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Conceived and deed the experiments: EC. Performed the experiments: EC. Analyzed the data: EC FD. Wrote the paper: EC FD. 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 Looking sex Willard by giving birth to more sons. While studies on mammals in general provide support for the hypothesis, studies on humans provide particularly inconsistentpossibly 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.
However, women who themselves are billionaires have fewer sons than women having Looking sex Willard 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.
Natural selection should favour adaptive variation in offspring Looking sex Willard 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 males in polygynous speciesthe 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 rateand 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 . For the predictions of the TWH to apply, the assumptions of the model must be met . Specifically, 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 males in humans.
The first two assumptions hold in humans. There is a strong association between birthweights of mothers and offspring which seems to be largely environmentally determined . Furthermore, birthweight is associated with survival and future reproductive success suggesting that condition advantages endure into adulthood.
However, a greater condition advantage for sons than daughters has only been shown in one study on humans and never in an industrialised population. To determine whether any sex ratio relationship is driven by success of resulting offspring in line with TWH or simply occurs because sons cost more to raise it is vital to determine if sons of mothers in good condition have higher reproductive success.
However, in contemporary societies status can be negatively related to fertility . Furthermore, most contemporary western societies have a monogamous social system, reducing differences in reproductive variance between sexes, although the change to monogamy appears to have been relatively recent .
A trivers-willard effect in contemporary humans: male-biased sex ratios among billionaires
Emerging evidence suggests that such an effect exists in mammals in relation to condition at conception . In humans, there has been extensive interest in sex ratio variation, but have been inconsistent. While some studies in traditional societies support the TWH [e. Similar to studies in other Looking sex Willard studies on contemporary humans have used a variety of indices of condition with variable timing in the reproductive cycle, such as economic status female body shape  and size dominance, achievement or employment status and health .
In mammals, evidence suggests that sex ratios vary most consistently with condition around conception and physiological mechanisms may alter sex ratios during very early embryo development .
Recent evidence for sex ratio variation with diet at conception in humans  suggests similarity with other mammals. Furthermore, while the human birth sex ratio is male-biased  human in vitro raised embryos are more male-biased [e. Excess glucose kills female blastocysts but enhances male development . However, alternate hypotheses in humans suggest that hormone levels  or masculinity  are important regulators of human sex ratios.
For example, levels of maternal testosterone are hypothesised to cause high-achieving women to give birth to more sons . We examined whether the assumptions of the TWH, apply to the super-rich, using Looking sex Willard information on family size among billionaires.
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This enabled us to distinguish between biases resulting from the costs  or the advantages  of raising sons. We then compared the offspring sex ratio of billionaires with the global population, and use information on these billionaires to test whether maternal condition influences sex ratios. Lastly, we test whether work achievement, as an index of testosterone levels [following 23]or paternal masculinity  explain variation in sex ratios.
We then searched the billionaire's name online using Google, and used the resulting s to determine the sex of children. Looking sex Willard sex ratio analysis we used those billionaires for whom we could ascertain the sex of every child male billionaires, 49 female billionaires.
We compared the sex ratio of billionaires with the population sex ratio using a chi-squared test. We then divided region of citizenship and residency into western Europe, eastern Europe, North America, South America, Asia, Africa, and Australasia, to control for cultural differences. Most billionaires were from monogamous societies, with fewer than 20 from Arab countries. Some Asian countries limit the of children born, which could also influence .
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We fitted a full model including interaction effects between gender and region of citizenship and gender and region of residency and the main effects, and used Akaike's Information Criterion  to select the most parsimonious model. We square root transformed of children to Looking sex Willard closely approximate normality  and excluded two male billionaires that had grossly outlying of offspring 37 and 61since the rest of the population each had less than 20 children. We used a generalized linear logistic model with a logit link function  to test if offspring sex ratio differed with the source of wealth using the proportion of sons as the response variable, and gender and wealth-source as factorial predictors.
researchers have used work achievement as a proxy for testosterone levels .
We fitted a model including all interaction effects and used AICs to select the minimal adequate model. We identified 14 families where an original fortune had been made 2 generations Looking sex Willard, and where the fortune-maker male in all cases had at least one son and at least one daughter. This enabled us to trace the resulting grandchildren within a single family. The original fortune-maker left more grandchildren through his sons than through his daughters mean of 1.
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Therefore the assumptions of the model are met, Looking sex Willard the TWH predicts that billionaires should give birth to more sons than daughters, and more sons than the non-billionaire population. Therefore, mothers in the highest economic bracket had more sons than expected from the population average. Among all billionaires we show that male billionaires produce more children than female billionaires. Furthermore, when we investigated the grandchildren of an original fortune-maker, we found that he had more grandchildren through his sons than through his daughters, supporting the assumption that reproductive success would be enhanced by giving birth to more sons than daughters.
Finally, parental allocation by the fortune-maker seemed to be higher into their sons than their daughters since sons ranked higher up the billionaire list than daughters. This is the first study to show that males have Looking sex Willard reproductive success in a sample from a contemporary, industrialized, monogamous society, and it supports research showing that high-income men have more children than either high-income women or low income men .
It also means that the assumptions of the TWH apply in this subset of society. Therefore, any differences in the production of sons and daughters are unlikely to be explained solely by differences in the cost of producing sons and daughters even though human sons seem to be more costly to produce . Rather, our suggest that there is a benefit to producing Looking sex Willard for billionaires . Hypotheses to explain biased sex ratios in humans include those relating levels of testosterone to offspring sex ratios .
For example, Grant argues that maternal testosterone determines offspring sex ratios and that testosterone levels are indicated by work achievement . Other studies have confirmed a link between testosterone and work achievement and people with entrepreneurial tendencies have higher levels of testosterone .
Contrary to expectations based on findings women who were themselves billionaires, indicating high motivation, had fewer sons, and this was most marked among self-made women and those that were working and expanding their inheritance. Therefore, work achievement, was not a strong predictor of offspring sex ratios, suggesting that either maternal testosterone was not associated with sex ratio, or, more likely, that work achievement is a poor index of testosterone.
Recent studies on other mammals have suggested that maternal testosterone levels are related to offspring sex ratios .
However, theory also suggests that particularly high achieving women may have more influence over the future success of their daughters than their sons, or daughters may inherit aspects of their mother's rank as seen in other primate species .
studies have suggested a similar hormonal or masculinity effect in males, whereby high achieving males give birth to more sons, either because of hormone levels  or brain masculinity both of which may be indexed by work achievement. However, there was no difference in sex ratio between inherited or high work-drive male billionaires, Looking sex Willard that male work achievement does not explain the observed sex ratio bias. Finally, if sex ratio variation arose due to variation in maternal condition, we would predict that all Looking sex Willard in the sample were in good condition, and no difference in sex ratio between the groups would be predicted.
This was confirmed for all except the high-achieving billionairesses, although this may have been due to the small sample size in this category.
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Therefore, contemporary humans in the highest economic bracket show a ificant sex ratio bias in favour of sons as predicted by the TWH, and these sons leave more grandchildren for the parents than daughters. It seems likely that similar physiological mechanisms operate in human females as in other mammalian species since sex ratio variation seems to arise around conception in relation to maternal condition . The manuscript was improved by comments from J. Gaillard and an anonymous reviewer. Competing Interests: The authors have declared that no competing interests Looking sex Willard.
The funders had no role in study de, data collection and analysis, decision to publish, or preparation of the manuscript.