Breast milk isn’t magic, it IS an important evolutionary feature in humans

I started the day off yesterday by upsetting some colleagues.

You see, Dr. Katie Hinde (@Mammals_Suck) is a human evolutionary biologist (I like to say anthropologist) at Harvard whose expertise is on mammalian lactation. Dr. Elizabeth Quinn (@Quinnanthrowman) is an anthropologist with expertise in the same area at Washington University. And what I had done was point out this editorial by Dr. Emily Oster (@profemilyoster), an economist at Brown University. In that essay, Oster concludes:

Many women find breastfeeding to be an enjoyable way to bond with their babies. There is certainly no evidence that breastfeeding is any worse for a baby than formula. And maybe there are some early-life benefits in terms of digestion and rashes, which you may or may not think are important. But what the evidence says is that the popular perception that breast milk is some kind of magical substance that will lead your child to be healthy and brilliant is simply not correct.

Breast milk is not magical. But breast milk is the product of a long evolutionary heritage shared by mammals, and something that does do some pretty surprising and amazing things for infants.

Oster raises some good points in her essay, most notably that we should all calm down a little bit about breastfeeding. There is plenty of “mother-shaming” that is already going on, we don’t need to contribute to it, particularly when many women find themselves in positions where they are actively constrained from the kinds of parenting decisions they can make because of legal, socio-economic, or simply culturally discriminating practices. But as someone teaching a course about human evolution to a public audience of 5000 people at the moment, and having just made the argument that understanding human evolution is important because it matters today, and because I really like Katie and Elizabeth and feel bad for ruining their day…I wanted to respond to some of the points Prof. Oster leaves out.

To begin…Prof. Oster writes this:

The purported benefits of nursing (here is one list from the California Department of Public Health) extend to better mother-infant bonding, lower infant mortality, fewer infections in infancy, higher IQ, higher wages in adulthood, less cancer and on and on. If one takes the claims seriously, it is not difficult to conclude that breastfed babies are all thin, rich geniuses who love their mothers and are never sick a day in their lives while formula-fed babies become overweight, low-IQ adults who hate their parents and spend most of their lives in the hospital.

“…it is not difficult to conclude…” well…except that such a conclusion would be wrong. Suggesting breast milk has advantages does not mean it is an automatic path to a determined outcome. It means that it is, on average, likely to accrue positive benefits. This means that breast feeding might shift the possible distribution of outcomes in a positive direction, which is good. I think of as a site grounded in sound statistical and probabilistic reasoning, so I was surprised that Prof. Oster would falsely relay such a probabilistic shift as leading to a kind of assumed certainty. That is not what the research says, even though Prof. Oster seems to imply that this is inevitably how people will read it.

In poor countries where water quality is very poor, these benefits may be very large since the alternative is to use formula made with contaminated water. In developed countries — the main focus of the discussion here — this isn’t an issue.

Here the anthropologist in me just has to jump up and point out that poor water quality is not a condition confined to or inherent to “poor” countries. This kind of construction–assuming a problem found in “other” places does not extend to “us” here in the wonderful US of A–is very common and not at all helpful (in addition to being wrong).

And if you are planning to be home with your baby for an extended period of time, breastfeeding can be convenient and inexpensive (if you are planning to return to work, this is largely not the case, given the time and costs of pumping).

Here my issue is that there is an assumption embedded here that working and breastfeeding cannot go together without pumping. This is part of that larger structural problem with how we “normalize” particular kinds of maternal parenting behaviors. Not everyone can nurse at work, but far more people can than are allowed to because of overly restrictive understandings of what breast feeding is held by many in this country (and many other countries). I can eat a sandwich during a lunch meeting, why can’t a baby nurse? It really is not a big deal.

It is not that the claims about benefits are completely made up. They are mostly based on some data. The trouble is that the evidence they are based on is often seriously biased by the fact that women who breastfeed are typically different from those who do not. Breastfeeding rates differ dramatically across income, education and race.

Yes! This is a good point. And it is a reason to continue to press to study breast feeding more and in more different contexts. But it is not a reason to disregard to the results of those studies that have been done. And, oh by the way, anthropologists have been studying breast feeding in other contexts for a long time.

In the U.S. (and most developed countries), white, wealthy women with a lot of education are much, much more likely to nurse their babies than the rest of the population. But these demographic characteristics are also linked to better outcomes for infants even independent of breastfeeding. This makes it very difficult to infer the actual causal effect of breastfeeding. Sure, there is a correlation between nursing and various good outcomes — but that doesn’t mean that for an individual woman, nursing her baby would improve the child’s life.

Excellent…correlation does not equal causation. To understand causation, you actually want to look at the evolutionary and biological mechanisms associated with lactation, breastfeeding, and infant health. That is exactly the kind of work that Katie and Elizabeth (and many others) do! And here is my real rub…there is a lot of value in putting together pieces that synthesize a large body of research for a public audience. I try to do some of that here. But if you are trying to explain the “science” behind scientific studies you need to do so in a comprehensive and thorough fashion. Implying the multiple correlations make it impossible to trust the results of studies showing positive outcomes associated with breastfeeding is only fine if we don’t have other studies which actually demonstrate the causal mechanisms underlying those outcomes. And we do!

(As an aside, I am not even going to go into the use of IQ as an evaluative property of an individual here, but I have strong feelings about why this is a poor understanding of what IQ is that I have written elsewhere)

Oster continues:

To actually learn about the impacts of breastfeeding, we need to rely on studies in which breastfeeding is assigned randomly (the best option) or, in the absence of that experiment design, studies that somehow fully adjust for differences across women.

I am fortunate to conduct most of my research in the serene environment of paleolandscapes with long dead hominin fossils. But in the real world, the awesome people who study these kinds of topics are working with real people. Real mothers and real infants. And real IRB boards, and real study sampling challenges, and real ethical considerations for how we construct “ideal” samples. These are not mere obstacles in the way of getting at “the true data,” they are valuable, legitimate, and necessary components of the research process.

Given how much interest there is in this topic, it is perhaps surprising that we have only this one large randomized trial of breastfeeding. It’s not clear to me why this is the case. People may be so convinced of the benefits of breastfeeding that they see no need for further testing. Or it may be that a large enough study is too daunting and expensive to run. Whatever the reason, the randomized evidence is limited to this single case.

See above. And, we do not only gather knowledge about health from randomized trials. There are lots of studies about the biology, evolution, and variation surrounding lactation, nursing, and infant health that are available. LOTS. As one example, there is the Cebu Longitudinal Health and Nutrition Survey that has been looking at breastfeeding among a variety of other health behaviors in the Philippines for more than two decades.

So what does evolution have to offer to our understanding of breastfeeding and its importance? I will just give you the basics, and leave the rest to those colleagues of mine who are actual legitimate experts in this area. First of all, the infant developmental period is a hugely critical window in our lives. For at least the past two million years and probably the past four million years, human childhood has been getting longer. The reason for this is that childhood has become an increasingly evolutionarily important period for the development and transmission of life characteristics associated with adult morbidity and fertility. We don’t just learn how to be adults in childhood, our bodies learn how to interact with the food, pathogens, and other biological and social aspects of the world around us. And evolution has been making this more and more the case for a long time. Humans are not born as little adults. We are born as infants with a basic, but vastly incomplete paradigm of who we might become. We rely on life, our parents, and the world around us to help fill that in.

In this context, milk is not merely a food product for infants, it is a transmission mechanism between mother and offspring. Milk contains not just macronutrients used for fuel, but also hormones and other physiologically active endocrine communicators to convey evolutionarily valuable information to an infant. These signals play a role in the development of infant immune systems, organ function, metabolic pathways, genetic expression, and on and on and on…

There are lots of ways to be a great mother. Breastfeeding can be part of that. It would be great if we had public policies and cultural norms that made breastfeeding easier for mothers. From an evolutionary perspective, breastfeeding is not as important as many people think…it is MORE important than most people think. Lots of studies, not just of the outcome of breastfeeding, but of the biology and evolutionary perspectives that underlie nursing in mammals, demonstrate this.

And if you don’t trust me, trust the experts:

Mega mammal milk analysis, by Dr. Katie Hinde
Getting the message via milk, by Dr. Katie Hinde
When to wean, by Dr. Katie Hinde
Of mice and milk, mind and memory, by Dr. Katie Hinde
Sweet mother monkey milk cortisol, reloaded, by Dr. Katie Hinde
Nursing patterns and mothers milk, by Dr. Katie Hinde
Human milk has a microbiome, by Dr. EA Quinn
Milk remembers, by Dr. EA Quinn
Milk responds, by Dr. EA Quinn

About Adam Van Arsdale

I am biological anthropologist with a specialization in paleoanthropology. My research focuses on the pattern of evolutionary change in humans over the past two million years, with an emphasis on the early evolution and dispersal of our genus, Homo. My work spans a number of areas including comparative anatomy, genetics and demography.
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