An alternative direct proof of Gibbard’s random dictatorship theorem

Abstract.

We present an alternative proof of the Gibbard’s random dictatorship theorem with ex post Pareto optimality. Gibbard(1977) showed that when the number of alternatives is finite and larger than two, and individual preferences are linear (strict), a strategy-proof decision scheme (a probabilistic analogue of a social choice function or a voting rule) is a convex combination of decision schemes which are, in his terms, either unilateral or duple. As a corollary of this theorem (credited to H. Sonnenschein) he showed that a decision scheme which is strategy-proof and satisfies ex post Pareto optimality is randomly dictatorial. We call this corollary the Gibbard’s random dictatorship theorem. We present a proof of this theorem which is direct and follows closely the original Gibbard’s approach. Focusing attention to the case with ex post Pareto optimality our proof is more simple and intuitive than the original Gibbard’s proof.