Stress and birth outcomes 

 You might recall explanations of a gender bias at birth due to simple and sophisticated discrimination, or even infectious disease like hepatitis B.   Last week’s Economist  reports that in industrialized countries, the probability of getting a boy is slightly higher than getting a girl.  More surprisingly, that extra chance of having a boy has been decreasing.   

 One new cause put forward is mother’s stress, acute or chronic, and there seems to be evidence that stressed mothers are more likely to give birth to girls.  The explanation could be pathological or adaptive: the article suggests that in hard and stressful times, it makes evolutionary sense to have more girls.   


 I suspect that there are many omitted variables related to stress and other health behaviors.  Apparently some studies find similar effects of stress using variation from natural disasters or terrorist attacks.  Still, this won’t explain a pro-boy bias in developing countries (since stress should generally be higher there we would expect more girls to be born).  But it’s an interesting aspect of a growing literature that takes psychological and environmental stress seriously.