Instant-runoff Voting - Proportionality

Proportionality

IRV is not a proportional voting system. Like all winner-take-all voting systems, IRV tends to exaggerate the number of seats won by the largest parties; small parties without majority support in any given constituency are unlikely to earn seats in a legislature, although their supporters will be more likely to be part of the final choice between the two strongest candidates. A simulation of IRV in the 2010 UK general election by the Electoral Reform Society concluded that the election would have altered the balance of seats between the three main parties, but the number of seats won by minor parties would have remained unchanged.

Australia, a nation with a long record of using IRV for election of legislative bodies, has had representation in its parliament broadly similar to that expected by plurality systems. Medium-sized parties, such as the National Party of Australia, can co-exist with coalition partners such as the Liberal Party of Australia, and can compete against it without fear of losing seats to other parties due to vote splitting. IRV is more likely to result in legislatures where no single party has an absolute majority of seats (a hung parliament), but does not generally produce as fragmented a legislature as a fully proportional system, such as is used for the House of Representatives of the Netherlands or the New Zealand House of Representatives, where coalitions of numerous small parties are needed for a majority.