Why are males and females 50%-50%?

[napoleon_in_rags]

17. No here is the answer, Fisher's principle.

http://en.wikipedia.org/wiki/Fisher%27s_principle

1) Suppose male births are less common than female.
2) A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
3) Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
4) Therefore the genes for male-producing tendencies spread, and male births become more common.
5) As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
6) The same reasoning holds if females are substituted for males through-out. Therefore 1:1 is the equilibrium ratio.

No bottlenecks, no magic, just a simple formula. Its interesting to look at this playing out economically as well, as your OP says. The fascinating thing is to watch these forces drive culture, like in China where the dislike of female children will clearly become less and socially acceptable as these forces kick in with the male population.