Abstract

This is a slightly longer version of an entry prepared for the 2nd edition of The New Palgrave Dictionary of Economics, edited by Steven Durlauf and Lawrence Blume (Palgrave-Macmillan, forthcoming). Since the New Palgrave does not include acknowledgments, I should use this chance to thank Roger Backhouse, Philippe Fontaine, Daniel Kahneman, Kyu Sang Lee, Ivan Moscati, and Vernon Smith for their help and suggestions in preparing this paper.