Like almost all new parents, when my wife and I were in the first flush of parenthood four decades ago, we were possessed to do the best we could for our babies. We were also skeptical of conventional wisdom. In those days, that combination meant we would opt for breast-feeding. It’s a little shocking to say this now, but at that time almost no one chose to breast-feed. Our pediatrician gently tolerated our eccentricity but told us, apologetically, that he couldn’t help—he knew nothing about it. Fortunately, both grandmas were knowledgeable and supportive and things worked out.

Our reasoning about the merits of nursing was pretty simple: Humans are closer to humans than they are to cows or soys, so evolution would surely produce a better-fitting product in breast milk than manufacturers have with infant formula, which is made according to recipes based on cow’s milk or soy proteins. That reasoning still seems pretty plausible to me. Like most doctors—and the American Academy of Pediatrics, which strongly recommends nursing for the first year of a baby’s life—I have a strong suspicion that breast-feeding is preferential. But because it is inherently difficult to do research on humans, especially infants, good data in support of this position are harder to obtain than you’d think. The often-exuberant claims that breast-feeding makes babies smarter, sleeker, and healthier may be right. But we don’t really know.

Nursing is credited with preventing infants from getting cancer, allergic diseases, Crohn’s disease, cavities, SIDS, and with improving IQ. For mothers, it’s also asserted to prevent diabetes, certain cancers, and postpartum depression. In most cultures, however, vast differences—economic, educational, ethnic, psychological, biological—separate women who choose to breast-feed from women who choose formula-feeding. These differences are exaggerated when researchers compare, as they commonly do, the babies of women who breast-feed exclusively for six months and those who exclusively formula-feed for that length of time. The difficulty of doing research on humans thus poses a particular problem for studies of breast-feeding. Breast-fed babies may on average have higher IQ scores, say, but is the difference because of the breast-feeding or some other factor, like coming from a family with a higher income level or more education or fewer siblings? In the studies that have been done to date, untangling the observed effects is a nearly impossible exercise in subjective judgment. That’s especially the case for evaluating subtle effects like IQ level, or the much later development of childhood cancer, allergies, or tooth decay.

Other benefits of breast-feeding seem pretty clear and incontrovertible. Large-scale studies in the developing world have reported a striking drop in infant mortality as formula-feeding is replaced by nursing. But while the role of breast-feeding in preventing infection is real, it is also widely misunderstood.

When you ask a bunch of doctors about how breast-feeding prevents infection, they get it wrong—I know they do, because I’ve asked the question. Doctors tell you that colostrum (produced in the first three days or so after a baby is born) and breast milk are full of maternal antibodies. Next, doctors say that these maternal antibodies are absorbed into the infant’s blood circulation and thus serve to protect infants from disease.

That’s the correct description of the immunology of breast-feeding for most mammals. It’s also true that human colostrum and milk are rich in maternal antibodies—colostrum is pretty much antibody soup. And babies take in these antibodies as they nurse. But human babies are never able to absorb maternal antibodies from milk or colostrum into the bloodstream, except perhaps in the minutest amounts. Maternal antibodies in milk and colostrum protect against infection—but only locally, working inside the baby’s gastrointestinal tract.

This information will surprise farmers, veterinarians, and strongly invested proponents of breast-feeding. After all, if a newborn piglet is deprived of its mother’s colostrum for the first eight hours of life, it is almost guaranteed to become sick and die. Similarly, newborn horses, cats, dogs, and most other mammals are not likely to survive long if they are deprived of colostrum. The reason is simple: Most mammals are born without any antibodies, or only the tiniest amounts, circulating in their blood. That leaves them defenseless at birth against viruses, bacteria, and other pathogens. Fortunately, for a brief period after birth, the antibody molecules in colostrum can easily pass through the bowel walls of babies of each of these species.

But human newborns, it turns out, differ from most other mammals in the way they acquire maternal antibodies. (Before the creationists get too excited, I should point out that everything I am about to say applies to monkeys as well as to people.) Newborn infants get their maternal protection before birth, via an active transport system in the human placenta that carries maternal antibodies from the mother to the fetus. Unlike all those other animals, human babies are born with all the maternal antibodies they will ever have. That’s why we don’t need to absorb maternal antibodies from colostrum. And it’s why formula-fed babies are not at a disadvantage, compared with breast-fed babies, in their supply of circulating maternal antibodies.

None of this is my discovery. It was well-known, even commonplace, in the immunological literature of 40 years ago. But as the field turned to other matters, these findings just sort of fell out of fashion (though I’ve certainly come upon plenty of modern papers whose authors understand the idea). Because of the modern aversion to looking at older research, a surprisingly large number of doctors, especially nonimmunologists, have either forgotten this aspect of human immunity or never knew about it. And perhaps nobody wanted to bring the older findings to light for fear that doing so might discourage breast-feeding. (I can assure you that I feel some trepidation as I write this.)

Before breast-feeding’s able defenders come after me with pitchforks, I’d like to reiterate that I’m only talking here about the incorrect idea that maternal antibodies in milk or colostrum protect against diseases by being absorbed in the baby’s blood. The breast milk antibodies are present in the babies’ intestines, and while they’re not absorbed, they still protect against diarrheal disease, and perhaps other infections as well. This is a huge boon in parts of the world where sanitation is poor and refrigeration nonexistent.

Breast-feeding protects against diarrheal ailments in three ways. Infants who nurse are not drinking formula, which is a perfect medium for bacteria if it’s not refrigerated and if it is reconstituted with contaminated water, as is typically the case in nonindustrialized countries. And if infants are exclusively breast-fed during the vulnerable first six months of life, they’re also not taking in contaminated food. Finally, the maternal antibodies in a baby’s gut deactivate swallowed bacteria and viruses that might otherwise infect the cells that line the intestine or penetrate the lining. Most often this prevents bacterial or viral gastroenteritis, which presently kills 1.6 million children worldwide a year.

Fifteen years ago, the Innocenti Declaration in support of breast-feeding was adopted internationally. Since then, breast-feeding has increased dramatically around the globe. In 2005, UNICEF estimated that 6 million infant lives worldwide (most in the developing world) are saved annually by policies that have replaced formula with breast-feeding. It is likely that a substantial fraction of the 1.6 million infants who still die of diarrheal disease could be saved by breast-feeding as well.

What should we make of the facts about the immunobiology of lactation? First, it bears repeating that even if the immunological benefits are often overstated, there is clear and obvious benefit to breast-feeding in most of the developing world. Second, though it is harder to demonstrate in a scientifically satisfying way, there are probably other biological benefits. And there are surely economic reasons to give babies human milk instead of formula, which costs between $1,500 and $3,000 a year. In the developing world, the economic case against formula-feeding might be as potentially lifesaving as the immunological one: Money stolen from a poor family’s budget for formula will not be available for food, housing, education—or even soap .

In the end, though, I find myself falling back on the same logic (or lack of logic) that appealed to me when my babies were born. Biologically speaking, it seems as if breast-feeding ought to be better for babies. At the same time, I am strongly convinced that there are two kinds of nutrition, physical and psychological, and that both are equally important. This conviction persuades me that it’s better for a mother to formula-feed her baby pleasurably than to breast-feed and hate it. Fortunately, the majority of mothers enjoy nursing. But not all. Some women don’t like to nurse, and others, even with the best help, find it physically difficult or daunting or intolerably uncomfortable. Sometimes, also, babies just aren’t good nursers. In the end, I always encourage a mother to choose the feeding method that is most satisfying to her.