> Nonetheless, the standard percentage to tip waitstaff has risen over the decades. According to a PayScale study, the median tip is now 19.5%. In recent years, some waiters and restaurants have suggested that 25% or even 30% is the proper gratuity level, and that a 20% tip, once considered generous, is just average today. As recently as 2008, though, an Esquire tipping guide stated "15 percent for good service is still the norm" at American restaurants. An American Demographics study from 2001 found that three-quarters of Americans tipped an average of 17% on restaurant bills, while 22% tipped a flat amount no matter what the bill, and the gratuity left averaged $4.67. Meanwhile, in 1922, Emily Post wrote, "You will not get good service unless you tip generously," and "the rule is ten per cent."

Presumably as a result of most people not wanting to leave a below average tip, which clips the low side of average from the distribution and thereby raises the average over time.