Fisher's principle

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. It was famously outlined by Ronald Fisher in his 1930 book The Genetical Theory of Natural Selection (but incorrectly attributed to Fisher as original). Nevertheless, A. W. F. Edwards has remarked that it is "probably the most celebrated argument in evolutionary biology". Specifically, Fisher couched his argument in terms of parental expenditure, and predicted that parental expenditure on both sexes should be equal. 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. Many eusocial wasps, such as the Polistes fuscatus and the Polistes exclamans seem to exhibit such a ratio at times.

W.D. Hamilton gave the following basic explanation in his 1967 paper on "Extraordinary sex ratios", 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).

In Chapter 6 "Sexual Reproduction and Sexual Selection", Fisher wrote under the heading "Natural Selection and the sex-ratio" (page 141) the explanation described by Eric Charnov and James J. Bull as being "characteristically terse" and "cryptic":

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