politicsanalysisWikipedia1950s
Single-winner elections tend to produce two main parties, a pattern political scientists call Duverger's law
Duverger's law is the observation that single-member districts with plurality voting, where the candidate with the most votes wins, tend to produce two-party competition. The French political scientist Maurice Duverger said this happens for two reasons. The mechanical effect is that smaller parties struggle to win seats, so their support is wasted. The psychological effect is that voters and parties coordinate around the candidates they see as most viable. Duverger also proposed that proportional representation tends to support many parties. He stressed these are tendencies rather than fixed rules, since district size, thresholds and geography all change the result.
In a single-winner district, a voter who backs a third party may split the vote and help a rival they dislike more. Over time, voters drop small parties and parties merge or fold into larger ones to avoid losing. Proportional systems reward each party with seats in line with its vote share, which removes that pressure and lets smaller parties survive.
The law helps explain why countries with first-past-the-post systems often have two dominant parties, while those with proportional systems have coalitions. It matters for debates about electoral reform because the rule shapes which parties can win power and how voters choose.
Similar patterns appear across democracies: the United Kingdom and United States have long been dominated by two parties under plurality voting, while many European countries with proportional systems govern through coalitions. Electoral reforms, such as changes to district size or thresholds, have repeatedly shifted these tendencies. Duverger's own writing in the 1950s set the terms that political scientists still use.
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