Anybody wondering should watch the video that the always helpful Porphyrogenitus posted. It is a recent presentation by Gary North that adresses exactly this issue. He is guided by Deirdre McCloskey is the authority on this topic. She has demolished all prominent theories so far in her book Bourgeois Dignity which can be partly read on Google Books
Hans Hermann Hoppe essentially agrees with IQ hypothesis and has built on it in his new book The Great Fiction. The book can be found on google books and I believe the chapter can be read there online.
McCloskey has answered the IQ argument as made by George Clark. Here that part is: