I just read recently alot of the small studies suggesting breastfed babies are "smarter" and hit milestones faster are flawed and don't look at economical situations on the advantages of breast vs formula. Its shown women who breastfeed in the US tend to be higher income families who are able to stay home with their kids, homeschool, etc, while most of the formula fed kids where lower income, mom had to go back to work quickly and leave baby at a daycare, etc. Entirely different first years have different outcomes.