Spoiler effect

The spoiler effect is a term to describe the effect a candidate can have on a close election, in which their candidacy results in the election being won by a candidate dissimilar to them, rather than a candidate similar to them.

It can also refer to a similar phenomenon in sports: when a team has failed to win enough games to make the playoffs, it can often affect the playoffs anyway, by beating a more successful team before the end of the season.

One often cited example of the spoiler effect at work was the 2000 U.S. Presidential election. In that election, George W. Bush and Al Gore had a very close election in many states, with neither candidate winning a majority of the votes. In Florida, the final certified vote totals show Bush won just 537 more votes than Gore, thus winning the state and the Presidency (see Florida election results). Many Gore supporters hypothesized that most of the 97,421 votes that went to Ralph Nader in that state would have likely been votes for Gore had Nader not been in the election. They contend that Nader's candidacy "spoiled" the election for Gore, by taking away enough votes from Gore in Florida and many other states (in particular, New Hampshire being the allegation most statistically supportable) only to allow Bush to win. Nader supporters say he had to run to protest Gore's positions, and the punishment of defeat is a powerful weapon that democracy allows and encourages. Without Nader on the ballot angry anti-Gore voters might as well have voted for Bush to punish Gore. However, Nader himself and many of his supporters argue that most Nader voters would have chosen another minor party candidate, or refrained from voting altogether, had he not been on the ballot. Some observers began to refer to the spoiler effect as the Nader effect after the 2000 election, but this term has been fading from use, particularly following Nader's much less significant showing in the 2004 election.

A similar effect was observed in 1992, when the conservative-republican vote split between George H.W. Bush and H. Ross Perot, allowing the more liberal Bill Clinton to take office with only forty-three percent of the popular vote.

The spoiler effect is one of the components contributing to the effect known as Duverger's law, which states that the first-past-the-post election system creates and preserves a two-party system.

The effect is removed in jurisdictions using preferential voting, where a voter can vote for a minor party candidate and still record a preference between the major party candidates. (In the Nader example above, Nader voters could have voted 1 Nader, 2 Gore, 3 Bush, and then once Nader was eliminated their votes would have transferred to Gore.)

Mathematical definitions

Possible mathematical definitions for the spoiler effect include failure of independence of irrelevant alternatives (IIA) and vote-splitting.

Independence of irrelevant alternatives

Arrow's impossibility theorem shows that rank-voting systems are unable to satisfy the independence of irrelevant alternatives criterion without exhibiting other undesirable properties as a consequence. However, different voting systems are affected to a greater or lesser extent by IIA failure. For example, instant runoff voting is considered to have less frequent IIA failure than First Past the Post. The local independence of irrelevant alternatives criterion is similar to IIA, but whch can be passed by some ranked ballot methods.

Vote-splitting

Voting methods that fail independence of clones may suffer from vote-splitting, teaming, or crowding. Vote-splitting happens when adding similar or clone candidates decreases the chance of any of them winning. Methods that suffer from vote-splitting include First Past the Post and two-round runoff.

List of American spoilers in presidential elections

These are third-party candidates who feasibly could have denied victory to a major nominee.

• Charles Cotesworth Pinckney, 1800
• James Birney, 1844
• Martin van Buren, 1848
• Theodore Roosevelt, 1912
• Henry Ross Perot, 1992 and 1996
• Ralph Nader, 2000

The spoiler effect also sometimes occurs in congressional elections and elections for state offices.

Split vote

When a spoiler effect occurs, it is sometimes said that a split vote has occurred.

Vote splitting as an issue is usually confined to single-winner voting systems such as those used by the United Kingdom, the United States and Canada. However, it can also be an issue in countries which use proportional representation with election thresholds such as Germany or Turkey - in these countries, "fringe" parties which do not meet the threshold can be seen to take away votes from larger parties with similar ideologies.