Sex Ratio - Fisher's Principle

Fisher's Principle

Fisher's principle explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher's argument in his 1967 paper on "Extraordinary sex ratios" as follows, given the assumption of equal parental expenditure on offspring of both sexes.

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 throughout. Therefore 1:1 is the equilibrium ratio.

In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS).