- Spoiler effect
The spoiler effect describes the effect a minor party candidate with little chance of winning has in a close election, when that candidate's presence in the election draws votes from a major candidate similar to them, thereby causing a candidate dissimilar to them to win the election. The minor candidate causing this effect is often referred to as a spoiler.
If preferential ballots are not used, the spoiler candidate takes votes away from the viable similar candidate (an effect called vote splitting). In some cases, even though the spoiler candidate cannot win themselves, their influence upon the voters may allow them to deliberately determine which of the viable candidates wins the election—a situation known as a kingmaker scenario.
In a preferential voting system, a voter can vote for a minor party candidate as their first choice, and in addition, they can record a preference between the remaining candidates, whether they are in a "major party" or not. For example, voters for a very left-wing candidate might select a moderately left-wing candidate as their second choice, thus minimizing the chances that their vote will result in the election of a right-wing candidate. Approval voting can also reduce the impact of the "spoiler effect".
Possible mathematical definitions for the spoiler effect include failure of the independence of irrelevant alternatives (IIA) axiom, and vote splitting.
Arrow's impossibility theorem states 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 independence of Smith-dominated alternatives (ISDA) criterion is similar to IIA; unlike IIA, some ranked-ballot voting methods can pass ISDA.
The spoiler effect in American elections
Bush, Gore, and Nader (2000 U.S. presidential election)
The 2000 U.S. Presidential election is often cited as an example of the spoiler effect. In that election, Al Gore, the Democratic candidate, received more popular votes than George W. Bush, the Republican candidate, but lost in the electoral college. In the state of Florida, the final certified vote count showed Bush with just 537 more votes than Gore. Because Bush defeated Gore in Florida, he won the state, received more votes in the electoral college, and became president of the United States.
Gore supporters pointed out that, had candidate Ralph Nader, a liberal, not run in the election, the majority of the 97,421 votes he received in Florida would have been cast for Gore. Gore supporters contend that Nader's candidacy spoiled the election for Gore by taking away enough votes from Gore in Florida to swing the election to Bush. Their argument is bolstered by a poll of Nader voters, asking them for whom they would have voted had Nader not run, which said 45 percent of Nader voters would have voted for Gore, 27 percent would have voted for Bush, and the rest would not have voted.
Nader himself and many of his supporters argued that most Nader voters would have chosen another minor party candidate, or abstained from voting, had he not been on the ballot. Some observers began to refer to the spoiler effect as the "Nader effect" after the 2000 election.  
Other alleged spoilers
These are third-party candidates who have been accused of denying victory to a major nominee:
Spoiler candidate Election "Denied victory" to Winning candidate Charles Cotesworth Pinckney 1800 John Adams Thomas Jefferson James Birney 1844 Henry Clay James Knox Polk Martin van Buren 1848 Lewis Cass Zachary Taylor Peter Cooper 1876 Samuel J. Tilden Rutherford B. Hayes John St. John 1884 James G. Blaine Grover Cleveland Theodore Roosevelt 1912 William Howard Taft Woodrow Wilson George Wallace 1968 Hubert Humphrey Richard Nixon Ross Perot 1992 George H. W. Bush Bill Clinton Ross Perot 1996 Bob Dole Bill Clinton Ralph Nader 2000 Al Gore George W. Bush
- In 1994 moderate Republican Marshall Coleman ran for the U.S. Senate as an independent, receiving over 11 percent of the vote in an election where Democrat Chuck Robb defeated Republican nominee Oliver North by only three percent of the vote.
- As a result of the 2011 Wisconsin protests and subsequent recall elections, the Wisconsin Republican Party is openly encouraging spoiler candidates to run in the recall elections on the Democrat ticket in order to force the Democrats into a Primary election. Republicans argue that this will even the playing field in the recalls, as incumbents facing recall will not have the time to campaign due to their work load in the state senate.
The spoiler effect has also been seen in other countries, such as Trinidad and Tobago. There, a minority party with no chance of winning, Congress of the People, took votes from its former party, the United National Congress and made it lose seats, thus allowing the incumbent People's National Movement, highly disfavoured for the elections, to take the Government role again.
In New Zealand, there have been two notable cases of the spoiler effect. In the 1984 general election, the free-market New Zealand Party deliberately ran for office in order to weaken support for the incumbent Prime Minister Robert Muldoon. Later on, the 1993 general election saw the New Zealand Labour Party's vote split by The Alliance, which has been attributed to the vagaries of the first past the post electoral system.
In sports, the "spoiler effect" refers to a similar phenomenon, in which a team or individual has failed to win enough games or competitions to make the playoffs or qualification rounds like the finals, but affects the playoffs or finals anyway by beating a more successful team or individual before the end of the season. For example, a baseball team that is ten games out of contention for a playoff berth could defeat a team that has a playoff berth several times. This could cause the would-be playoff team to be passed by in the rankings by the team directly behind it before the final positions at the end of the season are determined.
In individual participant sports, such as automobile racing, a racer with no hope of obtaining a championship title could prevent a racer with a chance at the title by defeating them, preventing the contending racer from earning critical points toward winning the title. Instead, the title would go to the contender directly behind him in the rankings, provided that second-tier racer is close enough to surpass and they win their own competition.
- ^ Public Disclosure Division (December 2001). "2000 Official Presidential General Election Results". Federal Election Commission. http://www.fec.gov/pubrec/2000presgeresults.htm. Retrieved August 30, 2010.
- ^ Rosenbaum, David E. (February 24, 2004). "Relax, Nader Advises Alarmed Democrats, but the 2000 Math Counsels Otherwise". The New York Times (New York). http://query.nytimes.com/gst/fullpage.html?res=9B03E4D6173CF937A15751C0A9629C8B63. Retrieved August 30, 2010.
- ^ Bacon Jr., Karen; Tumulty (May 31, 2004). "The Nader Effect". Time. http://www.time.com/time/magazine/article/0,9171,994287,00.html. Retrieved August 30, 2010.
- ^ Kuhn, David Paul (February 23, 2004). "The Nader Effect". CBSNews.com. CBS News. http://www.cbsnews.com/stories/2004/02/23/politics/main601802.shtml. Retrieved August 30, 2010.
- ^ Cook, Charlie (March 9, 2004). "The Next Nader Effect". The New York Times. http://www.nytimes.com/2004/03/09/opinion/the-next-nader-effect.html. Retrieved August 30, 2010.
- ^ http://host.madison.com/wsj/news/local/govt-and-politics/elections/article_df8f9e60-9056-11e0-8101-001cc4c03286.html
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