Single non-transferable vote
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The Single Non-Transferable Vote or SNTV is an electoral system used in multi-member constituency elections.
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Voting
In any particular legislative constituency election, each voter casts one vote for one candidate in a multi-candidate race for multiple legislative seats. Those candidates receiving the most votes win office. Thus, in a four seat constituency, the four candidates receiving the largest numbers of votes individually would win office.
An example
Three seats to be filled, five candidates: Yamada, Sato, Ishibashi, Morimoto, and Ashida.
- 100 Voters choose Yamada
- 1500 voters choose Sato
- 700 voters choose Ishibashi
- 8000 voters choose Morimoto
- 1300 voters choose Ashida
Sato, Morimoto and Ashida are the winning candidates.
Proportional representation
SNTV may result in roughly proportional representation only when political parties have accurate information about their relative levels of electoral support and nominate candidates in accordance with their respective level of electoral support. If there are n candidates to be elected, Candidate A can guarantee being elected by receiving one more than 1/(n+1) of the votes (the Droop quota), because n other candidates cannot all receive more than Candidate A. However, it can become very difficult for parties to receive representation proportional to their strength, because they are forced to judge their strength prior to deciding how many candidates to field (strategic nomination). If they field too many, their supporters votes might be split across too many candidates, evenly diluting their share to the point where they all lose to a slightly less diluted opposing party. Conversely, if the party fields too few candidates, then they might not win seats proportional to their hypothetical true level of support and excess votes would be wasted on their winning candidates.
The relative risks of strategic nomination are typically not the same for parties in different positions of electoral success. A large party with a majority of seats, for example, would have much more to lose from the split vote effect than they would have to gain from avoiding the wasted vote effect, and so would likely decide to err on the side of fielding fewer candidates. Conversely, a small party with little representation would be more risk-tolerant and err on the side of too many candidates, potentially gaining seats greater than their proportion of the electorate by winning with narrower margins of victory than the candidates from larger parties.
SNTV electoral systems typically produce more proportional electoral outcomes as the size of the electoral districts (number of seats in each constituency) increases.
Potential for tactical voting
The potential for tactical voting is large. Receiving only one vote, the rational voter must only vote for a candidate that has a chance of winning, but will not win by too great a margin. This also creates a gigantic opportunity for tactical nominations, with parties nominating candidates similar to their opponents' candidates in order to split the vote.
SNTV also results in complicated intra-party dynamics because in a SNTV system, a candidate must not only run against candidates from the other party, he or she must also run against candidates from their own party.
Because running on issues may lead to a situation in which a candidate becomes too popular and therefore steals votes away from other allied candidates, it has been argued that SNTV encourages legislators to join factions which consist of patron-client relationships in which a powerful legislator can apportion votes to his or her supporters. It has been argued that many of the characteristics of the Kuomintang in Taiwan and the Liberal Democratic Party of Japan arise because of this.
In addition, parties must ensure that their supporters evenly distribute their votes among the party's candidates. Historically, in Taiwan, the Kuomintang did this by sending members a letter telling them which candidate to vote for. With the Democratic Progressive Party, vote sharing is done informally, as members of a family or small group will coordinate their votes. The New Party had a surprisingly effective system by asking party supporters to vote for the candidate that corresponded to their birthdate. This led to a system of vote allocation which had been adopted by all parties for the 2004 ROC Legislative elections.
Puerto Rico
In Puerto Rico, where SNTV is known as at-large representation ("representación por acumulación" in Spanish), political parties vary the ballot order of their candidates across electoral divisions, in order to ensure each candidate has a roughly equal chance of being elected. Since most voters choose the candidates placed at the top of their party lists on the ballots they receive, at-large candidates from the same party usually obtain approximately equal vote totals.
The two major Puerto Rican political parties, the Popular Democratic Party and the New Progressive Party, usually nominate six candidates for each chamber, while the much smaller Puerto Rican Independence Party runs single-candidate slates for both the Senate and the House of Representatives. However, the overall distribution of legislative seats is largely determined by the results for the sixteen Senate and forty House district seats, elected by Plurality voting.
Taiwan
In the Republic of China, the party structure is further complicated by the fact that while members of the Legislative Yuan and municipal assemblies are elected by SNTV, executive positions are elected by a First Past the Post. This has created a party system in which smaller factionalized parties, which SNTV promotes, have formed two large coalitions that resembles the two party system which First Past the Post rewards.
Japan and South Korea
SNTV was once but is no longer used to elect the parliaments of Japan and South Korea. It is still used for prefectural assemblies and municipal assemblies in Japan.