In a study conducted in Pennsylvania, servers in various restaurants wrote Thank you on randomly selected bills before presenting the bills to their customers  Tips on these bills were an average of three percentage points higher than tips on bills without the message
Ttherefore, if servers in Pennsylvania regularly wrote Thank you on restaurant bills, their average income from tips would be significantly higher than it otherwise would have been
Virtually all patrons of the Pennsylvania restaurants in the study who were given a bill with Thank you written on it left a larger tip than they otherwise would have.