Seat linkage

A seat linkage is a mechanism used in some mixed and proportional electoral systems in which seat won in one part (subsystem) of the electoral system are taken into account in the apportionment of the other seats. Seat linkage is a usually key element in achieving mixed-member proportional representation, but any system using leveling seats typically also relies on seat linkage.

Different compensatory seat linkage systems are sometimes conflated under the term mixed-member proportional representation (MMP), or (in the UK) additional member system. Seat linkage presumes the use of party lists, although these might be so open, that they work in the background "invisible" to the voter, such as the "best near-winner" model of MMP used in Baden-Württemberg.

↵Systems with seat linkage are further divided into compensatory and majority-assuring systems.

Seat linkage is distinct from parallel voting, where there is no link between two subsystems (they are independent) and vote linkage, a different type of connection between two parts of the system.

Method for mixed-member proportional representation

In typical seat linkage mixed systems, voters get two votes: one to elect the winner in single-seat constituency (usually by first-preference plurality), and another for the list of political party. Some seat linkage systems use a mixed single vote, which brings its own advantages and disadvantages.

Seats are first awarded to the successful constituency candidates, and then top-up seats and second, by party candidates based on the percentage of nationwide or region-wide votes that each party received. party-list proportional representation.

MMP differs from parallel voting in that the nationwide seats are allocated to political parties

proportional election results across all seats (not just the additional seats).

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.

Countries

Seat linkage used for MMP

• Bolivia
• Germany (most state parliaments, formerly for Bundestag - now localized PR)
• Lesotho (one vote)
• London (UK)
• New Zealand
• Scotland (UK)

Seat linkage used for MMM (highly limited compensation)

• Mexico (conditional)
• South Korea (supermixed)

Seat linkage used for PR

• Denmark
• Germany

Abolished

• Albania
• Romania
• Thailand
• Wales
• Venezuela

Tactical voting

Decoy lists

So-called "decoy lists" are a trick to unhinge the compensation mechanisms contained into the proportional part, so to de facto establish a parallel voting system.

For the 2020 South Korean legislative election the electoral system was changed and a partial use of AMS was implemented. In response, there were two satellite parties that only ran in the proportional part, the Future Korea Party (controlled by the United Future Party) and the Platform Party (controlled by the Democratic Party of Korea). Both merged with the parent party after the election.

Proposals, mixed ballots and hybrids

Baden-Württemberg/Bavaria hybrid

A system using a mixed single vote in dual member districts (with local lists up to two candidates) called dual-member proportional has been proposed for Canada by Sean Graham. DMP uses both vote transfer (within districts) and seat linkage.

An open list variant of MMP (modified Bavarian MMP) has been proposed by Jameson Quinn which uses a mixed ballot and vote linkage combined with classical seat linkage.

1. If a ballot supports the winner in the local district, the list part of that ballot is counted for the party of that local winner. (This is inspired by exhausting ballots in STV, and reduces the chances of "overhang".)
2. If a ballot supports two different parties on its two halves, and exactly one of those two parties is nonviable (cannot win any seats), then it is counted as if both parts of that ballot supported the viable party. (This is inspired by transferring ballots in STV, and reduces the chances of "wasted/sub-threshold" voting power.

The vote linkage and seat linkage hybrid has been further refined by Markus Schulze, where in the proposed system uses STV on the local tier, and the votes for the local winner are only counted as votes for that candidate's party to the proportion as it is necessary to eliminate overhang seats. The proposal also contains a flexible number of leveling seats to ensure full proportionality.