How Electoral Rules Shape the Number of Parties

In a single-member district plurality system, each district elects exactly one representative, and whoever gets the most votes wins everything else gets nothing. That all-or-nothing payoff changes how people vote. A voter whose true favorite is a third party starts to notice that the candidate has no realistic path to winning the district, so backing them feels like throwing the vote away. Over several elections, that voter and others like them drift toward whichever of the two leading candidates sits closer to their own views. Parties respond to the same pressure, since donors, media attention, and ambitious politicians all gravitate toward the two camps that can actually win. The result is a steady squeeze toward two dominant parties, built from millions of individually rational decisions rather than any rule banning third parties outright.

This squeeze is not a law of physics. A third party whose support is thin but spread evenly across the whole country wins nowhere, since it never places first anywhere. But a party whose support is concentrated in a specific region, tied to a distinct ethnic group, language, or local identity, can dominate those particular districts even while remaining a minor force nationally. That is why regional parties keep winning seats in systems otherwise known for two-party competition.

This sets up the next comparison: proportional representation systems convert vote share directly into seat share, so they don't punish spread-out minor parties the same way, and mixing proportional seats into a plurality system tends to push the party count upward.