TriversWillard

Trivers-Willard is the shorthand for the Trivers-Willard hypothesis, an evolutionary theory proposed by Robert Trivers and Dan Willard in 1973. The hypothesis explains how parental condition can influence the allocation of investment between male and female offspring in species where parental care and the payoff to offspring differ by sex and where there is substantial variation in male reproductive success.

The central claim is that parents in good condition should bias investment toward the sex with the greater potential payoff when conditions are favorable—typically sons in species with high variance in male reproductive success—whereas parents in poorer condition should bias toward daughters, who often have more certain reproductive prospects. This bias can take the form of differential care, provisioning, or other investments, and, in some cases, may involve mechanisms that alter the sex ratio at birth or embryo survival. However, altering the primary sex ratio at conception is not universal or easy to detect across taxa.

Empirical support is mixed. Some studies in mammals with polygynous mating systems report patterns consistent with the hypothesis—high-condition mothers producing more sons and investing more in them; in humans, results are variable and context-dependent, with findings ranging from associations between maternal condition and offspring sex or investment to null results. Critics note difficulties in measuring “condition” and reproductive value, and point to alternative explanations such as environmental factors, stress, parental age, or random variation.

Today, the Trivers-Willard hypothesis remains a controversial yet influential framework in evolutionary biology and behavioral ecology, guiding research on sex allocation and parental investment across species.