 Fisher (1930) first argued that, because every individual has a mother and a father, the mean fitness of males must be equal to the mean fitness of females multiplied by the sex ratio, expressed as the number of females to males (i.e., the mean number of mates per male; see also Shuster and Wade 2003, Chapter 1). As a result, fitness increases with rarity, and, in this circumstance, whenever the population sex ratio deviates from unity, a gene that increases the numbers of the minority sex at birth will have a selective advantage. Thus, a sex ratio of unity is a stable, balanced polymorphism, achieved in many species by chromosomal determination of sex, which Ford referred to as a “‘built-in’ genetic switch-mechanism”, characteristic of other genetic polymorphisms, like Batesian mimicry. In general, the fitnesses of two different types constituting a phenotypic polymorphism must be equal to be maintained within a population by natural selection at a intermediate equilibrium frequency (a point recognized in Darwin 1871, Vol. II, Chapter 14). However, the balance of selective forces for non-sex related (or even sex-linked) polymorphisms is very different from that required to maintain an equal sex ratio. Using the existence of the separate sexes as an example of a balanced polymorphism is misleading or, at least unrepresentative, of the selective forces necessary to sustain balanced polymorphisms in general.