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… the principles of sexual selection … have been confirmed as the key to understanding the evolution and here - and - now proximate expression of sex differences across a vast array of species , ranging from insects and plants to fish and mammals ( Andersson , 1994 ) .
I mistook Geary here as seeming to say that sexual selection is the only mechanism by which sex differences arise. But elsewhere he says for example that the core of what makes women different is that:
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women who are successful in developing and maintaining social networks have healthier children and more surviving children.
So survival pressures play a part directly too.
Here I think what we’re seeing is that women comparatively have to moderate and change the nature of their within sex competition because having good social ties improves the health and survival of their offspring. So this is survival pressures moderating the instincts to compete for mates and changing the nature of female competition, I think.