Single non-transferable vote

Unlike block voting or limited voting, where each voter casts multiple votes (multiple non-transferable vote (MNTV)), under SNTV each voter casts just one vote. This usually produces semi-proportional representation at the district level, meaning small parties, as well as large parties, have a chance to be represented. Single transferable vote (STV) is a more proportional alternative to SNTV. Under STV, ranked voting allows unused votes (placed on winners or losers) to be transferred to other candidates.

Proportional representation[edit]

SNTV facilitates minority representation, that is, it produces mixed representation of large and small parties where no party takes all the seats.^[2]


In fact, SNTV would elect the same people as are elected in STV contests where the vote transfers do not move an initially-lower-placing candidate over an initially-higher-placing one. It is common even after STV vote transfers to elect the same as would be elected under SNTV. But not having transfers, SNTV sees more votes wasted than under STV due to votes being placed on un-electable candidate or due to surpluses received by successful candidate over and above the quota used in STV elections that are not able to be transferred under SNTV.^[3]


SNTV produces representation that is most proportional (proportional representation) when political parties have accurate information about their relative levels of electoral support, and nominate candidates in accordance with their respective levels of electoral support or when all parties suffer from poor information of that sort. Knowing the portion of the votes a party can take allows it to avoid vote waste due to lessening the chance of vote splitting and inefficient placement of party support. Under 'perfect' tactical voting and strategic nomination, SNTV would be equivalent to the D'Hondt (Jefferson) method of proportional representation.^[4]


But under SNTV even inefficient distribution of votes allows more balanced representation than would be elected under either single-member plurality or Block voting.


Given ${\displaystyle n}$ candidates to be elected, Candidate A can guarantee success by receiving one more than ${\displaystyle {\frac {1}{n+1}}}$ of the votes (the Droop quota), because ${\displaystyle n}$-1 other candidates cannot each receive more than Candidate A.


But as SNTV is a plurality system, it is possible to win with less than that quota. To determine the successful candidates, candidates' vote tallies are compared with the vote tallies of others, not with a theoretical threshold. In the 2020 Vanuatuan general election, using SNTV, as few as 5 percent of the vote was enough to be elected in a seven-seat district.^[5]


Vote spitting due to poor information on voters' behaviour may deny a popular party its due share of representation. (Single Transferable Voting does not suffer from this handicap as votes are transferable and many are transferred and used that are wasted under SNTV.) Parties organizing slates of multiple candidates may nominate many candidates and then learn on election night that the party was not as popular as they thought.


If every party does that, all suffer the same inefficiency and the final result is proportional. If one party is more prudent, it may do better than the others. Because votes cannot be transferred, there is more chance of vote wastage than under STV.


But in elections that use SNTV, representation is usually mixed. It is rare for one party to make a sweep of a city's seats, a thing common in First past the post elections. The number of wasted votes in an SNTV election is generally lower than in First past the post elections as well.^[6][7]


Under SNTV, parties often do not receive representation exactly proportional to their strength, because it is difficult to accurately judge their strength when deciding how many candidates to field (strategic nomination) and difficult to direct party supporters as a whole to spread their votes efficiently. If they field too many, supporters' votes might be split across too many candidates. The party votes might spread their vote numbers to the point where all of a party's candidates lose to a less thinly spread opposing party.


If a party fields too few candidates, they might elect all their candidates but not win seats proportional to their level of support, and the winning candidates would have more support than necessary and thus wasting votes.


The risks of poor strategic nomination are not equal for parties of various strengths. A large party would have much more to lose from the split vote effect than to gain from avoiding the wasted vote effect, and so would likely decide to err on the side of fielding fewer candidates (but probably not less than their existing number of seats). A small party with little representation would be more risk-tolerant and err on the side of too many candidates, hoping to gain as many seats as possible, perhaps even winning more than its proportion of the electorate if they can edge out candidates from larger parties with just a few votes. As well, a small party running just one candidate would not suffer from vote spitting, while a larger party running four or more may suffer from that.


SNTV electoral systems, like STV and proportional electoral systems generally, typically produce more proportional electoral outcomes as the size of the electoral districts (number of seats in each constituency) increases.^[8]

Potential for tactical voting[edit]

The potential for tactical voting in a single non-transferable vote system is large. Casting only one vote, a rational voter wanting to maximize the number of seats captured by his party should vote for a candidate of the party that has a chance of winning, but one that will not win by too great a margin and thus take votes away from party colleagues. This creates opportunities for tactical nominations, with parties nominating candidates similar to their opponents' candidates in order to split the vote. Like all multiple-winner selections, parties find it advantageous to run a range of candidates in SNTV elections.


SNTV has been measured through the lens of such concepts as decision-theoretic analysis. Professor Gary W. Cox, an expert on SNTV, has studied this system's use in Japan.^[9] Cox has an explanation of real-world data finding the, "two systems [plurality and semi-proportional] are alike in their strategic voting equilibria."^[9] His research found that voters use the information offered in campaigns (polls, reporting, fundraising totals, endorsements, etc.), to rationally decide who the most viable candidates are and then vote for them.


SNTV can result in complicated intra-party dynamics because in a SNTV system, a candidate runs against candidates from their own party as well as against candidates from the other party. SNTV elections are not zero-sum contests. Just because one particular candidate is elected does not mean that another specific candidate will not be. They both can be elected.


Because running on issues may lead to a situation in which a candidate becomes too popular and therefore draws votes away from other allied candidates, SNTV may encourage legislators to join factions that consist of patron-client relationships in which a powerful legislator can apportion votes to his or her supporters.


In addition, parties will do best if 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.