Hanna Rosin's article on breastfeeding in the latest Atlantic is excellent and would make a topical and accessible introduction to causality studies in an econometrics or statistics class. (And lest that sound damning it's also a great read.)
The general point will be familiar to the audience at Marginal Revolution. The studies that show breastfeeding leads to lower weight, fewer ear infections, less allergies, less stomach illnesses and so forth are almost all observational studies.
An ideal study would randomly divide a group of mothers, tell one half to breast-feed and the other not to, and then measure the outcomes. But researchers cannot ethically tell mothers what to feed their babies. Instead they have to settle for “observational” studies. These simply look for differences in two populations, one breast-fed and one not. The problem is, breast-fed infants are typically brought up in very different families from those raised on the bottle. In the U.S., breast-feeding is on the rise–69 percent of mothers initiate the practice at the hospital, and 17 percent nurse exclusively for at least six months. But the numbers are much higher among women who are white, older, and educated; a woman who attended college, for instance, is roughly twice as likely to nurse for six months.
Moreover, the better we control for other factors that might account for differences in child outcomes between mothers who breastfeed and those who do not, the less evidence there is for breastfeeding's benefits. Even looking at children within the same family (still far from the gold standard of randomization), shows many fewer benefits from breastfeeding than studies that look across families. Some modest evidence suggests a gain in IQ and better evidence suggests minor improvements in avoiding some diarrhea. Rosin does not discount these benefits (so the title of her piece is unnecessarily sensationalistic) but she very appropriately does point to opportunity cost.
The debate about breast-feeding takes place without any reference to its actual context in women’s lives. Breast-feeding exclusively is not like taking a prenatal vitamin. It is a serious time commitment that pretty much guarantees that you will not work in any meaningful way. Let’s say a baby feeds seven times a day and then a couple more times at night. That’s nine times for about a half hour each, which adds up to more than half of a working day, every day, for at least six months. This is why, when people say that breast-feeding is “free,” I want to hit them with a two-by-four. It’s only free if a woman’s time is worth nothing.
One final point, Rosin's article is also usefully read as a study in propaganda and social psychology.