There's a bit of back and forth between intrinsic and extrinsic motivation, where an external reward can reduce intrinsic motivation more than it increases extrinsic motivation. That is, if the reward is known but small, someone could go form, "I'll submit this since I want to help them out," to, "I'm not going to work for below minimum wage!"

http://en.wikipedia.org/wiki/Motivation#Intrinsic_and_extrin...