He explains: 'The generalized Trivers-Willard hypothesis (gTWH) (Kanazawa, 2005) 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 (Add Health) confirm both of these hypotheses. Very attractive individuals are 26 per cent less likely to have a son, and women are significantly more physically attractive than men in the representative American sample.'