Does Breastfeeding Affect Intelligence?
Economist Emily Oster examines the questionable studies that have led to the myth that breastfeeding increases IQ.
The following is an excerpt of Cribsheet: A Data-Driven Guide to Better, More Relaxed Parenting, from Birth to Preschool by Emily Oster.
Cribsheet: A Data-Driven Guide to Better, More Relaxed Parenting, from Birth to Preschool
Most of the academic research on breastfeeding focuses on early-life outcomes—infections, for example, in the time period in which you might actually be breastfeeding. In the popular discourse, however, the focus seems to be much more on the long-term benefits. This is where the guilt stacks up.
You rarely hear people say, “It’s great to breastfeed since it lowers the chances of diarrhea in the next six months!” Rather, they say things like, “It’s great to breastfeed since that gives your kid the best start; they’ll be smarter, taller, thinner!” This problem isn’t limited to random people on the street: One woman told me her doctor had told her that by quitting breastfeeding, she was costing her child three IQ points.
The idea that choosing not to breastfeed might be something your child would suffer from for their whole life is far worse as a parent than simply thinking they might get one more ear infection.
The good news for guilt-ridden moms is that, even more than in the case of early-life health issues, I have not seen any convincing evidence for these long-term impacts.
We can begin with the set of outcomes studied in PROBIT—the Promotion of Breastfeeding Intervention Trial, a randomized trial run in Belarus in the 1990s. These researchers have continued to follow the children in the trial through the age of seven. They find no evidence of any long-term health impacts: no change in allergies or asthma, cavities, height, blood pressure, weight, or indicators for being overweight or obese.
The results on obesity are worth pausing on, as this benefit of breastfeeding gets a lot of attention. (When I was pregnant with Finn, there was a very large poster in my midwife’s office claiming that breastfeeding lowered obesity, a message underscored by the image of two ice cream scoops, each topped with a cherry so they looked like breasts. It was a neat visual, although the point it was illustrating remains unclear to me. I suppose the idea was that you could eat more ice cream if you were breastfed.)
It is certainly true that obesity and breastfeeding are correlated, as kids who are breastfed are less likely to be obese later in life. But this correlation doesn’t show causation—it doesn’t prove that those kids who go on to become obese do so because they weren’t breastfed. The randomized data from PROBIT shows no impact of breastfeeding on whether the child is obese at the age of seven or, in the latest follow-up, at close to eleven. Bolstering this, studies that compare siblings who are breastfed to those who are not show no differences in obesity. These studies often demonstrate that breastfeeding seems to matter when you compare across families, but not within a family. This suggests that something about the family, not the breastfeeding, is impacting the likelihood of a child becoming obese. In fact, when researchers look at many studies of obesity and breastfeeding together to get a fuller picture, they find that studies that carefully adjust for maternal socioeconomic status, maternal smoking, and maternal weight—even if they cannot compare siblings—also show no association.
All these results come with some statistical error. Can we say for sure that breastfeeding does not impact obesity? No. But we can say that nothing compelling in the data supports a significant link.
Media attention tends to miss the nuance of published literature, even when the literature itself is good, which is often not the case.
A few long-term outcomes—for example, juvenile arthritis and urinary tract infections—could not be studied in PROBIT, but at least one or two studies have shown some link between these conditions and breastfeeding. The evidence on most of these links is simply very limited. A significant relationship shows up in only one of many studies, or the research design is poor, or the population is very unusual—basically, we cannot learn anything from the data about whether there is a relationship.
More has been written on two more serious illnesses—type 1 diabetes and childhood cancer—but, again, given the limitations of the data, I do not think we learn much. More on these two in the endnote.
In many of these cases—like others in the breastfeeding arena—even very limited and poorly done studies get a lot of attention. Media attention tends to miss the nuance of published literature, even when the literature itself is good, which is often not the case. We see, again and again, aggressive headlines that often overstate the claims of the articles they report on.
Why is this?
One reason is that people seem to love a scary or shocking narrative. “Report: Formula-Fed Children More Likely to Drop Out of High School” is a more clickable headline than “Large, Well-Designed Study Shows Small Impacts of Breastfeeding on Diarrheal Diseases.” This desire for shock and awe interacts poorly with most people’s lack of statistical knowledge. There is no pressure on the media to focus on reporting the “best” studies, since people have a hard time separating the good studies from the less-good ones. Media reports can get away with saying “A new study shows…” without saying “A new study, with very likely biased results, shows…” And other than the few of us who get our dander up on Twitter, people are mostly none the wiser.
It is hard to sort out study quality from this initial media coverage, although it’s probably easier in the age of the internet. Many media reports will now link to the original study. If the “Formula-Fed Children More Likely to Drop Out of High School” article is based on a study of 45 people surveyed about their breastfeeding behavior when their now 20-year-old children were infants, you can probably let it go.
Breast milk is optimal for brain development, right? Nurse your way to a successful child! So they say. But is this true? Will breast milk make your kid smarter?
Let’s start by returning from the land of magical breast milk to reality. Even in the most optimistic view about breastfeeding, the impact on IQ is small. Breastfeeding isn’t going to increase your child’s IQ by twenty points. How do we know? Because if it did, it would be really obvious in the data and in your everyday experience.
The question is, really, whether breastfeeding gives children some small leg up in intelligence. If you believe studies that just compare kids who are breastfed to those who are not, you find that it does. I talked about one example of these studies on page 68, and there are others. There is a clear correlation here—breastfed kids do seem to have higher IQs.
But this isn’t the same as saying that breastfeeding causes the higher IQ. In reality, the causal link is much more tenuous. We can see this by looking carefully at a number of studies that compare children who were breastfed to their siblings who were not. These studies tend to find no relationship between breastfeeding and IQ. The children who were nursed did no better on IQ tests than their siblings who were not.
This conclusion differs fundamentally from the studies without sibling comparisons. One very nice study gives us an answer to why. The key to this study is that the authors analyze the same sample of kids in a bunch of different ways. First, they compare children who are breastfed with those who are not with a few simple controls. When they do this, they find large differences in child IQ between the breastfed kids and those who are not. In the second phase, they add an adjustment for the mother’s IQ, and find that the effect of breastfeeding is much smaller—much of the effect attributed to breastfeeding in the first analysis was due to differences in the mothers’ IQs—but does still persist.
But then the authors do a third analysis where they compare siblings—children born to the same mother—one of whom was breastfed and one who was not. This is valuable because it takes into account all the differences between the moms, not just their performance on one IQ test. In this analysis, researchers see that breastfeeding doesn’t have a significant impact on IQ. This suggests that it is something about the mother (or the parents in general), not anything about breast milk, that is driving the breastfeeding effect in the first analysis.
PROBIT also looked at the relationship between breastfeeding and IQ. For this sample, the measurement of IQ was done by researchers who knew whether a child was in the breastfeeding-encouraged treatment group. There were no significant effects of breastfeeding on overall IQ or on teachers’ evaluations of the children’s performance in school. The researchers did see small impacts of breastfeeding on verbal IQ in some of their tests, but further analysis suggested that this may have been driven by the people doing the measurement—knowing which children were breastfed might have influenced their evaluation. Overall, therefore, this study doesn’t provide especially strong support for the claim that breastfeeding increases IQ.
In conclusion, there is no compelling evidence for smarty-boobs.
From Cribsheet: A Data-Driven Guide to Better, More Relaxed Parenting, from Birth to Preschool by Emily Oster. Reprinted by arrangement of Penguin Press, part of the Penguin Random House company. Copyright (c) 2019 by Emily Oster.
Emily Oster is the author of Cribsheet: A Data-Driven Guide to Better, More Relaxed Parenting, from Birth to Preschool (2019, Penguin Press) and Expecting Better (2014, Penguin Books). She’s also a professor of Economics at Brown University in Providence, Rhode Island.