Mixed member proportional voting

Mixed member proportional voting (MMP) is a voting system that combines some features of a proportional voting system and some of a (usually) first past the post (FPTP) system. It tends to create widely representative parliaments with many parties catering to the preferences of broad sections of the voters. It frequently results in minority governments and governing by consensus. It is currently used in the national parliaments of Bolivia, Germany, Lesotho, New Zealand, and Thailand. It is also used for elections to the devolved governments of London, Scotland, and Wales within the UK, and for South African local elections.

Method
There are two ways to be elected to parliament under this system, the first is by personally winning a seat (by winning the most votes in a district). The second is through being high enough on a party's list so as to qualify for election. The list is made up in advance of the election. In general, approximately half of the seats available are electoral seats and half are list seats. Each ballot consists of two votes, one for political parties and one for candidates in the particular electorate the voter resides in. The number of members of parliament (MPs) each party receives is proportional to the number of votes the party gets. The party first appoints those MPs who won their electoral seats, and any shortfall is made up from the list.

MMP in action, a hypothetical example
The parties range from left wing (A) through to right wing (D) with two centrist parties.

In a 100 seat parliament with 50 electoral seats:


 * Party A receives 11% of the vote and has 5 MP's win electoral seats
 * Party B receives 35% of the vote and has 21 MP's win electoral seats
 * Party C receives 5% of the vote and 1 electoral seat
 * Party D receives 49% of the vote and has 24 electoral seats

Party A receives 11 MPs: its 5 electoral MPs and the 6 members highest on Party A's list. This is repeated for the other parties. Note that no party has a majority in the house and must rely on one or both of the other parties to form a government. This result is called a "hung parliament". Note here, the very small Party C has a disproportionate amount of power compared to its electoral support. It can in effect dictate either to Party D or to an alliance of parties A and B what it wants in return for its support in parliament.

Support
The system allows for a wider range of political views to be addressed than under a traditional first past the post system. As an example, when New Zealand moved from FPTP to MMP in 1996, it changed from having a two party system in its parliament, to, by 2002, having 8, ranging from far left to far right parties. Non-proportional systems often result in a large number of electoral votes not being translated into political power. Under FPTP all votes for losing candidates are ignored while under MMP those votes are included in the makeup of the parliament. MMP also results in greater representation in parliament for women and minorities

This system also tends to require a consensus to govern effectively, thus providing an effective check on dramatic changes of policy. For instance, the above party B can moderate any potential radicals from the left or right wing parties.

Criticisms
The system can also produce unstable governments. Hung parliaments mean that coalition governments are required, and alliances can change quickly. One way to counter tendencies towards fragmentation is the imposition of an often defined as a minimum percentage of the national vote, which a party needs to cross to enter the parliament. However, it's often still possible for parties or candidates who win a constituency outright to enter parliament without being able to pass the national threshold.

Also, very small parties can wield a large amount of power in determining policy. This is said to be having the balance of power. An example of this occurred in 1996 in New Zealand when the party "New Zealand First" who had campaigned on the promise that "a vote for them was the only way voters could keep National out of power" held the balance of power and after a month of backroom dealing returned National to power. The coalition agreement resulted in a number of inexperienced MP's from New Zealand First becoming cabinet members, including an unsuitable treasury minister, leading some to suggest an auction was taking place. This example shows that better proportionality can sometimes have the impact of lessening the importance of the election results.

Proportional representation systems of this kind can have extreme consequences, and it is sometimes claimed that the system assisted Hitler in coming to power. This argumentum ad Hitlerum argument loses much of its force when it's recalled that the largest parties tend receive a greater share of the seats than their proportion of the votes, e.g. in the 2015 UK general election, the Tories received an absolute majority based on about a third of the votes cast. Had Weimar had a FPTP system, the Nazis, as the biggest party in the Reichstag (receiving between 33.1% and 43.9% of the vote in the three elections between July, 1932 and March, 1933), would likely have been able to muster an absolute majority.