However, the data that supports breast feeding also finds that breast feeding is much more common among educated, wealthy white women with high IQs. And being born to such a woman probably affords a wealth of socioeconomic advantages beyond simply breast milk. These issues, as well as mixed research findings, are reviewed in Emily Oster's "Everybody calm down about breast feeding", written for fivethirtyeight.com.
I think that this would be a good way to introduce the idea of co-variates/third variable problem to your students. For example, what are we to conclude when breast feeding is associated with higher IQ in children. But women with high IQs are more likely to breast feed?
It is also an example of a situation when creating a truly randomized study is exceedingly difficult. This could also be used as a bigger discussion point of how influential socio-economic status is in our life choices. Not to brag, but I'm a highly educated woman with a high IQ. I also happen to be white. And I work in an environment in which I control my own schedule, have an office with a door that shuts, I have enough money to pay for a breast pump, etc. It is relatively easy for me to breast feed. What is a poor woman to do in a work environment in which an employer is not obliged to give her time off to pump, a discreet location to pump, or a secured refrigerator or freezer to store her breast milk?
Another thing to love about this story are all of the links to cited research. If you combined the fivethirtyeight article with an original research article, it might be a way to give your students a security blanket as they take on a big, bad research article.