Characterization of Group-Fair Social Choice Rules under Single-Peaked Preferences

TL;DR

This paper extends the theory of social choice rules under single-peaked preferences by incorporating group fairness notions, providing characterizations of rules that ensure fairness within and across groups, including random and deterministic rules.

Contribution

It introduces novel group-fairness concepts in single-peaked social choice, with characterizations of fair rules, including convex combinations of deterministic rules, and sharpens individual-fairness results.

Findings

Characterizations of group-fair social choice rules

Convex combinations of deterministic rules satisfy group-fairness

Sharper results for individual-fairness without groups

Abstract

We study fairness in social choice settings under single-peaked preferences. Construction and characterization of social choice rules in the single-peaked domain has been extensively studied in prior works. In fact, in the single-peaked domain, it is known that unanimous and strategy-proof deterministic rules have to be min-max rules and those that also satisfy anonymity have to be median rules. Further, random social choice rules satisfying these properties have been shown to be convex combinations of respective deterministic rules. We non-trivially add to this body of results by including fairness considerations in social choice. Our study directly addresses fairness for groups of agents. To study group-fairness, we consider an existing partition of the agents into logical groups, based on natural attributes such as gender, race, and location. To capture fairness within each group, we introduce the notion of group-wise anonymity. To capture fairness across the groups, we propose a weak notion as well as a strong notion of fairness. The proposed fairness notions turn out to be natural generalizations of existing individual-fairness notions and moreover provide non-trivial outcomes for strict ordinal preferences, unlike the existing group-fairness notions. We provide two separate characterizations of random social choice rules that satisfy group-fairness: (i) direct characterization (ii) extreme point characterization (as convex combinations of fair deterministic social choice rules). We also explore the special case where there are no groups and provide sharper characterizations of rules that achieve individual-fairness.