There must be a wealth of research on this. Admins and Old School teachers are quite likely to claim that only some students are intrinsically motivated and that a behaviourist carrot/stick extrinsic motivation is the only way to make them do what's good for them. Yet I'm also convinced that there's a wealth of solid evidence behind this notion that we can help learners develop their own intrinsic motivation by shifting the "reward system" to the thing itself. I keep going back to the gym analogy. Usually because of the sense of entitlement demonstrated by many customer-like students. It also works here: the work of a fitness coach builds intrinsic motivation, even if it addresses external rewards.