed skin of the upper inner arm, possibly because of sexual selection by men for lighter-skinned women. If this hypothesis is true, human skin color should become more sexually dimorphic with increasing distance from the equator, since sexual selection for lighter skin in women would be less constrained by natural selection for darker skin in both sexes. Yet when Madrigal and Kelly (2006) analyzed skin reflectance data from 53 different samples, they found that the most dimorphic human populations were actually those of medium skin color at medium latitudes.

There is a discussion about this issue in the older blog post about Madrigal and Kelly's study with some comments from Peter Frost himself. That article also contains a very useful table of skin reflectance data in the end. Some Caucasoid (incl. some Indian ones) data points from the table (at wavelength 685), with females first, males second, and higher values designating lighter skin (more reflectance). In these data points it looks to me that Iranians and Kurds have the highest sexual dimorphism. Are Iranian males really much darker than females?

Punjab 53.64 54.52
Sikh 55.52 53.2
Jirel 56.64 49.12
Yemen 56.94 53.15
Algiers 58.04 58
Iran 60.1 51.2
Kurdish 60.02 54.89
Europeans 63.1 61.5
Belgians 63.65 64.51
Viscaya (Basques) 65.78 65.25
Belgians 65.9 67.3
Guipuzcoa (Basques) 66.38 65.53

UPDATE (March 7): Racial Reality also had a table of skin reflectance data.


Unknown said...

Sper interesting this post about the colors of the skin, it hurts to know nothing about this topic to add something more.

Stephanie said...

Ancient Egyptians, as most will know, had standards for depiction of living creatures (and just about everything else). Women, traditionally, are painted with a yellow-based pigment, whereas males are painted with a red/brown-based pigment. That is a good indicator both of the standard of beauty (cultural recognition of sexual selection across a given population), and perhaps a reality of the nuances between male and female complexions in this "bronze skinned" culture.