Combatting Gerrymandering with Ranked Choice Voting: An experimental analysis of Multi-member Districts in the United States

TL;DR

This paper explores the use of multi-member districts with ranked choice voting to reduce gerrymandering and improve proportionality in U.S. elections, demonstrating promising results through large-scale simulations.

Contribution

It introduces a computational framework for analyzing multi-member districts with various voting rules, showing potential to enhance fairness and reduce partisan gerrymandering.

Findings

Three-member districts with Single Transferable Vote improve proportionality.

Fairness-minded commissions can achieve proportional outcomes in all states.

Partisan gerrymandering is significantly curtailed with MMDs and ranked choice voting.

Abstract

Every representative democracy must specify a mechanism under which voters choose their representatives. The most common mechanism in the United States -- Winner takes all single-member districts -- both enables substantial partisan gerrymandering and constrains `fair' redistricting, preventing proportional representation in legislatures. We study the design of \textit{multi-member districts (MMDs)}, in which each district elects multiple representatives, potentially through a non-Winner takes all voting rule. We carry out large-scale empirical analyses for the U.S. House of Representatives under MMDs with different social choice functions, under algorithmically generated maps optimized for either partisan benefit or proportionality. Doing so requires efficiently incorporating predicted partisan outcomes -- under various multi-winner social choice functions -- into an algorithm that optimizes over an ensemble of maps. We find that with three-member districts using Single Transferable Vote, fairness-minded independent commissions would be able to achieve proportional outcomes in every state up to rounding, \textit{and} advantage-seeking partisans would have their power to gerrymander significantly curtailed. Simultaneously, such districts would preserve geographic cohesion. Through simulation, we find that the insights are robust to cross-party voting. In the process, we advance a rich research agenda at the intersection of social choice and computational gerrymandering.