Increasingly popular around the world, mixed-member proportional systems (MMP) attempt to combine the local representation afforded by one-seat districts with the fairness provided by proportional representation. Voters in an MMP system typically have two votes at the ballot box: one for their local representative and one for their favored political party. The two votes are used to elect two separate but related sets of legislators. The first vote applies only to the geographic district in which the voter lives, and is used to elect a district representative in a traditional plurality, winner-take-all election -- the highest vote-getter wins. The second vote works very differently. It is tallied on a national (or regional) level in multi-seat districts. In the proportional representation form of MMP, the percentage of the second vote that a political party wins determines the overall percentage of seats that party will hold in the final legislature.

How MMP is tallied: First, all the district votes are counted, and the district representatives elected. Then the national party votes are tallied, and the remaining seats in the legislature are allotted to the various political parties to bring their total share of the legislature in line with their national (or regional) support. A party’s share of seats go to candidates according to its "party list" – through an internal process (sometimes a primary, convention or local caucus), a party establishes an ordered list of candidates before the election.

An example will make the process clearer. Imagine an MMP legislature to fill 100 seats, 50 of which would be filled through district representatives and 50 through the party vote. Suppose Party A polled 60% of the national vote and won 40 of the 50 district elections: it would be allotted 20 more seats from its party list to bring its total to 60 seats, which is a share of the total seats equal to its share of the total votes. If Party B polled 25% of the national vote but won only 10 district seats, it would earn 15 more party list seats to bring its total to 25 seats. The methodology is both simple and effective at returning a legislature that affords both good local representation and full proportionality.

Where MMP and variations are used: Recent worldwide electoral trends show a rise in popularity of the mixed member system because of how it combines geographic, district-based representation with proportional representation. For decades, West Germany was the only nation to use MMP, having adopted it after World War II. In the 1990’s, however, MMP was adopted for elections in New Zealand, Scotland and Wales, and a modified form of MMP was proposed by the Jenkins Commission to elect the British House of Commons.

Several major countries in the 1990’s adopted "parallel" mixed member systems that share several features with MMP, but are classified as semi-proportional systems. The allocation of the party list seats is done in proportion to the party vote no matter what the results in the district elections, meaning that the largest party tends to win a disproportionally high share of seats. Among countries using parallel mixed member systems are Mexico, Russia and the Ukraine. Italy and Hungary also use mixed member systems that are classified as semi-proportional.

Evaluation: By seeking to combine the best of the two major electoral principles, mixed member PR has clear attractions. But some fear it combines the worst of these two models, creating two classes of representatives with competing interests. Others are concerned about its relative complexity, and those suspicious of parties may have difficulty accepting the party’s role in establishing its list of candidates. Given its recent adoption in several nations, their experience with the system warrants close scrutiny.