From The Atlantic:
Restaurant servers who leave a piece of candy with the check make 18 percent more in tips than servers who don’t. (Leaving two pieces of candy increases a tip even more.) Building on—believe it or not—more than thirty years of research on tip enhancement (which has established that “briefly touching one’s customers, squatting during the initial contact, making additional nontask visits, and displaying a maximal smile when introducing oneself to one’s customers have all been associated with increases in tip amounts”), four sociologists undertook two studies in an effort to determine which of several competing theories most accurately explains why “unexpected food treats” produce larger gratuities. Does the treat increase the “perceived friendliness” of the server? Does it increase the customer’s identification with the server? Or does a “positive affect” produced by the treat make the customer’s assessment of the server rosier? Actually, the authors conclude, writing in the Journal of Applied Social Psychology, it’s none of the above; what generates the larger tip is the “norm of reciprocity.” The unexpected treat from the server makes the customer feel obligated to respond with a “friendly gesture” in kind.