To actually learn about the impacts of breastfeeding, we need to rely on studies in which breastfeeding is assigned randomly (the best option) or, in the absence of that experiment design, studies that somehow fully adjust for differences across women.
This leaves us with a small but informative set of studies. In the first camp -- the randomized trial camp -- we have one very large-scale study from Belarus. Known as the PROBIT trial, it was run in the 1990s and continued to follow up as the children aged.1 The study randomized women into two groups, one in which breastfeeding was encouraged and another in which it wasn't, and found that the encouragement treatment increased breastfeeding rates. The trial has studied all sorts of outcomes, including infant and child health and cognitive development.
Infants in the treatment group -- who, remember, were more likely to be breastfed -- had fewer gastrointestinal infections (read: less diarrhea) and were less likely to experience eczema and other rashes. However, there were no significant differences in any of the other outcomes considered. These include: respiratory infections, ear infections, croup, wheezing and infant mortality.
In other words, the evidence suggests that breastfeeding may slightly decrease your infant's chance of diarrhea and eczema but will not change the rate at which he gets colds or ear infections and will not prevent death.
So, you know, mom...mebbe you could try to breastfeed just a little? Think of all the laundry you could be saving yourself without all that messy diarrhea happening. Also, i would like to point out that rashes are no fun. I have cortisone cream stashed all over my house for just in case i get itchy.
Sadly, it there's no proof that breastfeeding makes you smarter, either, so quit your mom-shaming, people!
First, researchers looked at all the kids in the study. For this sample, the evaluation of IQ was done by evaluators who knew whether or not a child was in the breastfeeding-encouraged treatment group. There were no significant effects of breastfeeding on overall IQ. In addition, breastfeeding had no effect on teachers' evaluation of the children's school performance. But the researchers observed large effects of breastfeeding on verbal IQ.
Because the researchers were concerned about evaluator bias, they also had a subset of children evaluated by independent evaluators who did not know which children were breastfed. The differences in verbal IQ disappeared. This, in combination with the teacher evaluations, makes it seem likely that the overall effect was driven by the evaluators, not by true differences among children because of breastfeeding.
This explanation seems especially likely since the effects observed in the full sample are too large to be plausible. Taking into account the impact of the program on breastfeeding rates, the results suggest that nursing increases child IQ by about 24 IQ points, which is far outside of what any other study -- even one seriously biased by differences across mothers -- would suggest. Overall, as others have noted, this study doesn't provide especially strong support for the claim that breastfeeding increases IQ.
Comparisons among siblings (i.e., this and this) also show no IQ impacts. Again, these studies make clear that if you ignore differences across mothers, you can find large impacts of breastfeeding on IQ. It is only when you compare within the same family that you reveal the fact that it really doesn't seem to matter.
Although, really, i feel a lot better now because i wasn't breastfed and i had been all sad about my missed potential. Now i can rest assured that i didn't actually have any potential to lose. Woot!