Overhang seats can arise in elections under the traditional mixed member proportional system, when a party's share of the nationwide party votes entitles it to fewer seats than the number of constituencies it won.
How overhang seats arise
Under MMP, a party is entitled to a number of seats based on its share of the total vote. If a party's share entitles it to ten seats and its candidates win seven constituencies, it will be awarded three list seats, bringing it up to its required number. This only works, however, if the party's seat entitlement is not less than the number of constituencies it has won. If, for example, a party is entitled to five seats, but wins six constituencies, the sixth constituency seat is referred to as an overhang seat.
Two mechanisms to earn many overhang seats
The two mechanisms that together increase the number of overhang seats are:
winning many constituencies;
decreasing the number of party votes and therefore the number of seats to which the party is proportionally entitled.
In many countries, overhang seats are rare - a party that is able to win constituency seats is generally able to win a significant portion of the party vote as well. There are, however, some circumstances in which overhang seats may arise relatively easily:
Few major parties, large number of minor parties - When there are only one or two major parties, but a relatively large number of minor parties that, combined, achieve a significant share of the total proportional vote, but fail to elect any constituency seats, the large parties often end up with overhang seats.
Similar lead across constituencies - If one party wins all or most constituencies of a relevant area with a rather low margin, it would be more prone to overhang mandates than if different parties lead in different constituencies, with the same overall share of each of the parties. As such, if there is less difference between constituencies, overhang seats are more probable.
A large number of constituencies compared with the total number of seats - If too many seats are used for constituencies, the remainder are less likely to ensure strict proportionality.
Unevenly sized constituencies - Candidates that win small constituencies by a narrow margin do not generate enough votes to justify their full seats under a proportional system.
Low turnout in some constituencies - This has the same effect as small constituencies. Furthermore, turnout and party preferences may be highly correlated, e.g. rural vs. urban areas.
Small number of seats to be allocated - The higher the absolute number of constituencies, the more likely it is that different reasons for overhang seats will balance out between parties. E.g., after the German federal Bundestag election in 2013, the main reason for overhang seats was that some compensation happens for party's state lists instead of at the federal level only.
Individual candidates with strong local followings - Sometimes, a particular politician will have strong support in their own constituency, but will belong to a party with very low support, even in their own area. The candidate will be elected based on their own qualities, but the party they belong to will not receive enough votes to justify the candidate's seat. In the case of independent candidates, this is usually guaranteed - they have no party at all, and so obviously cannot win votes under MMP's party-list proportional representation. However, some countries, such as New Zealand, have special rules dealing with independents - seats won by these candidates are exempted from the proportional system altogether.
Regional parties - Parties based in a particular region may win a substantial number of constituency seats in that region without necessarily gaining a large share of the national vote. Parties focused on particular ethnic or religions minorities may also come under this category, particularly if seats are reserved for these groups.
Tactical voting - Voters in countries such as Germany may cast two votes and they need not be for the same party. A voter might support one party in the list vote but vote for the candidate of another party in the local vote, perhaps because the former party lacks a candidate in his or her constituency or it has a candidate but he or she has little chance of winning. Parties that win many local seats but attract a reduced list vote may receive an overhang as a result.
Decoy parties - Party labels in the constituencies can be deliberately mismatched with those in the proportional vote in an attempt to induce tactical voting. In Italy in 2001, two lists won a significant majority of the total number of first-past-the post seats, despite winning almost none of the proportional vote. The system, nicknamed scorporo, was not a type of additional member system in which overhangs could occur, it resulted in a significant distortion to the desired compensatory nature.
Dealing with overhang seats
Overhang seats are dealt with in different ways by different systems. The four main methods are:
Examples
In New Zealand, the Māori Party won one overhang seat in 2005 and 2011, and two overhang seats in 2008. In 2005 their share of the party vote was under 2% on the initial election night count, but was 2.12% in the final count which included special votes cast outside the electorate. On election night it appeared that the party, whose candidates had won four electorate seats, would get two overhang seats in Parliament. However with their party vote above 2% the party got an extra seat and hence needed only one overhang seat. National got one less list seat in the final count, so then conceded defeat. In Germany, the electoral system requires that a party needs 5% of the popular vote to win any seats in state and federal elections, while every overhang seat is granted. Unless a party wins more than three district seats, it is completely excluded from the election threshold. This happened in 1994 and 2002 to PDS, which they failed to reach 5% of party-list votes.
