This is one of those instances where citing to Wikipedia will get you the groupthink rather than the facts. Many studies show no link between breastfeeding and development, once accounting for various confounding factors: http://pediatrics.aappublications.org/content/early/2017/03/...; https://www.nhs.uk/news/pregnancy-and-child/no-significant-l...; http://time.com/9917/sibling-study-shows-little-difference-b...; http://healthland.time.com/2013/03/13/breastfeedings-role-in...

From a 2017 study:

> RESULTS: Before matching, breastfeeding was associated with better development on almost every outcome. After matching and adjustment for multiple testing, only 1 of the 13 outcomes remained statistically significant: children’s hyperactivity (difference score, –0.84; 95% confidence interval, –1.33 to –0.35) at age 3 years for children who were breastfed for at least 6 months. No statistically significant differences were observed postmatching on any outcome at age 5 years.

It is extremely difficult to perform good studies about breastfeeding in the U.S., because breastfeeding rates are highly correlated with other factors. For example over 90% of babies in California are ever breastfed. That figure is around 60% in West Virginia, Louisiana, and Mississippi. Breast feeding rates also vary dramatically by race (black mothers are far less likely to breastfeed). Breastfeeding rates are also much lower among younger mothers and single mothers. So it's very easy to design a study that shows nothing more than that kids of wealthier, white married couples tend to be healthier.