Trivers's definition and sexual selection
Charles Darwin (1871) used sexual selection to explain competition within a sex and mate choice, but did not fully answer why males usually compete and females usually choose. In fruit fly experiments, Angus Bateman (1948) reported that male reproductive success rose steeply with number of mates while female success did not. Robert Trivers (1972) generalized this by defining parental investment as any investment by the parent in an individual offspring that increases the offspring's chance of surviving and reproducing at the cost of the parent's ability to invest in other offspring. He argued that the sex investing more pays a higher cost for a poor mate choice and so chooses carefully, while the sex investing less competes for access to the investing sex. In mammals females usually invest more because of gestation and lactation, but the theory's core variable is relative investment, not sex as such.
Parent–offspring conflict and sex ratio adjustment
Applying the logic of kin selection (Hamilton, 1964), that parents and offspring share only half their genes, Trivers (1974) proposed parent–offspring conflict: offspring want more investment for themselves, whereas parents seek to spread investment across current and future offspring, so their interests diverge over matters such as weaning. David Haig (1993) used this logic to explain physiological tugs-of-war between fetus and mother during pregnancy. Trivers and Willard (1973) proposed that mothers in good condition may be selected to produce more sons, whose reproductive success varies more, and mothers in poor condition more daughters, extending Fisher's (1930) sex ratio theory. Tim Clutton-Brock (1991) synthesized patterns of parental care across animals.
Tests, extensions and critiques
Sex-role reversal in species such as seahorses and jacanas, where males brood and care for young and females are more ornamented and competitive, is often cited as strong support for the theory. Clutton-Brock and Vincent (1991) proposed that the potential reproductive rate, how quickly each sex can breed again, predicts the direction of competition better than investment size. David Buss (1989) tested sex differences in mate preferences derived from the theory across 37 cultures. By contrast, Kokko and Jennions (2008) argued that models must include feedback from sex ratios and mating opportunities on investment decisions, and Tang-Martinez (2016) criticized the stereotype of coy females and eager males, citing methodological problems in Bateman's experiments and evidence of female multiple mating. Today parental investment theory is a key framework linking natural selection and sexual selection, refined to account for its many exceptions.
FAQ
- What is parental investment theory in simple terms?
- It says the sex that puts more into producing and raising offspring is choosier about mates, while the sex that puts in less competes harder to get mates.
- Do females always invest more?
- No. In species such as seahorses and jacanas, where males do more of the care, sex roles reverse and females become more competitive and ornamented.
Sources reviewed: 2026-10-26
References
- Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. London: John Murray.
- Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Oxford: Clarendon Press.
- Bateman, A. J. (1948). Intra-sexual selection in Drosophila. Heredity, 2(3), 349–368.
- Trivers, R. L. (1972). Parental investment and sexual selection. In B. Campbell (Ed.), Sexual Selection and the Descent of Man, 1871–1971 (pp. 136–179). Chicago: Aldine.
- Trivers, R. L. (1974). Parent-offspring conflict. American Zoologist, 14(1), 249–264.
- Trivers, R. L., & Willard, D. E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science, 179(4068), 90–92.
- Hamilton, W. D. (1964). The genetical evolution of social behaviour. I & II. Journal of Theoretical Biology, 7(1), 1–52.
- Clutton-Brock, T. H. (1991). The Evolution of Parental Care. Princeton: Princeton University Press.
- Clutton-Brock, T. H., & Vincent, A. C. J. (1991). Sexual selection and the potential reproductive rates of males and females. Nature, 351(6321), 58–60.
- Kokko, H., & Jennions, M. D. (2008). Parental investment, sexual selection and sex ratios. Journal of Evolutionary Biology, 21(4), 919–948.
- Buss, D. M. (1989). Sex differences in human mate preferences: Evolutionary hypotheses tested in 37 cultures. Behavioral and Brain Sciences, 12(1), 1–14.
- Haig, D. (1993). Genetic conflicts in human pregnancy. Quarterly Review of Biology, 68(4), 495–532.
- Tang-Martinez, Z. (2016). Rethinking Bateman's principles: Challenging persistent myths of sexually reluctant females and promiscuous males. Journal of Sex Research, 53(4–5), 532–559.