For species where one sex has more variable reproductive success (males in polygynous species), the TWH predicts that 1) a mother with more resources to invest would be advantaged by producing a son, as a successful son would out-compete a successful daughter (constrained to a less variable reproductive rate), and 2) a mother with less resources to invest would be advantaged by producing a daughter, as her daughter would out-reproduce an unsuccessful son. Alternatively, if sons are more costly than daughters, only mothers in good condition could bear this cost .
PLoS ONE doi: 10.1371/journal.pone.0004195
A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires
Elissa Z. Cameron, Fredrik Dalerum
Natural selection should favour the ability of mothers to adjust the sex ratio of offspring in relation to the offspring's potential reproductive success. In polygynous species, mothers in good condition would be advantaged by giving birth to more sons. While studies on mammals in general provide support for the hypothesis, studies on humans provide particularly inconsistent results, possibly because the assumptions of the model do not apply.
Here, we take a subset of humans in very good condition: the Forbe's billionaire list. First, we test if the assumptions of the model apply, and show that mothers leave more grandchildren through their sons than through their daughters. We then show that billionaires have 60% sons, which is significantly different from the general population, consistent with our hypothesis. However, women who themselves are billionaires have fewer sons than women having children with billionaires, suggesting that maternal testosterone does not explain the observed variation. Furthermore, paternal masculinity as indexed by achievement, could not explain the variation, since there was no variation in sex ratio between self-made or inherited billionaires.
Humans in the highest economic bracket leave more grandchildren through sons than through daughters. Therefore, adaptive variation in sex ratios is expected, and human mothers in the highest economic bracket do give birth to more sons, suggesting similar sex ratio manipulation as seen in other mammals.