There's a data set called the National Longitudinal Survey of Youth. The kids who are breastfed tend to do better on IQ tests than those who aren't. But there are still potentially unmeasured differences between women that are both contributing to their decision to breastfeed and contributing to their kids IQ.
In this special episode of Freakonomics, M.D., host Bapu Jena looks at a clever new study that could help answer one of parenting’s most contentious questions.