Participation Incentives in Approval-Based Committee Elections

TL;DR

This paper investigates voter participation incentives in approval-based committee voting, revealing that scoring rules generally promote participation while many sequential rules do not, though some conditions can mitigate this issue.

Contribution

It is the first study to analyze participation incentives in ABC voting rules, highlighting differences between scoring and sequential rules and exploring conditions for participation.

Findings

All ABC scoring rules satisfy group participation.

Most sequential rules fail participation, but some satisfy it under specific conditions.

Deciding whether abstaining benefits a voter is NP-hard.

Abstract

In approval-based committee (ABC) voting, the goal is to choose a subset of predefined size of the candidates based on the voters' approval preferences over the candidates. While this problem has attracted significant attention in recent years, the incentives for voters to participate in an election for a given ABC voting rule have been neglected so far. This paper is thus the first to explicitly study this property, typically called participation, for ABC voting rules. In particular, we show that all ABC scoring rules even satisfy group participation, whereas most sequential rules severely fail participation. We furthermore explore several escape routes to the impossibility for sequential ABC voting rules: we prove for many sequential rules that (i) they satisfy participation on laminar profiles, (ii) voters who approve none of the elected candidates cannot benefit by abstaining, and (iii) it is NP-hard for a voter to decide whether she benefits from abstaining.