Instant-runoff voting in the context of "Ranked voting"

The two-round system (TRS or 2RS), sometimes called ballotage, top-two runoff, or two-round plurality, is a single-winner electoral system which aims to elect a member who has support of the majority of voters. The two-round system involves two rounds of choose-one voting, where the voter marks a single favorite candidate in each round. The two candidates with the most votes in the first round move on to a second election (a second round of voting). The two-round system is in the family of plurality voting systems that also includes single-round plurality (FPP). Like instant-runoff (ranked-choice) voting and first past the post, it elects one winner.

The two-round system first emerged in France and has since become the most common single-winner electoral system worldwide. Despite this, runoff-based rules like the two-round system and RCV have faced criticism from social choice theorists as a result of their susceptibility to center squeeze (a kind of spoiler effect favoring extremists) and the no-show paradox. This has led to the rise of electoral reform movements which seek to replace the two-round system with other systems like rated voting, particularly in France.

Ranked-choice voting (RCV) can refer to one of several ranked voting methods used in some cities and states in the United States. The term is not strictly defined, but most often refers to instant-runoff voting (IRV) or single transferable vote (STV), the main difference being whether only one winner or multiple winners are elected. At the federal and state level, instant-runoff voting is used for congressional and presidential elections in Maine; state, congressional, and presidential general elections in Alaska; and special congressional elections in Hawaii. Since 2025, it is also used for all elections in the District of Columbia.

Single transferable voting, only possible in multi-winner contests, is not currently used in state or congressional elections. It is used to elect city councillors in Portland, Oregon, Cambridge, Mass., and several other cities.

The politics of Australia has a mild two-party system, with two dominant political groupings in the Australian political system, the Australian Labor Party and the Liberal Party of Australia. Federally, 13 of the 150 members of the lower house (Members of Parliament, or MPs) are not members of major parties, as well as 20 of the 76 members of the upper house (senators).

The Parliament of Australia has a number of distinctive features including compulsory voting, with full-preference instant-runoff voting in single-member seats to elect the lower house, the Australian House of Representatives, and the use of the single transferable vote to elect the upper house, the Australian Senate.

A center squeeze is a kind of spoiler effect shared by rules like the two-round system, plurality-with-primaries, and ranked choice voting. In a center squeeze, the majority-preferred and socially optimal candidate is eliminated in favor of a more extreme alternative in an early round, while there are still spoilers. Systems with center squeeze are sometimes called centrifugal ("center-fleeing") because they encourage political polarization.

Candidates focused on appealing to a small base of core supporters can "squeeze" broadly-popular candidates trapped between them out of the race, by splitting the first-preference votes needed to survive earlier rounds. This effect was first predicted by social choice theorists in the 1940s and 50s, and has since been documented in various countries using plurality-style electoral systems.

In social choice theory and politics, a spoiler effect happens when a losing candidate affects the results of an election simply by participating. Voting rules that are not affected by spoilers are said to be spoilerproof and satisfy independence of irrelevant alternatives.

The frequency and severity of spoiler effects depends substantially on the voting method. First-past-the-post voting without winnowing or primary elections is sensitive to spoilers. And so, to a degree, are Instant-runoff or ranked-choice voting (RCV) and the two-round system (TRS). Majority-rule (or Condorcet) methods are only rarely affected by spoilers, which are limited to rare situations called cyclic ties. Rated voting systems are not subject to Arrow's theorem, allowing them to be spoilerproof so long as voters' ratings are consistent across elections.

A first-preference is a voter's most-preferred candidate. In certain ranked systems such as first preference plurality, ranked-choice voting (RCV), and the single transferable vote, first preferences for a candidate are considered most important and prioritized heavily. This incentivizes pandering to the political base or "core support" as a result of the center squeeze effect. Methods like Condorcet voting, rated voting, and the Borda count do not exhibit such effects. Methods like anti-plurality voting and Coombs' method have the opposite effect, being dominated by a voter's bottom rankings and so tending to elect the "least offensive" candidates.

First-preference votes are used by psephologists and the print and broadcast media to broadly describe the state of the parties at elections and the swing between elections. The term is much-used in Australian politics, where ranked voting has been universal at federal, state, and local levels since the 1920s.