You dine at Restaurant #1. You receive the bill, with no guidance on an appropriate tip.
You dine at Restaurant #2. You receive the bill, with a calculation of what a 15% and a 20% tip will be.
All things being equal, to which server will you give the larger tip?
Research suggests the latter. In a recent article entitled "Persuasion by Way of Example: Does Including Gratuity Guidelines on Customers' Checks Affect Restaurant Tipping Behavior?" the co-authors conclude that "parties who received the gratuity examples left significantly higher tips than did those receiving no examples" (emphasis added). The article appears in the January 2011 issue of the Journal of Applied Social Psychology, and an abstract is available here.
I wonder, though, if this is just further evidence that most of us are math-challenged.
I wonder if the study controls for the pre-tax/post-tax issue. I like to tip based on a percentage of the pre-tax bill, for one thing because I don't think sales tax variation should lead me to tip differently as I move into different areas. But if the calculation were done for me, I might not notice that it is based on the total, including tax.
This strikes me as a good example of the cognitive illusion known as anchoring: the values provided create an artificial baseline that (at least impliedly) directs patrons to tip above the suggested amount. My guess is that if a check arrived with a 50% tip calculation, patrons would tip even more, even if the check made clear that the 50% value was absurd and should not be taken seriously.
There is a nice legal connection here, too: Chris Guthrie, Jeffrey Rachlinski and Andrew Wistrich had a great article back in 2001 (Inside the Judicial Mind, 86 Cornell L. Rev. 777) that discusses the effects of anchoring and other cognitive illusions in judicial decisionmaking.