HOW MUCH does breastfeeding really affect IQ?

Even if breastfed babies really do grow up to have higher IQs and make more money, how do we know it’s because they were breastfed? “Breastfeeding is positively associated with socioeconomic standards,” acknowledged Horta, one of the authors of the Brazil study, in a podcast conversation with The Lancet about his work. “So there is always a question of whether [an outcome like higher IQ] is a consequence of breastfeeding by itself… The kids who are breastfed are wealthier.”

In other words, because mothers who breastfeed are also more likely to enjoy a higher socioeconomic status, maybe the breastfed babies have higher IQs and better income in life because of advantages—other than breast milk—associated with that higher socioeconomic status.

And maybe the mothers who breastfed were more likely to enjoy a higher socioeconomic status for the same reason their children had higher IQs and better income in life. Parental IQ correlates with both income and children’s IQ. See Charles Murray’s “Why the SAT Isn’t a ‘Student Affluence Test’” (original link here):

All high-quality academic tests look as if they’re affluence tests. It’s inevitable. Parental IQ is correlated with children’s IQ everywhere. In all advanced societies, income is correlated with IQ. Scores on academic achievement tests are always correlated with the test-takers’ IQ. Those three correlations guarantee that every standardized academic-achievement test shows higher average test scores as parental income increases.

But those correlations also mean that a lot of the apparent income effect is actually owed to parental IQ.

Earlier: Why Breastfed Babies Are So Smart.