That point I will stick to. I've worked at both Fedex and UPS, and Fedex pays more. We worked in a place where the two companies were on the same street. The Fedex guys always loaded and unloaded the trucks faster. Knowing that you are being paid more to do the same job makes you work harder.

Sample size is 1 and a confirmation bias exists.
Nonetheless, monetary incentives are way to make employees increase productivity, as several studies have shown. Find me a study showing a negative correlation with increased incentives and maybe I'll concede on that point as well.
Psychology 101