The limits of ex post implementation without transfers

Abstract

We study ex post implementation in collective decision problems where monetary transfers cannot be used. We find that deterministic ex post implementation is impossible if the underlying environment is neither almost an environment with private values nor almost one with common values. Thus, desirable properties of ex post implementation such as informational robustness become difficult to achieve when preference interdependence and preference heterogeneity are both present in the environment.

Voting with interdependent values: The Condorcet winner

“…This yields a contradiction. We note that in the present framework with a continuum of signals, a much stronger impossibility result holds: only constant social choice functions are ex-post implementable -see Feng et al [2022]. These authors also discuss why ex-post implementation is more permissive in models with discrete (e.g., binary) signals, a phenomenon we also observed in our model with binary signals analyzed above.…”

“…Feng et al [2022] focus on robust implementation with one dimensional signals and without monetary transfers Closest to our present paper, Gruener and Kiel [2004] and Rosar [2015] analyze static voting mechanisms in a setting where agents have quadratic, interdependent preferences, focusing on a comparison of the mean and the median mechanisms. Moldovanu and Shi [2013] analyze voting in a dynamic setting where multi-dimensional alternatives appear over time and where voters are only partially informed about some aspects of the alternative.…”