Each week, our sales department sends around a list of the best-selling books. The usual suspects usually battle it out for the top spots on the list–Animal Spirits, Lincoln on Race and Slavery, and the perennial favorite On Bullshit. But for the last two weeks, there has been a dark horse giving them a run for the money–Mostly Harmless Econometrics by Joshua Angrist and Jörn-Steffen Pischke. So what is driving these sales? The book has been lauded as “The hitchhiker’s guide to econometrics,” and reviewed in depth by statistician Andrew Gelman (a review that is then reviewed here). That’s all well and good, but secretly, I think it’s the t-shirts.