bin, i.e., women are less brown and less ruddy in com-
plexion (Edwards and Duntley, 1939). To explain this
sex difference in terms of natural selection, one must
postulate two separate selection pressures.
2. If an allele lightens women’s skin more than men’s, and
if we compare different populations with the same level
of solar UV exposure (see Jablonski and Chaplin, 2000;
68), the allele should increase in frequency as we go
from darker- to lighter-skinned populations. In addi-
tion, there should be more alleles that exhibit this kind
of sex linkage. The reason in both cases is that the
lighter-skinned populations would owe some of their
loss of skin pigmentation to sexual selection for lighter-
skinned women, and not simply to natural selection (ei-
ther stronger selection for vitamin D synthesis or
weaker selection for protection against sunburn or skin
cancer). Such sex-linked alleles would thus be over-rep-
resented even if they were less effectively expressed in
fair-skinned individuals. We can test this prediction by
examining alleles that 1) lighten skin color and 2)
appear to have been under positive selection, e.g., the
‘‘redhead’’ MC1R alleles, the derived SLC24A5 and
SLC45A2 alleles, and possibly OCA2, MYO5A, HPS7,
and TYRP1 variants (Sturm, 2006). If some or all of
these alleles affect one sex more than the other, they
should do so in the direction of lightening skin color
more in women than in men.
On a ﬁnal note, the sexual selection hypothesis does not
presuppose that human skin color became sexually dimor-
phic solely or even mainly via the action of sexual selection.
Some of its proponents argue that women’s lighter skin
ﬁrst arose through fortuitous causes (the differing effects
of male and female hormones on melanin production) and
that the male mind then came to use this visible female
trait, subconsciously, as a measure of hormonal status and
thus childbearing potential (van den Berghe and Frost,
1986). Others believe that a lighter skin color was ﬁrst part
of a complex of childlike traits (smoother skin, higher-
pitched voice, more pedomorphic face) that enabled women
to lessen aggressiveness in men and stimulate male provi-
sioning (Guthrie, 1970). Still others think that women ﬁrst
acquired a lighter skin to facilitate vitamin D synthesis
and thereby ensure more calcium for pregnancy and lacta-
tion (Jablonski and Chaplin, 2000). All three of these
explanations assign sexual selection a secondary, faculta-
tive role. Women’s lighter skin is thought to have ﬁrst
evolved for other reasons and only later became a criterion
that men could use for mate choice.
More importantly, regardless of the universality of this
mate-choice criterion, the extent to which it actually did
inﬂuence mate choice—and hence sexual selection—
would have depended on the availability of mateable
women and on the relative importance of other mate-
choice criteria. We thus come back to one of the ﬂaws in
Madrigal and Kelly’s model: the assumption that sexual
selection for lighter-skinned women was equally intense
in all human populations and was constrained only by
natural selection for dark skin. In fact, the intensity
may have varied considerably in response to a variety of
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Human Skin-Color Sexual Dimorphism:
A Test of the Sexual Selection Hypothesis.
Reply to Frost (2007)
Lorena Madrigal* and William Kelly
Department of Anthropology, University of South Flor-
ida, Tampa, FL 33620
We thank Frost (2007) for his comments. The evolu-
tion of human skin color has not received much atten-
tion in the physical anthropology literature recently, and
we are glad to see it discussed.
Frost’s comments focus on the following points: He
proposes that sexual dimorphism might not be able to be
expressed in light-skinned populations at higher lati-
tudes. Therefore, we should not expect these populations
to be more sexually dimorphic. We are aware of no data
*Correspondence to: Lorena Madrigal, Department of Anthropol-
ogy, University of South Florida, 4202 E. Fowler Avenue, SOC 17,
Tampa, FL 33620. E-mail: email@example.com
Received 14 December 2006; accepted 10 January 2007
Published online 26 February 2007 in Wiley InterScience
780 NOTES AND COMMENTS
American Journal of Physical Anthropology—DOI 10.1002/ajpa