Why do some parliaments mirror their country's vote almost exactly?

In some countries, if a party wins 30% of the vote, it gets roughly 30% of the seats — not by magic, but because the voting system is built to do exactly that. Proportional representation (PR) allocates seats so the makeup of a parliament tracks the makeup of the votes cast, instead of handing every seat to whichever candidate gets the most votes in their own small district. The European Union goes so far as to require it for its own Parliament: when n% of voters in a country back a party, that party gets roughly n% of that country's seats.

The idea is old — John Adams wrote in 1776 that a representative assembly "should be in miniature, an exact portrait of the people at large" — but a practical system came only in 1855. The Danish mathematician Carl Andræ devised one using the single transferable vote, a mix of preferential voting and multi-member districts, and Denmark first used it that year, making it the oldest PR system. Belgian mathematician Victor D'Hondt devised his seat-allocation method for party lists in 1878. Tasmania became an early pioneer of the single transferable vote, electing the world's first legislators under it in 1896.

PR has also been used as a political weapon against entrenched power. In 1936, a coalition of New York City reformers pushed through proportional voting specifically to break the grip of the Tammany Hall political machine on city government — and it worked, weakening Tammany's hold, though the city abandoned the system a decade later after only five elections under it.

No system achieves perfect proportionality except by chance, and PR isn't immune to manipulation either: in Hungary's 2014 election, the governing party combined old-fashioned gerrymandering with decoy party lists to turn 45% of the vote into a two-thirds parliamentary majority. Still, research suggests that once districts elect five or more representatives each, proportional systems become essentially immune to the kind of gerrymandering that plagues winner-take-all elections. Today party-list PR is used in 85 countries.