Yes!: 50 Scientifically Proven Ways to Be Persuasive: Noah J. Goldstein, Steve J. Martin, Robert B. Cialdini
In an experiment, waiters gave candy with the restaurant bill to diners in three different ways: one piece of candy for each guest; two pieces of candy to each guest; and one piece of candy for each guest followed by walking away from the table and then turning around to give a second piece of candy to each guest. Could you guess the average increase it waiter's tips in each case? The answer (and much more) is in the book.