Could you cite a study or other statistical evidence? I'm surprised to find that the median server gets tips >20% averaged over all customers as a voluntary contribution. It would be interesting to see a break down of the range of tips send which restaurants were in the study, etc..

Presumably other areas of devices, like clothing or general retail are getting demands from unions to move over to voluntary tipping to tap that generosity?

In UK it's relatively common to tip some hotel workers and taxi drivers -- do USA hotel room cleaners also benefit from this effect of tips>>charges?