As I recall... studies showing the US is an oligarchy, by comparing special interest lobbies being successfully voted into law, vs laws with general public support, and support of demographic sets of people (age/race/religion/sex etc)...
I had not actually seen this specific study... I was getting a BA in Poli Sci and a BS in Econ from 07-11.
It's been quite a while, I don't remember specifically what papers I read, but I definitely read them, and got into arguments with my professors over them.