Manipulation and Single‐Peakedness: A General Result

This article considers environments in which individual preferences are single‐peaked with respect to an unspecified, but unidimensional, ordering of the alternative space. We show that in these environments, any institution that is coalitionally strategy‐proof must be dictatorial. Thus, any nondictatorial institutional environment that does not explicitly utilize an a priori ordering over alternatives in order to render a collective decision is necessarily prone to the strategic misrepresentation of preferences by an individual or a group. Moreover, we prove in this environment that for any nondictatorial institution, the truthful revelation of preferences can never be a dominant strategy equilibrium. Accordingly, an incentive to behave insincerely is inherent to the vast majority of real‐world lawmaking systems, even when the policy space is unidimensional and the core is nonempty.