Greg Mankiw is strong evidence by authority for your claims. When a major authority in a field says "We don't know what's going on here, and nobody has really tried to find out," it sets off my atrophied grad student reflexes, and I start thinking "dissertation topic!" However were I still looking for a dissertation topic, the next thing I would do would be to spend a few hours in the library, a few hours with Google, and a few hours talking to folks in my department (not necessarily in that order) to see whether Mankiw was correct or not.

As reliable as Mankiw is, he's still just one economist; and my prior that economists as a group don't simply ignore obvious questions like this one is pretty strong. Show me ten respected economists from a diverse array of schools of economic thought that agree with Mankiw that there really hasn't been a lot of research or thought on this topic, and then I update further.