I think i read somewhere that money isn't the best motivator to get people to do the best work they can. And in general even with children they say positive enforcement should be preferred to punishment. So maybe someone doing a job cause they want to, to make more money and have a better standard of living, will get the job done better than someone being forced to do it cause otherwise he will be living on the streets. In my experience the main predictor of the quality of a job is weather the person wants to do it, or has to. The job is the same, it's the perception that's different.