The Trivers-Willard hypothesis of parental investment: No effect in the contemporary United States

The Trivers-Willard hypothesis (TWH) predicts that parents will bias their sex ratio toward sons when in good condition and toward daughters when in poor condition. Many human studies have tested the related hypothesis that parents' bias allocation of resources to existing sons and daughters according to the same principle. The present study used time diary and self-report data from the parents of 3200 children in the US to test the hypothesis that as status increases, parents will allocate more resources to sons vs. daughters. It finds no evidence that higher-status parents invest more in sons or that lower status parents invest more in daughters. This finding illustrates the specificity of situations in which the TWH effects should be expected. Only certain types of parental investment - such as protection and a bias in the sex ratio - may have been selected to vary according to parental condition. Optimal allocation of resources after the child is born, however, is achieved not by the simple bias predicted by the TWH, but by allocating resources among offspring in ways that yield the largest marginal inclusive fitness gains.