Girard and her co-authors used a method to try to tease apart the effects of breast-feeding from all the factors that shape these outcomes.

She points out that mothers who choose to breast-feed tend to share a whole range of characteristics and habits.

“For example, mothers who breast-feed typically have higher levels of education,” Girard says. And they tend to engage less in risky behaviors while they’re pregnant, such as smoking. In addition, there are factors such as IQ, and varying home environments. “How many books are in the home, how much time is spent reading?” Girard and her colleagues wanted to consider all these kinds of differences.

Now, what’s interesting is that before the researchers applied methodology designed to account for all these variables, breast-feeding was associated with better cognitive development outcomes almost across the board.

But when they accounted for all these socio-economic variables, the stand-alone effect of breast-feeding seemed to disappear.