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 him/her, thereby causing the opposing candidate to win. The minor candidate causing this effect is often referred to as a spoiler.

If ranked 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".

One of the main functions of political parties is to mitigate the effect of spoiler-prone voting methods by winnowing the contenders before the election. Each party nominates at most one candidate per office since each party expects to lose if they nominate more than one. (In some cases, a party can expect to "lose" if they nominate more than zero, since nominating one can cause spoiling when there are three or more parties; the party may prefer the candidate who would win if the party nominates zero.) For example, if the Democrats had nominated both Hillary Clinton and Barack Obama for U.S. President in 2008, it would have allowed the Republican candidate (John McCain) to easily win; the voters who preferred both Clinton and Obama over McCain could not have been relied on to solve the strategy coordination problem on their own. Thus, empirical observations of the frequency of spoiled elections do not provide a good measure of how prone to spoiling the voting method is, since the observations omit the relevant information about potential candidates (e.g., Hillary Clinton) who did not run because they did not want to spoil the election.