Abstract
BackgroundNatural 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.Methodology/Principal FindingsHere, 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.Conclusions/SignificanceHumans 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.
Highlights
Natural selection should favour adaptive variation in offspring sex ratio if alterations maximise the offspring’s potential reproductive success [Trivers-Willard hypothesis, TWH; 1]
For species where one sex has more variable reproductive success, 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, 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
Birthweight is associated with survival and future reproductive success [7,8], suggesting that condition advantages endure into adulthood
Summary
Natural selection should favour adaptive variation in offspring sex ratio if alterations maximise the offspring’s potential reproductive success [Trivers-Willard hypothesis, TWH; 1]. Maternal condition should influence offspring condition, the offspring’s condition should endure into adulthood, and any condition advantages should have a greater effect on the more reproductively variable sex [1], males in humans. To determine whether any sex ratio relationship is driven by success of resulting offspring in line with TWH [1], or occurs because sons cost more to raise [2], it is vital to determine if sons of mothers in good condition have higher reproductive success. Natural selection should favour the ability of mothers to adjust the sex ratio of offspring in relation to the offspring’s potential reproductive success. While studies on mammals in general provide support for the hypothesis, studies on humans provide inconsistent results, possibly because the assumptions of the model do not apply
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