The presidents of the United States have had, collectively, almost half again as many sons as daughters ((148 to 102 if I’ve counted correctly). Far more strikingly—because the sample size is so much larger—the people listed in Who’s Who have, collectively, about 15% more sons than daughters. (For the latter statistic, I rely on the testimony of the biologist Robin Baker, writing in his remarkable book Sperm Wars.)
Why do high-status parents have more sons? Presumably because high-status sons can give you lots of grandchildren (Baker points to an ex-emperor of Morocco with 888 children). A daughter is far more likely to give you about the average number of grandchildren. On the other side, low-status boys die childless more often than low-status girls. (On average, boys and girls have the same number of offspring—they must, because each offspring has one mother and one father. But girls are clustered around the average, while boys veer off to both extremes.)
So if you want a lot of grandchildren (and whether you want them or not, your genes do) you’ll want sons if you’re near the top of the status heap and daughters if you’re near the bottom.
Now: What’s the mechanism to accomplish all this? One suggestion from the biologists—and one that makes very good sense to an economist—is that a pregnant woman’s body, in deciding how much to invest in nourishing the embryo, takes account of the parents’ status and the embryos’ sex. High status mothers give more nourishment to male embryos; low status mothers give more nourishment to female embryos; better nourished embryos are more likely to be born alive.
How can a process as involuntary as nourishing an embryo respond to conscious information like the status of the father? Well, how can a process as involuntary as sweating with fear respond to conscious information like the approach of a tiger? Clearly this kind of thing happens all the time. More fundamentally, decisions like how much to nourish your embryo are among the most important economic problems the body ever faces. Is it really plausible that the body would simply throw away highly relevant information when it’s making a decision like that?
Incidentally, this ties into my earlier post about stress and daughters. There is evidence that stressed parents, like low-status parents, have more daughters, presumably for the same reason: stressful circumstances, like low-status parents, tend to depress reproductive success.