Quantitative Relaxations of Arrow's Axioms

TL;DR

This paper introduces a quantitative framework for relaxing Arrow's axioms in voting rules, measuring how closely rules adhere to IIA and Unanimity, and empirically evaluates various voting methods using real and synthetic data.

Contribution

It develops a novel probabilistic approach to measure the degree of compliance with Arrow's axioms, extending classical binary criteria to continuous metrics, and applies this to real-world and synthetic election data.

Findings

Borda rule scores highest on IIA and Unanimity metrics.

Quantitative metrics recover classical axioms at their extremes.

Empirical results align with theoretical predictions about Borda's properties.

Abstract

In this paper we develop a novel approach to relaxing Arrow's axioms for voting rules, addressing a long-standing critique in social choice theory. Classical axioms (often styled as fairness axioms or fairness criteria) are assessed in a binary manner, so that a voting rule fails the axiom if it fails in even one corner case. Many authors have proposed a probabilistic framework to soften the axiomatic approach. Instead of immediately passing to random preference profiles, we begin by measuring the degree to which an axiom is upheld or violated on a given profile. We focus on two foundational axioms-Independence of Irrelevant Alternatives (IIA) and Unanimity (U)-and extend them to take values in $[0,1]$. Our $\sigma_{IIA}$ measures the stability of a voting rule when candidates are removed from consideration, while $\sigma_{U}$ captures the degree to which the outcome respects majority preferences. Together, these metrics quantify how a voting rule navigates the fundamental trade-off highlighted by Arrow's Theorem. We show that $\sigma_{IIA}\equiv 1$ recovers classical IIA, and $\sigma_{U}>0$ recovers classical Unanimity, allowing a quantitative restatement of Arrow's Theorem. In the empirical part of the paper, we test these metrics on two kinds of data: a set of over 1000 ranked choice preference profiles from Scottish local elections, and a batch of synthetic preference profiles generated with a Bradley-Terry-type model. We use those to investigate four positional voting rules-Plurality, 2-Approval, 3-Approval, and the Borda rule-as well as the iterative rule known as Single Transferable Vote (STV). The Borda rule consistently receives the highest $\sigma_{IIA}$ and $\sigma_{U}$ scores across observed and synthetic elections. This compares interestingly with a recent result of Maskin showing that weakening IIA to include voter preference intensity uniquely selects Borda.