Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value—namely, varying maternal condition.
In polyandrous species where some females mate with multiple males (and others get no matings) and males mate with one/few females (i.e., "sex-role reversed" species), these predictions from the Trivers–Willard hypothesis are reversed: parents in good condition will invest in daughters in order to have a daughter that can out-compete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get out-competed, and will instead invest in sons in order to gain at least some grandchildren.
"Condition" can be assessed in multiple ways, including body size, parasite loads, or dominance, which has also been shown in macaques (Macaca sylvanus) to affect the sex of offspring, with dominant females giving birth to more sons and non-dominant females giving birth to more daughters.
As blood glucose levels are highly correlated with access to high-quality food,[5] they may serve as a proxy for maternal condition.
Investigations in humans pose a number of practical and methodological difficulties,[6] but while a 2007 review of previous research found that empirical evidence for the hypothesis was mixed, the author noted that it received greater support from better-designed studies.