Is earning money detrimental to creativity?

Mark Twain had some relevant comments to make along these lines (or parallel ones, perhaps) in Tom Sawyer. It was the incident where Tom is set to whitewash the back fence, and manages to inveigle all of his friends and acquaintances to do the work - and pay to do it - by making out that it was an artistic oeuvre, not work.

Twain goes on to set out the differneces between amateur and professional, and boils it down to whether one is paid or not - the actual task and the quality of its execution, remain constant.