Duverger's Law
The political science theory linking electoral systems to party systems, and why FPTP tends to produce two parties while PR produces many.
From Rules to Parties
In 1951, French political scientist Maurice Duverger proposed what became the most famous generalization in political science: first-past-the-post (FPTP) systems tend to produce two-party systems, while proportional representation (PR) tends to produce multi-party systems. The 'law' (really a strong tendency) operates through two mechanisms.
The mechanical effect is direct: FPTP wastes votes for third parties by not converting their support into seats. A party winning 15 percent of the vote nationally but never finishing first in any district wins zero seats. The psychological effect follows: voters, knowing their preferred small party cannot win, vote strategically for the lesser evil among the two viable parties. Candidates and donors also anticipate this, starving small parties of money and talent before an election even begins. Over time, these effects compound and squeeze out third parties, consolidating the system around two dominant forces.
