This brilliant post on Psyblog explains why you should never pay volunteers or set fixed rewards for any behaviour.
What they had been motivated to do intrinsically, they were now being given an external, extrinsic motivation for. This provided too much justification for what they were doing and so, paradoxically, afterwards they drew less.
Not only this but rewards are dangerous for another reason: because they remind us of obligations, of being made to do things we don't want to do. Children are given rewards for eating all their food, doing their homework or tidying their bedrooms. So rewards become associated with painful activities that we don't want to do. The same goes for grown-ups: money becomes associated with work and work can be dull, tedious and painful. So when we get paid for something we automatically assume that the task is dull, tedious and painful—even when it isn't.
This is why play can become work when we get paid. The person who previously enjoyed painting pictures, weaving baskets, playing the cello or even writing blog posts, suddenly finds the task tedious once money has become involved.
When tasks are inherently interesting to us rewards can damage our motivation by undermining our natural talent for self-regulation.