Breastfeeding is negatively correlated with future fertility because nursing temporarily reduces fecundity and because mothers usually wean on becoming pregnant again. We model breastfeeding under son-biased fertility preferences and show that breastfeeding duration increases with birth order, especially near target family size; is lowest for daughters and children without older brothers because their parents try again for a son; and exhibits the largest gender gap near target family size, when gender is most predictive of subsequent fertility. Data from India confirm each prediction. Moreover, child survival exhibits similar patterns, especially in settings where the alternatives to breastmilk are unsanitary.
Does breastfeeding inhibits fertility?
. . . [N]ursing affects certain hormones that regulate ovulation. Breastfeeding appears to interrupt the release of the gonadotropin-releasing hormone, which triggers the pituitary gland to release high levels of luteinizing hormone (LH). This so-called LH surge marks the beginning of ovulation. There is also some evidence that breast- feeding increases levels of the hormone prolactin, which inhibits ovulation (Blackburn 2007).
. . . [N]ursing diverts calories from the mother to the infant. For mothers who consume a limited number of calories, this diversion can lead to malnutrition, which shuts down ovu- lation. This channel likely plays an important role in developing countries.Some results:
There appears to be a strong positive correlation between breastfeeding and birth order, consistent with the model’s prediction. As the average mother has about four children, the last child would be breastfed about 0.6 months longer than his oldest sibling. Similarly, children born once their mother has reached her ideal family size are breastfed 0.4 month longer than older siblings.
Yet another reason for the gender gap. Very interesting paper!!!