How Babies Work
Emergent Thinking About Emergent Humans

March 21 2013 12:20 PM

Wet-Nursing and the Catholic Church

Illustration by Robert Neubecker.

Illustration by Robert Neubecker

I mentioned wet-nursing yesterday, and that has a fascinating history all its own. Before wet-nursing died a slow, rubber-nipple-assisted death, it was common practice across Europe for centuries, especially in Catholic countries like France. Why there? At least in part because the Catholic Church effectively encouraged people to use wet nurses. Since lactation was a form of birth control, the Church, which taught that the only legitimate purpose of sex was procreation, frowned on intercourse for as long as a woman was nursing. The prohibition was, not surprisingly, widely ignored. But there was a way to comply with it: If a woman stopped nursing altogether and hired a wet nurse, it would be impossible for her to violate the ban—or to be suspected of violating the ban. Having mothers not nurse was the only way the church could be sure that mothers were not having sex while nursing. It was a very clever doctrinal end-run, as long as everyone involved avoided thinking too hard about the sex lives of the wet nurses.

What about the Protestants? They swung the other way: They regarded wet-nursing as a divine betrayal. Women who did not nurse were said to be selfish and even evil; ministers preached against their wickedness. The Puritans in particular were the most impassioned supporters of maternal breast-feeding who can be imagined. Their evangelical rhetoric was often just an elaborate nursing metaphor: Ministers were the breasts of God, and on Sundays parishioners would “suck the breast while it is open.” Eternal life was like “being laid in the bosom of Christ, when sucking the breasts of the grace of Christ.” Being weaned was losing faith. The great Puritan minister Cotton Mather wrote that women who don’t breast-feed “are dead while they live.”

But Mather, despite the sort of pronouncements that make La Leche League look wishy-washy, had at least some of his own children wet-nursed, and colonial America would hardly represent a clean break from the practice. In the 18th century, human milk was already said to be “the most frequently advertised commodity” in the land.


Nicholas Day's book on the science and history of infancy, Baby Meets World, from which part of this post was taken, will be published in April. His website is

March 20 2013 1:58 PM

Your Adorable Puppy Is Also a Breast Pump

Last time, we talked about the new science of milk and the work of Katie Hinde, who studies lactation in rhesus macaques. Her research is remarkable, but its focus is not unusual. Scientists have always been interested in how other mammals lactate, even species that have little light to shed on our own. We know the fat content of elephant seal milk, for example. (It’s basically fatty fat with fat sauce.) But our interest in the lactation of other animals extends well beyond studying it. For a lot of history, we’ve had a much more intimate relationship with the subject. I’m not talking about babies being fed cow’s milk. I’m talking about babies nursing from cows.

This didn’t happen all that often, in truth: Babies are small and fragile; cows are enormous. So goats were a better bet. To wit: The Goat as The Best and Most Agreeable Wet Nurse, published in 1816, by a Conrad A. Zwierlein. In the 19th century, there were hospitals in France with goats effectively on staff. Infants fed on their milk were slid under the goats on wooden trays built for that purpose. They were said to do well, and given that many infants not being fed breast milk died from poor sanitation, it was probably safer to go straight to the source, without any germ-infested buckets and pots getting in the way. (At the time, the biggest worry about baby/goat nursing wasn’t that the infant would be injured by the goat; it was that the infant would grow up goatlike.) This wasn’t exactly common but it was by no means rare: as I write in Baby Meets World, even Montaigne, from his home in the French countryside, took note of it.

The milk flow went in the other direction, too: Humans nursed other species. In the highlands of New Guinea, women nursed orphaned piglets; in South America, they breastfed baby deer, opossums, monkeys; in Japan, the Ainu people were said to suckle bear cubs. Adoptive mothers used the infants of other species to induce lactation. There are numerous accounts of puppies at work: For centuries in England, puppies were used to suck out a mother’s first milk, which physicians at the time thought was tainted. (Puppies were also called on in cases of engorgement.) When Turkish wet nurses traveled overseas for work, they brought puppies along to stimulate their milk production. The puppies were, in a sense, breast pumps. Walking, wagging breast pumps.


Nicholas Day's book on the science and history of infancy, Baby Meets World, will be published in April. His website is

March 20 2013 4:46 AM

The Secrets of Breast Milk

When we come out of the womb, we make our way to the breast. We enter the world knowing we’re mammals, with milk on our minds.

But even as grown-ups, we have never known exactly what’s in that milk—or, as strange as it may sound, what the point of it is. For decades, milk was thought of strictly in terms of nutrients, which makes sense—milk is how a mother feeds her baby, after all. But providing nutrients turns out to be only part of what milk does. And it might not even be the most important part.

“Mother’s milk is food; mother’s milk is medicine; and mother’s milk is signal,” says Katie Hinde, an assistant professor of human evolutionary biology at Harvard. (She also writes the fascinating blog Mammals Suck, which I suspect is the only place on the Internet where you can fill out a Mammal Madness bracket.) “When people find out I study milk, they automatically think we already know about it, or it’s not important. And I’m like, ‘No, we don’t know about it, and it’s super important.’”

But first, a disclaimer—because conversations about lactation always seem to require disclaimers, especially if you happen to be someone who will never ever lactate. (I’m pretty sure.) In my new book Baby Meets World, I write about how, contrary to myth, not nursing has never been a death sentence. Hundreds of years before halfway-decent formula, infants were fed gruesome substitutes for breast milk (mushed bread and beer, say)—and although many more died than those who were nursed, many also survived. So the lesson of the new science of milk isn’t that formula is some sort of modern evil. (It isn’t modern or evil.) It’s that milk is really complicated—and evolutionarily amazing.

Here’s how complicated: Some human milk oligosaccharides—simple sugar carbohydrates—were recently discovered to be indigestible by infants. When my son was nursing, those oligosaccharides weren’t meant for him. They were meant for bacteria in his gut, which thought they were delicious. My wife was, in a sense, nursing another species altogether, a species that had been evolutionarily selected to protect her child. (A relationship immortalized in the paper titled “Human Milk Oligosaccharides: Every Baby Needs a Sugar Mama.”) In effect, as Hinde and UC-Davis chemist Bruce German have written, “mothers are not just eating for two, they are actually eating for 2 × 1011 (their own intestinal microbiome as well as their infant’s)!” That’s what’s meant by milk serving as medicine, and that’s only scratching the surface.

But Hinde primarily studies the food and the signal elements of milk. “The signal is in the form of hormones that are exerting physiological effects in the infant,” she explains. “Infants have their own internal hormones, but they’re also getting hormones from their mother. They’re binding to receptors in the babies, and we’re just starting to understand what those effects are.”

Hinde works with rhesus macaques, and she’s tracked the effects of the hormone cortisol in their milk. Cortisol is often thought of as the stress hormone, but its function is far more varied, and Hinde has found that the amount and especially the variation of cortisol successfully predicts how the infant macaques go on to behave. It’s a stunning finding: The composition of early milk seems to mold infant temperament. But—and here’s the twist—the males were much more sensitive than the females. Roughly, the more cortisol, the more bold and exploratory the male rhesus macaques were.

Such sex-specific variations in milk, possibly “programmed” by the placenta during gestation, may be common. In humans, there’s early data suggesting that mothers produce fattier milk for boys than girls. But that may be only part of the story, as Hinde has found with rhesus macaques. “Just because sons are getting better milk doesn’t mean they’re getting more. It looks like they’re getting very similar total calories.” So why do sons get fattier milk? “In rhesus macaques, daughters stay in their social groups their whole lives,” Hinde notes. “They form a bond with their mother that only ends when one of them dies. So it might be that mothers are nursing their daughters more frequently and that helps establish this bond.” In contrast, the sons end up leaving the group—and fattier milk means they nurse less often, which means they can spend more time playing with strangers, developing skills they’ll need later in life. The milk, in other words, reflects and cements the social structure of rhesus macaques.

We think of milk as a static commodity, maybe because the milk we buy in the grocery store always looks the same. But scientists now believe that milk varies tremendously. It varies from mother to mother, and it varies within the milk of the same mother. That’s partly because the infants themselves can affect what’s in the milk. “Milk is this phenomenally difficult thing to study because mothers are not passive producers and babies are not passive consumers,” Hinde says. Instead, the composition of milk is a constant negotiation, subject to tiny variables.

For example, she notes, in humans skin-to-skin contact appears to trigger signals that are sent through the milk. “If the infant is showing signs of infection, somehow that’s being signaled back to the mother and she up-regulates the immune factors that are in her milk. Now is that her body’s responding to a need of the baby? Maybe. Is it that she also has a low-grade infection that she’s just not symptomatic for and so her body’s doing that? Maybe. Is it partially both? Maybe. We don’t know. It’s brand-new stuff.”

The new awareness of this sort of signaling is why there’s been a paradigm shift in the study of milk. Scientists have gone from seeing it only as food to seeing it far more expansively—as a highly sensitive variable that plays a wide range of developmental roles.

This new perspective should change how we look at formula, too, Hinde says. Instead of comparing breast milk and formula, we should accept how little we actually know about breast milk. “We need to go back to square one and look at all the variation in breast milk and where it’s coming from and what it does,” she says. “Because how could we possibly know what the difference between breast milk and formula is if we aren’t even keeping track of what the variation in breast milk is doing? And so the more that we understand about what is in milk, and what predicts how it varies, the more opportunity there is for formula to better emulate what breast milk is.”

Almost 150 years after the first infant formula, the splendidly named Liebig’s Soluble Food for Babies, was proclaimed to be “virtually identical” to human milk, we now know how much we don’t know about milk. It’s a deeply intimate mystery. And the scientists who study it are a lot like almost any parent gazing down at their sucking child: They too are full of wonder.

Nicholas Day's book on the science and history of infancy, Baby Meets World, will be published in April. His website is

March 20 2013 4:45 AM

Introducing How Babies Work

Illustration by Robert Neubecker.

Illustration by Robert Neubecker

Welcome to How Babies Work, the blog for my new book Baby Meets World. Why don’t you collapse over there on the futon? You’ve clearly been up all night.

First, let’s get this straight: I’m not here to help. Baby Meets World isn’t a conventional baby book. It has no advice to offer. Instead, it’s a new, off-center story about our first experience of the world. It takes a few fundamental activities in infancy (sucking, smiling, touching, toddling) and looks at each from a variety of angles: scientific, historical, cross-cultural, personal. Our understanding of infancy and parenthood tends to be very prescriptive and parochial. But the world of babies and child-rearing is gloriously strange. Rather than skip over that strangeness, Baby Meets World wallows in it. 

Nicholas Day
Author and baby

Photo by Anya Bernstein

Over the next couple of months, while you’re sprawled out over there on the futon, I hope to do that here too. With any luck, this blog will tell you things about infancy you didn’t even know to be curious about. Those things won’t all come from me: I’m a writer who found himself with a baby, not an expert (although neither are all the experts). They’ll come from interviews with people who look at babies in new ways and from a wide range of perspectives—developmental psychology, history, biological anthropology, that sort of thing. I’ll try to avoid the deadening practicality of baby manuals (how to get your child to sleep!) and the breathless news of baby science (this fresh-from-the-lab, not-yet-replicated discovery about how your child sleeps!). There are more than two ways to talk about a baby. Here we’ll talk about some big philosophical things (like how you can see the foundation of human social cognition in an infant pointing) and some small weird things (like the history of humans nursing other species altogether). To start with, I’ll discuss the amazing new science of breast milk. Turns out you haven’t just been feeding your baby—you’ve been feeding the amazing bacteria in his guts.

I should add that I won’t be cannibalizing the book. Most of what I discuss here never made it into Baby Meets World. So don’t worry that you’ve read all the good bits already. And I’ll try not to give away the ending.