Psychologists now classify motivation as intrinsic vs. extrinsic - are you doing something because you want to, or because someone told you to/offered you something? Importantly, for creative tasks like knowledge work, extrinsic motivators like bonuses are weaker than people's concern for a job well done. Many studies in a variety of situtations have shown the counterinuitive result that adding bonuses to a task makes people perform worse, give up quicker, and not do it on their own initiative.

The book Drive by Daniel Pink is an excellent walk through the research.