Multivalued social choice functions and strategic manipulation with counterthreats

In this paper, we examine the manipulability properties of social decision rules which select a non-empty subset of the set of alternatives. Assuming that if an individual prefers x to y, then he prefers the outcome set {x, y} to {y}, and also {x} to {x, y}, we show that a wide class of scf's which allow ties even in pairwise choice violates one of the weakest notions of strategyproofness — a single individual can profitably misrepresent his preferences, even when he takes into account the possibility of countercoalitions. This class of scf's also violates exact consistency — no equilibrium situation gives the same outcome set as the ‘true profile’.