[FabHK]

This is really a quite surprising and profound fact, worth pondering.

Naively, one would think that if you have (in some antediluvian setting) village A with 50 men and 50 women, and village B with 10 men and 90 women (and children born with that gender ratio, respectively), that village B could "produce" way more offspring and grow faster.

A naive "group selection" view of evolution might even predict something like that to happen.

But, in village B, there is an incentive, so to speak, to cheat and have boys at a higher ratio, because that would increase expected total number of offspring. So, the gender ratio stabilises around 50/50 - a Nash equilibrium, if you will.

[hrolfgar]

Sort of like the prisoners' dilemma; at the end of the day, the selfish actors will choose to locally optimize.

Btw, this argument about the sex ratio stabilizing at 1:1 is known as Fisher's principle. Fisher made another really interesting observation about how extremely exaggerated male ornamentation, like in peacocks, reaches its equilibrium when the aesthetic advantages from a large, attractive tail becomes offset by the practical costs of being slower, needing more food to produce said tail, being highly visible, etc. This one's called "Fisherian Runaway". These purely aesthetic traits will "run away" all the way up until they becoming detrimental to survival.