It is a well known phenomenon that more male children survive childhood than female children in many developing countries. There have been many theories put forward to explain why there are so many “missing girls” in parts of Asia, which have ranged from structural neglect (Sen), hepatitis B vaccination (Oster), and selective use of abortion technologies (my friend Avi Ebenstein‘s theory). Without getting into some of the messiness of the arguments in this literature, I’ll just say that I think all of these factors could be important.

A new theory, however, has entered into the mix. It is reasonably well known fact that nursing mothers are less fertile (I knew this, and lets just say I know a whole lot less about fertility than most women my age). Breastfeeding inhibits ovulation via hormonal regulation as well as via calorie restriction (in particular in low calorie consumption countries). While not a perfect contraceptive, it does appear to delay fertility for at least a few months. Whether women in developing countries know this or not, is an important question for the validity of this theory, but my sense is that there are certain indicators available to women to evaluate fertility and it would be reasonable that women would notice the negative association between nursing and the return of these indicators.

Seema Jayachandran and Ilyana Kuziemko have just released an NBER working paper that argues that early weaning of female children might also account for some of the missing girls. The argument goes like this: breastfeeding is a really good thing to improve survival of children (this we know, don’t need to convince me), women know that they are less fertile while breastfeeding, so when they give birth to a female child but have a preference for male children they will wean girls earlier than they would wean boys to speed up when they can try for another child. As a result, female children are more prone to illnesses and will die at a higher rate.

The authors mine fertility survey data from India and do, in fact, find that female children are weaned sooner than boys. They also find that subsequent children who already have an older brother are weaned longer (less need to try for another child) and that higher birth order in general is associated with longer breastfeeding (potentially because women are relying upon breastfeeding to reduce their fertility). They also argue that women with a few children are the most likely to take advantage of this technology, as the stakes are higher at this point. They test these hypotheses, which are all supported by their data.

I like this theory, and I also really like that the authors have thought through some of the potential policy implications of these findings. They speculate that as modern contraceptive technologies become more available, than women will rely less upon breastfeeding to control their fertility and may breastfeed all children less. They argue that the roll out of contraceptives should be accompanied by information campaigns to stress the health benefits to children of breastfeeding (I dont know if this is currently done or not, but agree that this could be a potential problem that should be addressed).

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