What is "two-party system"

n. The availability or dominance of two political parties in a government

A two-party system is a party system where two major political parties dominate politics within a government. One of the two parties typically holds a majority in the legislature and is usually referred to as the majority or governing party while the other is the minority or opposition party. Around the world, the term has different senses. For example, in the United States, Jamaica, and Malta, the sense of two party system describes an arrangement in which all or nearly all elected officials belong to one of the only two major parties, and third parties rarely win any seats in the legislature. In such arrangements, two-party systems are thought to result from various factors like winner takes all election rules. In such systems, while chances for third party candidates winning election to major national office are remote, it is possible for groups within the larger parties, or in opposition to one or both of them, to exert influence on the two major parties. In contrast, in the United Kingdom and Australia and in other parliamentary systems and elsewhere, the term two-party system is sometimes used to indicate an arrangement in which two major parties dominate elections but in which there are viable third parties which do win seats in the legislature, and in which the two major parties exert proportionately greater influence than their percentage of votes would suggest.

The reasons why a country with free elections will evolve into a two-party system have been debated. A leading theory, referred to as Duverger's law, states that two parties are a natural result of a winner-take-all voting system. Consider a hypothetical race with three politicians running for an office. Two happen to be identical twins who promote policies that are different, but not by much. The third person holds very different views on almost all issues. The outcome of the election is that the twins get 30% of the vote each, and the third candidate wins with 40% of the vote. When it comes to re-election, all three decide to run again, but this time the people who had voted the 30%+30% have learned their lesson that their intentions can be more powerful if they band together by forming a party where they hold a primary election to select which of the twins they will send to go up against the incumbent. This time the selected twin wins with 60% of the vote, and a two-party system has emerged.