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.
[. . .] Here, we take a subset of humans in very good condition: the Forbe's billionaire list.
[. . .] 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.
Male-Biased Sex Ratios among Billionaires
Cameron EZ, Dalerum F (2009) A Trivers-Willard Effect in Contemporary Humans: Male-Biased Sex Ratios among Billionaires. PLoS ONE 4(1): e4195. doi:10.1371/journal.pone.0004195