Last week, a paper came out in a relatively obscure journal and got a lot of attention (or at least got its own #slatepitches headline). A sociologist at Ohio State published a study saying that the positive effects of breastfeeding essentially disappear if you look at within-sibling differences. That is to say if you compare two siblings from the same parents, one of whom was breastfed and one of whom wasn’t, there isn’t much in the way of statistically significant differences in their educational achievement, health status, or intelligence as measured by standardized tests.
In many ways, this isn’t surprising. We already know that the vast majority of our later life outcomes are determined by our parents’ incomes and education levels, where we grow up, how many words are said to us before we can even talk, and the myriad investments our parents make in our parents’ health. That breastfeeding doesn’t make a very big difference among siblings shouldn’t surprise us.
Perhaps even more important though, and this is really the kicker when trying to identify the effects of early childhood interventions, is that we don’t know what else the parents did differently for these two children. Given what we know about mothers who breastfeed—they tend to be wealthier and more educated, they get more assistance with breastfeeding education, they tend to have more flexible jobs that allow them to breastfeed for longer, or are staying at home with their kids, they’ve been told for years, etc.—we also expect them to be conscientious if they for some reason are not breastfeeding one of their children. Colen’s analysis can only control for time-invariant, mother-specific characteristics. An important omission is that she can’t control for mothers who supplement bottle-feeding with additional doctor’s visits, vitamins, extra care, more time spent together, or any other activity or characteristic that would act as a complement to breastfeeding. It ignores anything that might cancel out the fact the child isn’t getting the extra nutrients and other stuff that we ascribe to breastfeeding.
This omission is only important if it’s correlated with breastfeeding, and in all likelihood, it is. If you thought your child was missing out by not being breastfed, mothers with the ability to might try to compensate by increasing other investments.
It’s also worth noting that there are other studies using the same methodology that do find within-family effects for breastfeeding. This one, for instance, by Rees and Sabia.
Ultimately Colen wants to use the results to push for more family-friendly social policy, like increased maternity leave and more. But this is not the last word on breastfeeding. One momy blogger called it a “suspect methodology.” It’s not, it’s a perfectly valid methodology, but we need to be careful about what it’s actually showing.
Rees, Danial and Joseph Sabia. 2009. “The Effect of Breastfeeding on Educational Attainment: Evidence from Sibling Data.” University of Colorado at Denver Working Paper 09-03.
Colen, Cynthia and DM Ramey. 2014. “Is Breast Truly Best?:Estimating the effects of breastfeeding on long-term child health in the United States using sibling comparisons.”Social Science and Medicine.
Hart, B. & Risley, T.R. “The Early Catastrophe” (2004). Education Review, 77 (1), 100-118.