Statistical Modeling, Causal Inference, and Social Science

There are two immediate hypotheses. One is that the economical development resembles a war, either by removing men from the population or by increasing stress. The other is that the birth order or age at birth influences. The following two charts show that the dependence of sex ratio at birth on the birth order is more clean than the dependence on mother's age (the weird colored bars are actually confidence intervals):

There are several theories for this. A divorce is less likely if the wife bears a son than if she bears a girl. In that sense, bearing sons earlier and daughters later would be adaptive to a woman. In 1997, Manning suggested that the war implies large age difference between men and women. Dominant men have more boys and dominant females more girls. For example, there is an overwhelming predominance of boys being born to US presidents, but there are more girls born to attractive parents. Finally, a very comprehensive discussion by Martin et al from 1994 suggests that it all has to do with coital frequency.

Who would know. But with the right data, these hypotheses could be evaluated statistically. To me, the most persuasive explanation is indeed dominance.