Let’s consider three ideas.
First, breastfeeding reduces infant sickness and death in poor and tropical places, largely because it insulates the child from water-borne illness.
Second, breastfeeding naturally reduces a mother’s fertility, and so mothers that want to have another child may stop breastfeeding early.
Third, in places where a sons are preferred over daughters newborn girls may be weaned sooner than boys.
If all three propositions are true, then the gender gap in breast-feeding could explain the millions of “missing girls” in societies with a preference for sons. That’s exactly what Seema Jayachandran and Ilyana Kuziemko find using data from India:
Back-of-the-envelope calculations suggest that breastfeeding accounts for 14 percent of the gender gap in child mortality (deaths between ages one and five) in India, or 22,000 missing girls each year.
Son preference is the underlying cause of this excess female mortality, but in a subtle way: Rather than resulting from parents’ explicit decisions to allocate more resources to sons, the missing girls are mainly an unintended consequence of parents’ desire to have more future sons.
They also point out that breastfeeding is used consciously as a natural contraceptive. As modern contraceptive use expands, women might stop breastfeeding sooner, in spite of the health benefits for children. (Before the anti-contraceptive class gets too excited, they recommend a simple solution: couple contraception programs with breastfeeding promotion.)