There have also been studies that show people often put in more effort when not being paid than when being paid what they consider an under-market amount.
That's not necessarily inconsistent with the previous comment - they might put in a lot of dedicated effort when they are working on it, but might still not make it a priority when more important stuff interferes. contrariwise, if they were getting paid below market, they might not be as enthused or dedicated, but they might be more committed in the sense of making it something they will try harder to make the time for in the face of other life stuff.