Spoiler Susceptibility in Party Elections
Daria Boratyn, Wojciech S{\l}omczy\'nski, Dariusz Stolicki, Stanis{\l}aw Szufa

TL;DR
This paper extends the concept of spoiler effects to party elections, characterizes spoiler-proof rules, and introduces a measure to compare the susceptibility of various electoral rules to spoilers.
Contribution
It generalizes spoiler effects to party elections, characterizes spoiler-proof rules for zero-sum methods, and develops a measure to compare rules' spoiler susceptibility.
Findings
Identity is spoiler-proof for seats-votes functions.
Spoiler impact varies across different electoral rules.
Extended probabilistic models for multidistrict party elections.
Abstract
An electoral spoiler is usually defined as a losing candidate whose removal would affect the outcome by changing the winner. So far, spoiler effects have been analyzed primarily for single-winner electoral systems. We consider this subject in the context of party elections, where there is no longer a sharp distinction between winners and losers. Hence, we propose a more general definition, under which a party is a spoiler if their elimination causes any other party's share in the outcome to decrease. We characterize spoiler-proof electoral allocation rules for zero-sum voting methods. In particular, we prove that for seats-votes functions only identity is spoiler-proof. However, even if spoilers are unavoidable under common electoral rules, their expected impact can vary depending on the rule. Hence, we introduce a measure of spoilership, which allows us to experimentally compare a…
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Taxonomy
TopicsGame Theory and Voting Systems · Electoral Systems and Political Participation
