Fisher's Principle

Basic logic

Every offspring has exactly one father and one mother, so males as a class and females as a class contribute equally to the genes of the next generation. If males are rare, an average male leaves more offspring than an average female, and parents genetically inclined to produce sons are favored. Males then increase, and in the reverse situation a tendency to produce daughters is favored. Fisher (1930) argued that this frequency-dependent selection stabilizes at the point where total parental investment in sons equals that in daughters. If sons and daughters cost the same to raise, that point is a 1:1 sex ratio.

Historical sources and formalization

Edwards (1998) showed that the argument predates Fisher. Darwin (1871) discussed the sex ratio in the first edition of The Descent of Man but in the second edition left the problem to the future as too intricate, and the German scholar Düsing (1884) gave a mathematical treatment of self-regulating sex ratios. Shaw and Mohler (1953) formalized Fisher's argument as an explicit model of numbers of grandchildren. The principle is regarded as an early example of reasoning about evolutionarily stable strategies from the gene's point of view.

Exceptions and sex allocation theory

Hamilton (1967) showed that female-biased sex ratios evolve under local mate competition, when brothers compete with one another for mates in the same place, and Herre (1985) confirmed this in fig wasps. Trivers and Willard (1973) proposed that mothers in good condition may benefit from producing more sons. Trivers and Hare (1976) predicted a conflict in haplodiploid social insects between the sex ratio favored by workers and by the queen. Charnov (1982), Frank (1990) and West (2009) systematized these extensions into a general theory of sex allocation.

FAQ

What is Fisher's principle in simple terms?

When males are scarce, sons leave more grandchildren, and when females are scarce, daughters do, so evolution pushes toward producing sons and daughters in roughly equal numbers.

Does it mean sex ratios must always be 1:1?

No. The principle equalizes total investment in sons and daughters; unequal rearing costs or conditions such as local mate competition can produce ratios that depart from 1:1.

Sources reviewed: 2026-10-26

References

1. Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
2. Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. London: John Murray.
3. Düsing, C. (1884). Die Regulierung des Geschlechtsverhältnisses bei der Vermehrung der Menschen, Tiere und Pflanzen. Jenaische Zeitschrift für Naturwissenschaft, 17, 593–940.
4. Shaw, R. F., & Mohler, J. D. (1953). The selective significance of the sex ratio. American Naturalist, 87(837), 337–342.
5. Hamilton, W. D. (1967). Extraordinary sex ratios. Science, 156(3774), 477–488.
6. Trivers, R. L., & Willard, D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179(4068), 90–92.
7. Charnov, E. L. (1982). The Theory of Sex Allocation. Princeton: Princeton University Press.
8. Edwards, A. W. F. (1998). Natural selection and the sex ratio: Fisher's sources. American Naturalist, 151(6), 564–569.
9. West, S. A. (2009). Sex Allocation. Princeton: Princeton University Press.
10. Hardy, I. C. W. (Ed.). (2002). Sex Ratios: Concepts and Research Methods. Cambridge: Cambridge University Press.
11. Herre, E. A. (1985). Sex ratio adjustment in fig wasps. Science, 228(4701), 896–898.
12. Trivers, R. L., & Hare, H. (1976). Haplodiploidy and the evolution of the social insects. Science, 191(4224), 249–263.
13. Frank, S. A. (1990). Sex allocation theory for birds and mammals. Annual Review of Ecology and Systematics, 21, 13–55.