Aggregation of Antagonistic Contingent Preferences: When Is It Possible?

TL;DR

This paper analyzes the conditions under which a group of voters with conflicting preferences can successfully aggregate their preferences through voting, identifying critical thresholds for the proportion of majority-type voters needed for effective decision-making.

Contribution

It introduces sharp thresholds for voter composition that determine the feasibility of preference aggregation and proposes an optimal mechanism for achieving informed majority decisions.

Findings

Threshold $ heta_{ exttt{maj}}$ determines when strategic equilibria favor informed decisions.

Below the threshold $ heta_{ exttt{maj}}$, preference aggregation is impossible.

A mechanism with threshold $ heta^ ext{*}$ guarantees aggregation when the majority proportion exceeds it.

Abstract

We study a two-alternative voting game where voters' preferences depend on an unobservable world state and each voter receives a private signal correlated to the true world state. We consider the collective decision when voters can collaborate in a group and have antagonistic preferences -- given the revealed world state, voters will support different alternatives. We identify sharp thresholds for the fraction of the majority-type voters necessary for preference aggregation. We specifically examine the majority vote mechanism (where each voter has one vote, and the alternative with more votes wins) and pinpoint a critical threshold, denoted as $\theta_{\texttt{maj}}$, for the majority-type proportion. When the fraction of majority-type voters surpasses $\theta_{\texttt{maj}}$, there is a symmetric strategy for the majority-type that leads to strategic equilibria favoring informed majority decisions. Conversely, when the majority-type proportion falls below $\theta_{\texttt{maj}}$, equilibrium does not exist, rendering the aggregation of informed majority decisions impossible. Additionally, we propose an easy-to-implement mechanism that establishes a lower threshold $\theta^\ast$ (with $\theta^\ast \leq \theta_{\texttt{maj}}$) for both equilibria and informed majority decision aggregation. We demonstrate that $\theta^\ast$ is optimal by proving a general impossibility result: if the majority-type proportion is below $\theta^\ast$, with mild assumptions, no mechanism can aggregate the preferences, meaning that no equilibrium leads to the informed majority decision for any mechanism.