The spoiler effect is the effect of vote splitting between candidates or ballot questions who often have similar ideologies. One spoiler candidate's presence in the election draws votes from a major candidate with similar politics, thereby causing a strong opponent of both or several to win. The minor candidate causing this effect is referred to as a spoiler. However, short of any electoral fraud, this presents no grounds for a legal challenge.
The spoiler effect is a problem in plurality voting systems because they enable a candidate to win with less than half of the vote.
The spoiler effect can broadly be thought of as a situation where, when a particular candidate or set of candidate(s) are running, some candidate not in the set (call him X) wins, but when that particular set of candidate(s) aren't running, some different candidate who's also not in the set wins (call him Y). The Independence of irrelevant alternatives article has more information on this broad interpretation of the spoiler effect.
In some sense, much of voting theory is really just an attempt to mitigate the effects of various things that might be considered "spoiler effects". For example, Condorcet methods and many rated methods attempt to elect a candidate who can beat all other candidates in a head-to-head matchup (assuming voters cast the same ballots no matter which candidates are in the race); this arguably reduces the ability of losing candidates to drop out to impact the race.
The situation where there are only two or fewer candidates competing for a single seat i.e. to be the single winner (or more generally, any time there are N + 1 or fewer candidates competing for N seats) is often compared to the situation where there are more than two (or more than N + 1) candidates when discussing the spoiler effect. For example, majority rule guarantees a winner or a tie when there are two or fewer candidates, but when there are three or more, it is possible not to have a "majority winner" for any straightforward generalization of majority rule (i.e. maybe no candidate has a majority of 1st choices, or no candidate is a beats-all Condorcet winner, etc.), and moreover, in the event of a Condorcet cycle, if certain candidates in the cycle drop out, the winner can change.
- Examples are the first past the post electoral system and in the single transferable vote or similar systems that chooses what candidate to elect based on first-preference votes.
- Buchler, Justin (2011-04-20). Hiring and Firing Public Officials: Rethinking the Purpose of Elections. Oxford University Press, USA. ISBN 9780199759965.
a spoiler effect occurs when entry by a third-party candidate causes party A to defeat party B even though Party B would have won in a two-candidate race.
- King, Bridgett A.; Hale, Kathleen (2016-07-11). Why Don't Americans Vote? Causes and Consequences: Causes and Consequences. ABC-CLIO. ISBN 9781440841163.
Those votes that are cast for minor party candidates are perceived as taking away pivotal votes from major party candidates. ... This phenomenon is known as the 'spoiler effect'.
- Borgers, Christoph (2010-01-01). Mathematics of Social Choice: Voting, Compensation, and Division. SIAM. ISBN 9780898716955.
Candidates C and D spoiled the election for B ... With them in the running, A won, whereas without them in the running, B would have won. ... Instant runoff voting ... does not do away with the spoiler problem entirely, although it ... makes it less likely
- Heckelman, Jac C.; Miller, Nicholas R. (2015-12-18). Handbook of Social Choice and Voting. Edward Elgar Publishing. ISBN 9781783470730.
A spoiler effect occurs when a single party or a candidate entering an election changes the outcome to favor a different candidate.
- A term designed to appeal to a wider section of the public as a result of the widespread, often national support of political parties.