In the book that everyone is reading but I’m still only 2/3 through, Brian Doherty writes,

[David] Friedman points out the intriguing datum that tipping in taxi cabs–almost purely a situation of paying for a public good because of social expectations–is widespread and amounts to over $100 million a year. This indicates to Friedman that most people in reality don’t get carried away with the economic benefits of free riding and neglect public goods when a developed social rule dictates that we should to the right thing.

From p. 386 of Radicals for Capitalism, where I believe that Doherty is continuing a discussion of Friedman’s 1973 book, The Machinery of Freedom.

Obviously, my Taxes and Tipping metaphor was anticipated by Friedman’s taxis and tipping.