While no, there is no scientific study to back it up, my personal experience (and that of everybody I know) is my dataset. Couple that with the fact that people regularly service and replace external BB while servicing and replacing internal BBs used to be pretty much unheard of and I think my comment is substantiated. Mind you, I should probably clarify I'm talking about threaded bottom brackets. Press fit one's are all over the place right now in terms of durability.