Breast Milk, Twins, and Outcomes: A Difference-in-Differences Approach
By Bryan Caplan
Does breast-feeding really give your kids a leg up in life? It’s an important question, and there’s a lot of research on it. But most of the research is, at best, moderately convincing.
The key weakness: If parents falsely believe that breast-feeding is good for kids, then the (genetically and environmentally) “best” parents will be more likely to breast feed. When their kids turn out well, breast milk will undeservedly get the credit.
Sure, you can control for parents’ genetic and environmental quality. When you do, the effect of breast-feeding on e.g. IQ shrinks but doesn’t disappear. But you always have to worry that the lingering correlation reflects unmeasured differences in parental quality.
I’ve figured out a more convincing way to measure the effect of breast-feeding, using what economists call a difference-in-differences approach. The starting premise: Breast-fed twins get less breast milk because they have to share it. They probably get more than 50% of a normal dose, because milk production responds to nursing. But when nursing time doubles, milk production less than doubles. As a result, moms who want to breast-feed their twins are more likely to supplement with formula.
Given this fact, it’s tempting to compare the outcomes of twins and non-twins, then chalk up the difference to breast milk. But that’s no good. Different doses of breast milk are just one of many differences between twins and non-twins. (For starters, twins have lower birth weights). I propose a more sophisticated approach:
1. Measure the differences in outcomes between breast-fed twins and breast-fed non-twins.
2. Measure the differences in outcomes between bottle-fed twins and bottle-fed non-twins.
3. See if (1)>(2), and if so, by how much. The key intuition: If breast milk matters, the outcome gap between breast-fed (twins and non-twins) should exceed the gap between bottle-fed (twins and non-twins).
Of course, this only measures the marginal effect of breast milk, not the total effect. Perhaps a small dose of breast milk is all a baby needs to realize all the benefits. But to the best of my knowledge, any study along these lines would be a big advance over previous research.
P.S. Any researcher who wants to steal my idea is welcome to do so.