In a model with finitely many agents who have single-dipped Euclidean preferences on a disc in the Euclidean plane, a rule assigns to each profile of reported dips a point of the disc. It is proved that any strategy-proof and Pareto optimal rule is dictatorial. This framework models situations...

Impossibility theorems for preference correspondences based on a new monotonicity concept are discussed. Here monotonicity means that if preferences update in such a way that they get closer to an outcome then at the new situation this outcome remains chosen. Strong monotonicity requires further...