a1 University of Maastricht
a2 London School of Economics
Which rules for aggregating judgments on logically connected propositions are manipulable and which not? In this paper, we introduce a preference-free concept of non-manipulability and contrast it with a preference-theoretic concept of strategy-proofness. We characterize all non-manipulable and all strategy-proof judgment aggregation rules and prove an impossibility theorem similar to the Gibbard--Satterthwaite theorem. We also discuss weaker forms of non-manipulability and strategy-proofness. Comparing two frequently discussed aggregation rules, we show that “conclusion-based voting” is less vulnerable to manipulation than “premise-based voting”, which is strategy-proof only for “reason-oriented” individuals. Surprisingly, for “outcome-oriented” individuals, the two rules are strategically equivalent, generating identical judgments in equilibrium. Our results introduce game-theoretic considerations into judgment aggregation and have implications for debates on deliberative democracy.
* F. Dietrich, Dept. of Quant. Econ., Univ. of Maastricht, P.O. Box 616, 6200 MD Maastricht, NL. C. List, Dept. of Govt., LSE, London WC2A 2AE, UK. This paper was presented at the University of Konstanz (6/2004), the Social Choice and Welfare Conference in Osaka (7/2004), the London School of Economics (10/2004), Université de Caen (11/2004), the University of East Anglia (1/2005), Northwestern University (5/2005), the 2005 SAET Conference in Vigo (6/2005), the University of Hamburg (10/2005), IHPST, Paris (1/2006). We thank the participants at these occasions, the anonymous referees of this paper and the editor, Bertil Tungodden, for comments.