VOTING UNDER CONSTRAINTS

We consider a broad class of situations where 3 society must choose fr om a finite set of alternatives. This class includes, as polar cases, those where the preferences of agents are completely unrestricted and those where their preferences are single-peaked. We prove that strateg y-proof mechanisms in all these domains must be based on a generalizat ion of the median voter principle. Moreover, they must satisfy a prope rty, to be called the ''intersection property,'' which becomes increas ingly stringent as the preference domain is enlarged. In most applicat ions, our results precipitate impossibility theorems. In particular, t hey imply the Gibbard-Satterthwaite theorem as a corollary. (C) 1997 A cademic Press.