LW - Nursing doubts by dynomight

Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Nursing doubts, published by dynomight on August 30, 2024 on LessWrong.
If you ask the internet if breastfeeding is good, you will soon learn that YOU MUST BREASTFEED because BREAST MILK = OPTIMAL FOOD FOR BABY. But if you look for evidence, you'll discover two disturbing facts.
First, there's no consensus about why breastfeeding is good. I've seen experts suggest at least eight possible mechanisms:
1. Formula can't fully reproduce the complex blend of fats, proteins and sugars in breast milk.
2. Formula lacks various bio-active things in breast milk, like antibodies, white blood cells, oligosaccharides, and epidermal growth factor.
3. If local water is unhealthy, then the mother's body acts as a kind of "filter".
4. Breastfeeding may have psychological/social benefits, perhaps in part by releasing oxytocin in the mother.
5. Breastfeeding decreases fertility, meaning the baby may get more time before resources are redirected to a younger sibling.
6. Breastfeeding may help mothers manage various post-birth health issues?
7. Infants are often given formula while lying on their backs, which might lead to fluid buildup in the ears and thus temporary hearing loss during a critical development period?
8. Breastfeeding is cheaper??
Second, the evidence for breastfeeding is overwhelmingly observational: It's not based on experiments, but rather looking at the existing population and "observing" that breastfeeding is correlated with having mildly fewer infections (of many kinds) and slightly lower obesity. It may also be correlated with better outcomes in terms of allergies, diabetes, lymphoma, colitis, Crohn's disease, or later IQ.
Observational evidence is disturbing because correlations are bad. Even if breastfeeding did nothing, people think it's good, so the same parents who breastfeed more tend to have higher socioeconomic status and provide lots of other goodies too. Babies that wear baby Rolex watches are probably healthier on average. But that's because their parents are rich, not because Rolexes are good for you. Could breastfeeding be like that?
Of course, experts are aware of this issue. They try to compensate for it by "controlling" for upstream variables. The most-cited meta-analysis on breastfeeding and IQ collected 18 papers that each controlled for different things, like parental education, social status, or how much social interaction the baby got. The control variables seemed to matter a lot:
Among studies that…
Breastfeeding associated with a…
Did not control for maternal IQ
4.1 IQ point increase
Controlled for maternal IQ
2.6 IQ point increase
But what about paternal IQ? Might smarter dads convince mothers to breastfeed more? What if you forgot to control for something, or your data was noisy, or the relationship is nonlinear? (What if smarter babies manipulate their mothers into breastfeeding more?) If any of that happens, then correlations will probably exaggerate the causal impact of breastfeeding.
So there's been a small movement in recent years to push back against Big Nurse, to argue that, despite the public health messaging, there is no clear evidence that breastfeeding is beneficial. (See Stuart Richie at Science Fictions or Emily Oster at Five Thirty Eight or The Guardian for good versions of this argument.)
Naturally, I am sympathetic. Down with groupthink! Down with control variables! Down with putting so much pressure on mothers based on weak evidence!
Except…
Imagine you just gave birth on a desert island - one that for some reason has an unlimited supply of formula. You're considering breastfeeding your baby, but you can't read any studies. What should you do?
Well, there's an obvious evolutionary argument. Maybe the epidermal growth factor and obscure mix of fats in breast milk are crucial. Or maybe they aren't. But they're probably not bad...