The law and the hypothesis
Duverger argued that electoral rules shape party systems and made two claims. First, the simple-majority single-ballot system (single-member plurality) favours a two-party system (the law). Second, proportional representation and two-round runoff systems favour multipartism (the hypothesis).
The mechanical effect
Because only the first-placed candidate in each district wins a seat, a third party with evenly spread support that rarely comes first gets far fewer seats than its vote share. The rule itself favours large parties when votes are turned into seats.
The psychological effect
Voters who expect a minor party to lose see a vote for it as wasted and vote strategically for the less disliked major party. Politicians and donors also avoid small parties, so support concentrates on two.
Refinements and criticism
- Riker (1982) traced the history of the law and stressed that convergence on two candidates happens at district level, not necessarily nationally.
- Cox (1997) generalised it as the "M+1 rule": with district magnitude M, the number of viable candidates tends towards M+1.
- Exceptions: where regional parties are strong, as in Canada and India, plurality voting still yields national multipartism, so many scholars treat it as a tendency rather than a strict law.
Applying it to Korea
Korean National Assembly elections use a mixed system of single-member districts plus proportional seats. District seats dominate, and the strong two-major-party pattern is often cited as an example of Duverger's law. For the wider constitutional setting, see presidential and parliamentary systems and divided government.
References
- Duverger, M. (1954). Political Parties: Their Organization and Activity in the Modern State (B. North & R. North, Trans.). Methuen. (Original work published 1951)
- Riker, W. H. (1982). The two-party system and Duverger's law: An essay on the history of political science. American Political Science Review, 76(4), 753–766.
- Cox, G. W. (1997). Making Votes Count: Strategic Coordination in the World's Electoral Systems. Cambridge University Press.
- Lijphart, A. (1994). Electoral Systems and Party Systems. Oxford University Press.