Ignoring the genetic factors behind maintaining a 1:1 gender ratio for humans, Hamilton’s late 80’s article presents a rather innovative explanation for the regulation of this equilibrium. Provided that factors like polygamy or different mortality of the sexes remain uncorrelated to the gender ratio, the author’s points remain true surprisingly often. Hamilton’s argument, structured with each consequent point relying on those before it, is unexpectedly simple in spite of its intricate organization.

Suppose that female births are less common than males; as a result, obviously, the female population is smaller than that of the male population. With more males in circulation, the mating prospects of all women thus increase, and these newly popular women can expect to have more offspring. Accordingly, those parents genetically disposed to produce more women than average will tend to have their genes (and with them a greater likelihood to produce females) spread more widely. As the population approaches 1:1, the advantages associated with producing women disappear, and the system will once again attain stability.

Simply put, when the ratio is in equilibrium (at 1:1), the totals of effort spent producing both sexes is equal. If these totals are not the same, producers of the sex corresponding to the lower total have an advantage, and will thus force the system back into balance. Hamilton’s argument relies on some of the most basic supply and demand economic concepts; with higher demand, the output of producers will increase in order to achieve equilibrium, regardless of gender.

http://www.sciencemag.org/cgi/rapidpdf/156/3774/477.pdf