This refers to the hypothesis that the sex of mammalian offspring may be influenced by the condition of the mother during her pregnancy. In particular, it states that more boys are born when the pregnant mother lives in a favourable environment with ample access to resources. This is because rich sons can achieve greater reproductive success than daughters who can only produce a limited number of offspring during their reproductive age. The Trivers-Willard hypothesis was proposed by American biologist Robert Trivers and American mathematician Dan Willard in their 1973 paper “Natural selection of parental ability to vary the sex ratio of offspring”.
