Thursday, August 10, 2006

The fairer sex is fairer (hotness, not just paleness)   posted by Razib @ 8/10/2006 11:17:00 PM
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Satoshi Kanazawa has a paper in press at The Journal of Theoretical Biology which is of some interest. Here is the abstract:

The generalized Trivers-Willard hypothesis...proposes that parents who possess any heritable trait which increases the male reproductive success at a greater rate than female reproductive success in a given environment will have a higher-than-expected offspring sex ratio, and parents who possess any heritable trait which increases the female reproductive success at a greater rate than male reproductive success in a given environment will have a lower-than-expected offspring sex ratio. One heritable trait which increases the reproductive success of daughters much more than that of sons is physical attractiveness. I therefore predict that physically attractive parents have a lower-than-expected offspring sex ratio (more daughters). Further, if beautiful parents have more daughters and physical attractiveness is heritable, then, over evolutionary history, women should gradually become more attractive than men. The analysis of the National Longitudinal Study of Adolescent Health...confirm both of these hypotheses. Very attractive individuals are 26% less likely to have a son, and women are significantly more physically attractive than men in the representative American sample.


The Triver's-Willard Hypothesis is pretty straightforward, but, let's take the finding that females are, on average, better looking than males at face value (Kanazawa seems to have used bilateral symmetry as a proxy, and let's ignore the confounding effect of greater male developmental instability). What does this imply? Well, it implies that sexual selection upon women has operated for a long period of time, because dimorphism takes a relatively long time to evolve relative to non-sex differentiated traits.

Addendum: You can find the full manuscript in the gnxp forum files under 'Kanazawa."