Spoiler Susceptibility in Party Elections

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 number of multiwinner social choice rules according to their spoiler susceptibility. Since the probabilistic models used in COMSOC have been developed for nonparty elections, we extend them to generate multidistrict party elections.