Committees and equilibria: Multiwinner approval voting through the lens of budgeting games

Approval-based multiwinner voting, one of the central topics in computational social choice, addresses collective decision-making scenarios in which n voters select a committee of k candidates from a larger pool of alternatives. A fundamental aim is to ensure that the elected committee proportionately represents the preferences of the electorate. Consequently, much effort has gone into exploring various proportionality notions and developing voting rules to achieve them. A key intuition underlying many fairness axioms and voting rules is that an optimal outcome is attained when no subset of voters can improve their position by reallocating their endorsements. In this paper, we formalize this intuition by defining a new class of games, which we call budgeting games, where committees occur as a result of voters' decisions about how to allocate a given budget. Our primary contribution lies in introducing this new class of normal-form games and showing that key notions in multiwinner voting theory, such as priceability, the core and EJR (Extended Justified Representation) can be thought of as equilibria of budgeting games. Remarkably, our budgeting games do not just capture existing concepts, but also give rise to entirely new families of voting rules. These rules, which are guaranteed to satisfy desirable fairness axioms, are based on improving-move dynamics in the respective budgeting games, and include the well-known Method of Equal Shares. Finally, we showcase the applicability of our game-theoretic perspective by proving existence of strong equilibria in a restricted version of our budgeting games, which implies that the core in a novel special case of multiwinner elections is non-empty.

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