Indecision Theory: Weight of Evidence and Voting Behavior


  • Paolo Ghirardato, Collegio Carlo Alberto, Via Real Collegio 30, 10024 Moncalieri (TO), Italy ( Jonathan N. Katz, DHSS (228-77), California Institute of Technology, Pasadena, CA 91125 (

  • A more extended version of this paper, discussing some extensions of the model presented here, is available as Caltech Social Science Working Paper No. 1106R with the title “Indecision Theory: Quality of Information and Voting Behavior.” We are very grateful to Alessandro Lizzeri for many detailed comments, and to Doug Bernheim, Peter Bossaerts, Colin Camerer, Tim Feddersen, Michel Le Breton, Tom Palfrey, Ken Shotts, John Conley, and two referees, and to audiences at Caltech, UCLA, the Department of Political Science at UC-San Diego, the 1999 Wallis Conference on Political Economy, and the 2001 SITE Conference for helpful comments and discussion. Jonathan Katz thanks the John M. Olin Foundation for a Faculty Fellowship supporting his research. URLs: and


In this paper, we show how to incorporate weight of evidence, or ambiguity, into a model of voting behavior. We do so in the context of the turnout decision of instrumentally rational voters who differ in their perception of the ambiguity of the candidates' policy positions. Ambiguity is reflected by the fact that the voter's beliefs are given by a set of probabilities, each of which represents in the voter's mind a different possible scenario. We show that a voter who is averse to ambiguity considers abstention strictly optimal when the candidates' policy positions are both ambiguous and they are “ambiguity complements.” Abstaining is preferred since it is tantamount to mixing the prospects embodied by the two candidates, thus enabling the voter to “hedge” the candidates' ambiguity.