Equilibrium Computation in the Hotelling-Downs Model of Spatial Competition

TL;DR

This paper develops algorithms to compute pure Nash equilibria in the Hotelling-Downs model of spatial competition, addressing a key computational gap in understanding strategic candidate positioning.

Contribution

It introduces three algorithms for finding pure Nash equilibria in various versions of the model, including continuous and discrete voter distributions and candidate positions.

Findings

Algorithms can compute exact or near-exact equilibria.

Applicable to both continuous and discrete voter distributions.

Potential to extend to more complex models.

Abstract

The Hotelling-Downs model is a natural and appealing model for understanding strategic positioning by candidates in elections. In this model, voters are distributed on a line, representing their ideological position on an issue. Each candidate then chooses as a strategy a position on the line to maximize her vote share. Each voter votes for the nearest candidate, closest to their ideological position. This sets up a game between the candidates, and we study pure Nash equilibria in this game. The model and its variants are an important tool in political economics, and are studied widely in computational social choice as well. Despite the interest and practical relevance, most prior work focuses on the existence and properties of pure Nash equilibria in this model, ignoring computational issues. Our work gives algorithms for computing pure Nash equilibria in the basic model. We give three algorithms, depending on whether the distribution of voters is continuous or discrete, and similarly, whether the possible candidate positions are continuous or discrete. In each case, our algorithms return either an exact equilibrium or one arbitrarily close to exact, assuming existence. We believe our work will be useful, and may prompt interest, in computing equilibria in the wide variety of extensions of the basic model as well.