Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females.
Sex ratios that are 1:1 are hence known as "Fisherian", and those that are not 1:1 are "non-Fisherian" or "extraordinary" and occur because they break the assumptions made in Fisher's model.
[3][4] W. D. Hamilton gave the following simple explanation in his 1967 paper on "Extraordinary sex ratios",[3] given the condition that males and females cost equal amounts to produce: In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS).
From this it follows that the sex ratio will so adjust itself, under the influence of Natural Selection, that the total parental expenditure incurred in respect of children of each sex, shall be equal; for if this were not so and the total expenditure incurred in producing males, for instance, were less than the total expenditure incurred in producing females, then since the total reproductive value of the males is equal to that of the females, it would follow that those parents, the innate tendencies of which caused them to produce males in excess, would, for the same expenditure, produce a greater amount of reproductive value; and in consequence would be the progenitors of a larger fraction of future generations than would parents having a congenital bias towards the production of females.
As a result, those genes that cause parents to invest unequally in the sexes will tend to be selected against.
R.H. MacArthur (1965) first suggested applying to sex ratios the language of game theory,[7] and this was subsequently picked up by W.D.
[1][9][2] Carl Düsing of the University of Jena published a similar argument in three works between 1883 and 1884,[9] which is essentially identical to Shaw and Mohler's later model.