One rule explains why your country probably has two parties or ten
Maurice Duverger noticed a pattern in the 1950s: the way you count votes quietly decides how many parties can survive.
Compare the United States, with essentially two parties that have held power for over 160 years, to a country like the Netherlands, which regularly seats a dozen parties in its parliament. The difference isn't culture or temperament. It's arithmetic, and a French political scientist named Maurice Duverger described the mechanism so clearly in the 1950s that it now carries his name as a law.
In a system where each district elects one winner by simple plurality — most votes, take all, as in most U.S. House races — voting for a small party is close to throwing your vote away. Rational voters drift toward whichever of the two leading parties they dislike least, because a vote for anyone else helps the party they like even less win by splitting the opposition. Over enough elections, minor parties starve for votes and merge or disappear, leaving two dominant coalitions standing.
Proportional systems remove that penalty entirely. If parliament seats are handed out roughly in proportion to national vote share, a party that wins 8 percent of the vote gets roughly 8 percent of the seats — real power, not zero. So voters feel safe voting their actual preference, and more parties survive.
Nobody designed American politics to be a two-party system by decree. They designed single-winner districts, and the two-party system followed as a side effect, the way water follows the shape of whatever pipe you build.