Multiwinner Elections and the Spoiler Effect

David McCune; Jennifer Wilson

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arXiv · 2403.03228

Multiwinner Elections and the Spoiler Effect

David McCune·Jennifer Wilson

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Abstract

In the popular debate over the use of ranked-choice voting, it is often claimed that the method of single transferable vote (STV) is immune or mostly immune to the so-called ``spoiler effect,'' where the removal of a losing candidate changes the set of winners. This claim has previously been studied only in the single-winner case. We investigate how susceptible STV is to the spoiler effect in multiwinner elections, where the output of the voting method is a committee of size at least two. To evaluate STV we compare it to numerous other voting methods including single non-transferable vote, $k$-Borda, and the Chamberlin-Courant rule. We provide simulation results under three different random models and empirical results using a large database of real-world multiwinner political elections from Scotland. Our results show that STV is not spoiler-proof in any meaningful sense in the multiwinner context, but it tends to perform well relative to other methods, especially when using real-world ballot data.

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David McCune, Jennifer Wilson. 2024-03-03. Multiwinner Elections and the Spoiler Effect. https://arxiv.org/abs/2403.03228

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