First-past-the-post voting tends to squeeze countries into two big parties

French political scientist Maurice Duverger argued in 1951 that single-member, winner-takes-all elections push voters away from small parties, since a vote for a third-place candidate rarely wins a seat. Over time, politics drifts towards two dominant parties. The UK and US broadly fit, but regionally concentrated parties like the SNP show the 'law' is a strong tendency, not an iron rule.

— Maurice Duverger, Political Parties: Their Organization and Activity in the Modern State — 1951
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