I don't believe that getting paid more necessarily makes someone work harder. In order for money to be an incentive for someone to work harder the following must be true:
First, money must be a valuable reward to them for labor. Some people, however, find more value in being praised for working hard, or getting time off to spend with family for working hard.
If they DO find money to be a valuable reward for their work, they must then believe that the monetary amount being paid to them is satisfactory to the amount of work they are willing to do. If someone works a lot harder and only gets paid a little more, then they might not see a reason to work harder.
Last, they must have the confidence that they truly have the capability to work harder on the required task. If they feel that they have already worked as hard as possible, then it doesn't matter if they get paid more. They will more than likely not try to increase their efforts, thus not increasing their work.