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.
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.