The basic idea behind tipping, of course, is that service workers are getting rewarded for doing a good job, but the science simply doesn’t back this up. There’s decades’ worth of consumer-psychology research demonstrating that tipping hardly improves service at all. Michael Lynn, a Cornell University professor and one of the nation’s leading experts on the psychology of tipping, has studied this at length.  In one 2001 review of the literature, for example, Lynn analyzed 14 studies on more than 2,645 bills at 21 restaurants. “I found that the average correlation between tip percentages and service ratings was only .11,” Lynn wrote in a paper published in Cornell HRA Quarterly. “In other words, service ratings explained an average of less than two percent of the variation in a restaurant’s tip percentages.”

Better service did indeed translate to a better tip, in other words, but the correlation was minuscule. Another study, published in 2001 in the journal American Demographics, suggested that many people pretty much just tip what they’re going to tip, no matter what happens during their dining experience. That research found that about a quarter of Americans always tip the same percentage, regardless of service. And although the research once suggested that servers could improve their tips by scrawling a little “thank you” or a smiley face on the bill, the latest studies are now showing the opposite — that when servers do this, their customers tend to leave lower tips.