Judith Warner writes about breast feeding today in the Times, and she makes the important point that much of our knowledge about what matters for infant outcomes is circumscribed.
Informed consent from infants to participate in a scientific study, in which there are control and treatment groups, is inconceivable. We'll often never really know what a particular treatment in isolation really does to kids, because it would be unethical to find out.
I remember when the question of breast feeding came up among a group of health economists some time ago. While the prevailing wisdom seems to be that it's good, none of us knew of any science that could really sell anybody on it.
In terms of social welfare, a troubling aspect indeed is that breast feeding is much more costly for low-income moms. It would be nice to know the benefits with some degree of certainty.