In evolutionary biology, there is a hypothesis about sex ratios: Trivers-Williard hypothesis. It has been noted that males have higher dispersion in their fertility than females have, a finding that is also true in humans (one, two, three). If resources are related to fertility — which they are in many species and used to be in humans — and the species has parental investment in offspring (many don’t), then one could boost one’s number of grandchildren by having more males if one has a lot of resources, and more females if one is poor.

It’s one of those hypothesis that has a nice theoretical theme going for it, and so psychologists are prone seek out evidence for it. How about this paper?

Trivers–Willard at birth and one year: evidence from US natality data 1983–2001

Trivers & Willard (TW) hypothesized that evolution would favour deviations from the population sex ratio in response to parental condition: parents in good condition would have more sons and parents in poor condition would have more daughters. We analyse the universe of US linked births and infant deaths to white mothers 1983–2001, covering 48 million births and 310 000 deaths. We find that (i) married, better educated and younger mothers bore more sons and (ii) infant deaths were more male if the mother was unmarried and young. Our findings highlight the potential role of offspring sex ratio as an indicator of maternal status, and the role of infant mortality in shaping a TW pattern in the breeding population.

What did they find?

The coefficient indicates that married mothers were 0.2% (1.022/(0.513×1000)) more likely to give birth to a son than unmarried mothers. The other direct measure of the economic circumstances of the mother is her education level. We find that lower education was associated with a more female sex ratio. For instance, relative to a mother with some college, a mother without a high school degree was approximately 0.6% (3.064/(0.513×1000)) less likely to bear a boy. We find that mothers in the age group of 15–19 were more likely to give birth to sons and mothers older than 35 were more likely to give birth to daughters (compared with mothers in the age group of 20–34). Biologically, younger women may be in better condition, an observation that would bring this finding in line with the TW hypothesis. The negative gradient is also consistent with the observation (noted by TW) that sons are a more risky parental investment.

The first two fit with TW, but the effect sizes are tiny. The age pattern does not fit. Older women have a lot more resources than younger women, so should have a lot more boys, which in fact they don’t. What’s wrong? Simple alternative hypothesis: male fetuses are more vulnerable, in particular because they only have 1 X chromosome. Many genetic disorders are due to mutations on the X-chromosome and if they are recessive (you need 100% bad copies to get an effect), then male fetuses are a lot more prone to these disorders than females. Medical geneticists are very familiar with this. Other errors are due to other fuck-ups that also affect male fetuses more, and the rate of these fuck-ups increase with the mother’s age, giving rise to the age pattern seen.

Smarter, more educated etc. people have better genes, including having fewer X-linked genetic variants. So male fetuses of such people are less likely to die off before birth (spontaneous abortion). Estimates of the rate of spontaneous abortion for humans are not easy to get because most of them occur in the early stages where women don’t know they are pregnant, but a value of around 50% would not be surprising. So there’s a lot of room for bad male fetuses to die off selectively in a way that looks like TW.

With that said, Noah points me to a study of billionaires which finds large effects for the sex ratio, like 60% boys in billionaires. It could be a more extreme version of the above, but that would imply that more than 60% of conceptions are male, and that this imbalance happens to get down to about 51% in most people due to sex-linked spontaneous abortions. That doesn’t sound so likely. But note that the effect is only there in male billionaires, not female ones. That seems like a big clue.