Spoiler effect
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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.
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 winning just 537 more votes than Gore, thus winning the state and the Presidency (see Florida election results). Many Gore supporters contended that many 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 (though Nader himself argued otherwise). They contend that Nader's candidacy "spoiled" the election for Gore, by taking away enough votes from Gore in Florida and many other states to allow Bush to win.
Different voting systems are affected to a greater or lesser extent by the spoiler effect: in First Past the Post elections, the spoiler effect can be a defining feature of campaigns, whereas the Single transferable vote and most Condorcet methods are barely affected by it.
A voting system which satisfies the independence of irrelevant alternatives criterion is immune to the spoiler effect, but Arrow's impossibility theorem shows that rank-voting systems are unable to satisfy this criterion without exhibiting other undesirable properties as a consequence. Satisfaction of the independence of clone alternatives or local independence of irrelevant alternatives criteria also tend to indicate that a voting system is less susceptible to this effect.
Tactical voting, strategic nomination, and vote swapping are all common responses to the spoiler effect.
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.
List of American Spoilers (third-party candidates who feasibly could have denied victory to a major nominee)
- James Birney, 1844
- Martin van Buren, 1848
- Theodore Roosevelt, 1912
- Henry Ross Perot, 1992
- Ralph Nader, 2000