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STRATEGY-PROOF JUDGMENT AGGREGATION*

Published online by Cambridge University Press:  01 November 2007

FRANZ DIETRICH  and
CHRISTIAN LIST
FRANZ DIETRICH
Affiliation:
University of Maastricht
CHRISTIAN LIST
Affiliation:
London School of Economics
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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.

Type
Essay
Information
Economics & Philosophy , Volume 23 , Issue 3 , November 2007 , pp. 269 - 300
Copyright
Copyright © Cambridge University Press 2007

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