It has been known for years that couples in powerful or rich positions have more sons that daughters. The opposite is also true, poor parents have slightly more daughters than sons. This all makes excellent evolutionary sense because a poor daughter is very likely to have children anyway, and in human society can marry upwards.

Poor sons however, are far less likely to have children due to competition from other males. A rich son could have many children, possibly with many women. His rich sister however, can only have a few children and will not really be benefit from being rich. So if you are rich have more sons, if you are poor have more daughters. It makes good sense and the strategy can be seen in the data.

I always liked the results and the evolutionary logic of this little story. But I wondered what mechanism was used to impart knowledge of one’s social position on the choice of sex in a child. The Economist has this article showing a beautifully simple mechanism of doing this – stress! For instance data shows that of the babies conceived in New York the week after 9/11, significantly more were boys girls.

It’s so simple. If you are highly stressed during pregnancy have more daughters because they will probably have more children. If you are unstressed then have more sons – they can probably better take advantage of your situation.