The Atlantic magazine recently published an article with the provocative title "The Case Against Breastfeeding" (April, 2009). The author, Hanna Rosin, expressed her exasperation with the peer pressure she felt to breastfeed her third baby. In Rosin's social circle, breastfeeding seems to provide one of the bright lines dividing the "in-group" and the "out-group", or, to put it more bluntly, the upper classes and the rest of the people. Although the concerns felt about breastfeeding may be largely social-among Rosin's friends, anyway-the arguments used to support breastfeeding are often drawn from science rather than the Social Register.
Can science actually tell us whether breastfeeding is superior to bottle-feeding? For biological issues, like the comparative analysis of human milk and baby formulas, or immune responses in babies fed in different ways, yes, of course it can. But what about the claims we so often read, to the effect that breastfed babies grow up to be more intelligent than bottle-fed babies? Is it possible or practicable for scientific methods to provide reliable evidence about this kind of question?
Unfortunately, there is no practical way to do this. In order to show that breastfeeding caused a different outcome than bottle-feeding, we would have to do an experiment, a research study in which babies and mothers were randomly assigned to breastfeed or to bottle-feed, and somehow made to follow their assignment no matter what their own plans had been. This is not very likely to happen, of course, so we find we cannot use the only research design that meets the rules for finding that one thing causes another.
Very few studies of breastfeeding are in any way experimental. Instead, they use an alternative design-one which is good as far as it goes but which can't demonstrate any cause-and-effect relationship. These studies compare mothers and babies who are already breastfeeding with those who are bottle-feeding, which sounds like a good idea until you realize that there are a lot of differences between these two kinds of families. For one thing, in the United States breastfeeding mothers generally have more education than bottle-feeding mothers. Differences like that can affect many things about the baby's experience other than the feeding method, and probably play their own strong role in determining outcomes like tested intelligence. There is no way to figure out which of the factors, or which combination of factors, is actually responsible for any difference in development.
One very important issue about breastfeeding research is that breastfeeding is harder for babies to manage than bottle-feeding. A breast-feeding baby has to suck harder than one who is bottle-feeding, and of course there is no way to enlarge the holes in the mother's nipple (ouch!) to make the flow of milk easier. Milk is ejected from the breast into the baby's mouth, so the breastfed baby has to be good at co-ordinating sucking and swallowing, but the milk flows passively from the bottle and thus needs less co-ordination on the baby's part. If a baby has a birth defect like cleft lip, special nipples help it drink from a bottle, but sucking at the mother's breast is quite difficult. All this means that in any large group of bottle-fed babies, there are likely to be some who were premature, sick, injured, or handicapped, and who are being bottle-fed because they are too weak or immature to breast-feed effectively. Although some weaker babies can be helped to breastfeed, the chances are that in a large group of breastfed infants there are relatively few babies who are not completely healthy. The healthier group is more likely to develop well and to have more members with average intelligence or better. All this means that, in fact, a baby's health and good development may be one of the causes of breastfeeding rather than the other way around.
Before anyone gets angry, I want to say that I am a strong proponent of breastfeeding, and spent most of three years of my life doing it. I think breastfeeding is great, and fun too. But it's not very wise to assume that science can answer questions that are actually impossible because of practical or ethical considerations. On issues like breastfeeding, we need to realize that we make our decisions for our own reasons, and it's only being honest to limit our claims for scientific support.