Metric Distortion of Social Choice Rules: Lower Bounds and Fairness Properties

TL;DR

This paper investigates the limits of social choice rules under metric assumptions, disproves a conjecture about Ranked Pairs' distortion, and compares the performance of various rules in terms of worst-case distortion and fairness.

Contribution

It provides new lower bounds on the distortion of Ranked Pairs, shows Copeland's performance matches this bound, and introduces fairness ratios for Copeland and Randomized Dictatorship.

Findings

Ranked Pairs has a worst-case distortion of at least 5.

Copeland's worst-case distortion matches the lower bound of Ranked Pairs.

Randomization cannot reduce expected worst-case distortion below 3.

Abstract

We study social choice rules under the utilitarian distortion framework, with an additional metric assumption on the agents' costs over the alternatives. In this approach, these costs are given by an underlying metric on the set of all agents plus alternatives. Social choice rules have access to only the ordinal preferences of agents but not the latent cardinal costs that induce them. Distortion is then defined as the ratio between the social cost (typically the sum of agent costs) of the alternative chosen by the mechanism at hand, and that of the optimal alternative chosen by an omniscient algorithm. The worst-case distortion of a social choice rule is, therefore, a measure of how close it always gets to the optimal alternative without any knowledge of the underlying costs. Under this model, it has been conjectured that Ranked Pairs, the well-known weighted-tournament rule, achieves a distortion of at most 3 [Anshelevich et al. 2015]. We disprove this conjecture by constructing a sequence of instances which shows that the worst-case distortion of Ranked Pairs is at least 5. Our lower bound on the worst case distortion of Ranked Pairs matches a previously known upper bound for the Copeland rule, proving that in the worst case, the simpler Copeland rule is at least as good as Ranked Pairs. And as long as we are limited to (weighted or unweighted) tournament rules, we demonstrate that randomization cannot help achieve an expected worst-case distortion of less than 3. Using the concept of approximate majorization within the distortion framework, we prove that Copeland and Randomized Dictatorship achieve low constant factor fairness-ratios (5 and 3 respectively), which is a considerable generalization of similar results for the sum of costs and single largest cost objectives. In addition to all of the above, we outline several interesting directions for further research in this space.