Ambiguous social choice functions

Call a mechanism that associates each profile of preferences over candidates to an ambiguous act an Ambiguous Social Function (ASCF). This paper studies the strategy-proofness of ASCFs. We find that an ASCF is unanimous and strategyproof if and only if there exists a nonempty subset of voters, called the set of top voters, such that at each preference profile, the range of the selected act equals the set of top-ranked candidates of top voters. We provide a full characterization of the class of unanimous, strategyproof, and anonymous ASCFs, and provide a large subclass of ASCFs that satisfy the additional property of neutrality.