Forty years ago, when I was a 19 year old dishwasher and I went out with my wait staff friends, they made a point of tipping 20% instead of the then-customary 10% because they knew what their servers went through and wanted to show meaningful appreciation and help their friends keep their rent paid, etc. I picked up the 20% habit from them. I don't know if what we are seeing is 40+ years of word of mouth, but I don't know that it isn't.