This time from Bernard Fraga and Eitan Hersh. Once you think about it, it’s hard to imagine any nonzero treatment effects that don’t vary. I’m glad to see this area of research becoming more prominent. (Here‘s a discussion of another political science example, also of voter turnout, from a few years ago, from Avi Feller and Chris Holmes.)
Some of my fragmentary work on varying treatment effects is here (Treatment Effects in Before-After Data) and here (Estimating Incumbency Advantage and Its Variation, as an Example of a Before–After Study).