The Food Issue - Why Tip? - NYTimes.com:
"These psychological factors also go a long way in explaining the steady rise of the average tip in the United States from 10 percent in the early 20th century to 18.9 percent today, with little regional variation. “To overtip is to appear an ass: to undertip is to appear an even greater ass,” Benjamin Franklin reportedly noted during his stint in Paris, and his quandary continues to vex American diners."
