Strategy-proof preference aggregation: Possibilities and characterizations

- We propose a definition of strategyproofness applicable to preference aggregation rules.
- We discuss examples of manipulable rules.
- We study three classes of strategy-proof aggregation rules.

An aggregation rule maps each profile of individual strict preference orderings over a set of alternatives into a social ordering over that set. We call such a rule strategy-proof if misreporting one's preference never produces a different social ordering that is between the original ordering and one's own preference. After describing two examples of manipulable rules, we study in some detail three classes of strategy-proof rules: (i) rules based on a monotonic alteration of the majority relation generated by the preference profile; (ii) rules improving upon a fixed status-quo; and (iii) rules generalizing the Condorcet-Kemeny aggregation method.