Mixed-member proportional representation

Mixed-member proportional representation (MMP or MMPR) is a mixed electoral system in which votes are cast for both local elections and also for overall party vote tallies, which are used to allocate additional members to produce or deepen overall proportional representation.

In some MMP systems, voters get two votes: one to decide the representative for their single-seat constituency, and one for a political party. In Denmark and others, the single vote cast by the voter is used for both the local election (in a multi-member or single-seat district), and for the overall top-up.

Seats in the legislature are filled first by the successful constituency candidates, and second, by party candidates based on the percentage of nationwide or region-wide votes that each party received.

The constituency representatives are usually elected using first-past-the-post voting (FPTP) but the Scandinavian countries have a long history of using both multi-member districts (members elected through party-list PR) and nationally-based compensatory top-up seats.

The nationwide or regional party representatives are, in most jurisdictions, drawn from published party lists, similar to party-list proportional representation. To gain a nationwide representative, parties may be required to achieve a minimum number of constituency seats, a minimum percentage of the nationwide party vote, or both.

MMP differs from parallel voting in that the nationwide seats are allocated to political parties in a compensatory manner in order to achieve proportional election results. Under MMP, two parties that each receive 25% of the votes end up with about 25% of the seats, even if one party wins more constituency seats than the other. Depending on the exact system implemented in a country and the results of a particular election, the proportionality of an election may vary. Overhang seats may reduce the proportionality of the system, although this can be compensated for by allocating additional party list seats to cover any proportionality gap.

MMP was first used to elect representatives to the German Bundestag, and has been adopted by South Korea, New Zealand, and others. In Germany, where it is used on the federal level and in most states, MMP is known as personalized proportional representation (German: personalisiertes Verhältniswahlrecht).

In the Canadian province of Quebec, where an MMP model was studied in 2007, it is called the compensatory mixed-member voting system (système mixte avec compensation or SMAC). In the United Kingdom the semi-proportional implementation of MMP used in Scotland, Wales, and the London Assembly is referred to as the additional member system.

MMP produces proportional election results by a number of compensatory seats, sometimes known as leveling seats.

Some countries use single vote variants of MMP, although this article focuses primarily on dual vote versions of MMP.