what do you mean, biology?
You must mean this :
When there are no societal constraints women will compete for the most attractive men (alphas—strong social skills and physical appearance) but the alphas have so many women competing for them they have no need or desire to marry.
Women’s refusal to “marry down” is not just financial.
These women do not want to have sex with or marry betas (average looking guys with minimal social skills) who may have some financial resources—until the women “hit the wall” and are no longer fit and attractive.
In the old days women did not expect to marry very attractive guys. They were trained by peer pressure (everyone else is getting married now, I need to get married too, to whoever is interested in me) to marry average guys from their neighborhood or social group. My mother once confided in me: “I would never marry a very good looking guy because I knew other women would steal him from me.”
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Well my son is attractive, almost too good looking, women chase him, and he is frustrated by the younger women’s behaviors... They tend to be slutty. Women sometimes stop when he is doing somethign like gassing the car to say, God you’re beautiful...if I wasnt married... He finds it embarrasing. He does Karate, weightlifting, some MMA. Bigger guy with no fat.
As I said, he would make a nice woman a great husband and at home dad to their many I hope children.