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Two-party system

A two-party system is a political system having a system of electoral rules which happens to ensure that all, or nearly all, elected offices are held only by the candidates of the two highest vote-getting parties. Two-party systems are generally not designed to be two-party systems, but tend to favor two major political parties as an unintended consequence of Duverger's law. Important rules are:

Such systems have evolved in the United States and the United Kingdom, and Canada, as well as in many small or newly independent countries such as Jamaica. While Americans and citizens of the UK often see the two-party system as natural, based on their long experiences with it, it is in fact a product of the particular rules in place. The two parties that dominate thus have an incentive to keep the rules as they are, so as to prevent electoral losses to smaller parties.

Defenders of the two-party system claim that it produces more stable governance than multiparty systems. Because uncommon ideas are non-influential, policies and governments do not change rapidly. The system allows major parties to co-opt uncommon ideas as they become more common. While smaller parties find this exceptionally frustrating, it enhances stability while allowing for ideas that gain favor to become politically influential.

Critics of the two-party system point out that stability may not be an advantage, since there are many stable democratic republics, such as Germany, that have some form of proportional representation. They argue that it leads to the following flaws: