Economics and Philosophy

Essay

STRATEGY-PROOF JUDGMENT AGGREGATION*

FRANZ DIETRICHa1 and CHRISTIAN LISTa2

a1 University of Maastricht

a2 London School of Economics

Abstract

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.

Footnotes

* 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.