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Why do single-member-district elections often produce two-party systems?

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Rubbing of Chinese philosopher Confucius in traditional robes with a long beard, based on a design attributed to Wu Tao-hsuan, with Chinese characters above and beside the figure. — political science Stone rubbing (19th century?) of Confucius from a design attributed to Wu Daoxuan (flourished 700–760 CE).

Single-member-district elections, often referred to as "first-past-the-post" or "winner-take-all" systems, frequently produce two-party systems because they structurally disadvantage smaller parties. In these districts, the candidate who receives the largest number of votes wins the seat, while parties that fail to secure a majority or plurality in a specific constituency receive no representation at all.123

This dynamic, famously identified by French political scientist Maurice Duverger as "Duverger’s law," creates a strong incentive for voters and political organizations to consolidate around two major parties. Because third parties often face the certainty of defeat—capturing few or no seats despite potentially significant national support—they struggle to remain viable unless they merge with or are absorbed by larger parties.34

Conversely, major parties in this system are compelled to broaden their appeal to capture a plurality of votes, often acting as "big tents" that accommodate a wide range of ideologies. This process of compromise and coalition-building occurs within the parties themselves rather than through the formal parliamentary coalitions common in proportional representation systems.132