Additional-member system
The additional-member system (AMS) is a hybrid electoral system that combines first-past-the-post voting for constituency seats with proportional allocation of additional seats from party lists to achieve greater overall proportionality in legislative representation.[1] Voters cast two ballots: one selecting a candidate for a local district seat, and another choosing a political party for the list vote, which determines the distribution of compensatory seats to offset disproportional outcomes from constituency results.[1] Unlike fully proportional systems, AMS maintains a direct link between voters and local representatives while mitigating the winner-takes-all distortions of pure plurality voting.[2] This system is employed in subnational elections within the United Kingdom, including for the Scottish Parliament (73 constituency seats and 56 regional additional members), the Senedd (40 constituency and 20 regional seats), and the Greater London Assembly (14 constituency and 11 additional seats).[3][4] It has been credited with producing more diverse parliaments by allowing smaller parties to gain representation through list seats, though critics argue it can encourage strategic voting and create two tiers of legislators, with constituency members holding greater prestige.[5][6] In practice, AMS yields results more proportional than first-past-the-post but less so than pure list proportional representation, balancing local accountability against broader voter preferences.[1]Definition and Principles
Core Mechanism
The additional-member system (AMS) is a mixed electoral system that combines majoritarian and proportional elements through a two-vote process. Voters cast one vote for a candidate in a single-member constituency, where seats are allocated using the first-past-the-post (FPTP) method: the candidate receiving the most votes wins the constituency seat, irrespective of majority support. This component ensures direct local representation by linking elected members to specific geographic areas.[1] The second vote is for a political party on a regional list ballot, which determines the allocation of additional seats to achieve greater overall proportionality. These seats are distributed across multi-member regions using a highest averages formula, such as the D'Hondt method. Under this approach, each party's regional list votes are divided by successively increasing divisors starting from 1 plus the number of constituency seats already won by that party in the region; the highest resulting quotients receive the available additional seats until all are filled. This adjustment partially compensates for disproportionalities from FPTP constituency results by disadvantaging parties that have secured many local wins in the regional allocation.[1][7] Unlike fully compensatory systems such as mixed-member proportional representation (MMP), AMS does not guarantee exact proportionality to the party vote share across the entire assembly, as the formula applies regionally and the fixed number of additional seats limits full correction. The balance between constituency and additional seats—often around 70-75% constituency seats in implementations like Scotland's 73 out of 129 total seats—influences the system's overall proportionality, with more additional seats yielding closer alignment to party vote shares.[1][8]Distinction from Pure Proportional and Majoritarian Systems
The additional-member system (AMS) differs from pure majoritarian systems, such as first-past-the-post (FPTP), by incorporating a compensatory mechanism that allocates extra seats to parties based on their overall vote share, thereby mitigating the disproportionality that arises in winner-take-all constituency contests. In pure majoritarian systems, the candidate with the plurality of votes in each single-member district secures the seat, which can result in significant overrepresentation for large parties and underrepresentation for smaller ones; for instance, a party receiving 35% of the national vote might win over 50% of seats due to concentrated support in key districts.[9] AMS addresses this through additional list seats that "top up" the results to approximate proportionality, ensuring that parties' total seat shares more closely reflect their second-vote (party list) totals, though the system remains partially majoritarian due to the dominance of constituency seats.[10] In contrast to pure proportional representation (PR) systems, which allocate all legislative seats directly in proportion to parties' vote shares—often via closed or open party lists in multi-member districts or single transferable vote (STV) methods—AMS retains a substantial portion of seats (typically 50-70%) elected via majoritarian rules in single-member constituencies, preserving direct voter links to local representatives. Pure PR emphasizes overall proportionality across the entire legislature without district-level winners, which can dilute geographic accountability as candidates are selected from party lists rather than direct constituency contests; this approach has been implemented nationwide in countries like the Netherlands, where all 150 seats in the House of Representatives are distributed proportionally from national lists.[11] AMS, however, uses the proportional component solely for compensation, not to override constituency outcomes, leading to outcomes that are more proportional than pure majoritarian but less so than pure PR, with potential for overhang or underhang effects if constituency results deviate sharply from list votes.[10] This hybrid structure aims to balance local representation with broader fairness, though critics argue it can still favor larger parties due to the fixed ratio of seat types.[8]Historical Development
Origins in Mixed Electoral Systems
The additional-member system emerged as a hybrid approach within mixed electoral systems during the mid-20th century, seeking to merge the constituency-based representation of majoritarian voting with the vote-seat proportionality of list systems. This combination addressed shortcomings in pure systems: majoritarian methods like first-past-the-post often produced disproportional outcomes favoring large parties, while pure proportional representation risked excessive fragmentation and instability, as experienced in interwar Europe. The system's core innovation—allocating "additional" seats from party lists to supplement constituency winners—first materialized in West Germany, where it was formalized in the Electoral Law of 1949 for the inaugural Bundestag elections on 14 August 1949. There, approximately half of the 402 seats were filled via single-member districts using plurality rule, with the remaining seats distributed proportionally based on parties' second-vote shares to achieve overall proportionality, a mechanism termed personalized proportional representation (personalisiert proportionale Wahlsystem).[12] This German model arose from post-World War II constitutional deliberations under Allied occupation, balancing demands for local democratic legitimacy—echoing Weimar-era preferences for district ties—with safeguards against the multiparty chaos that contributed to the Republic's collapse in 1933. Christian Democratic Union (CDU) leaders, prioritizing governmental stability, pushed for majoritarian elements, while Social Democratic Party (SPD) advocates emphasized proportionality to amplify minority voices; the resulting compromise embedded two-vote ballots, with the constituency vote (Erststimme) electing local representatives and the list vote (Zweitstimme) determining additional allocations via the d'Hondt method, subject to a 5% national threshold. Empirical outcomes in 1949 validated its stabilizing effect: the CDU/CSU secured 31% of second votes but 43% of seats, while smaller parties gained fairer representation without dominating. No prior system fully integrated compensatory additional members in this linked manner, distinguishing it from earlier parallel hybrids like Japan's prewar partial list supplementation, which lacked binding proportionality.[12][13] Subsequent mixed systems drew directly from this framework, adapting it amid decolonization and democratization waves. For instance, Italy's 1953 law introduced a similar structure for its Chamber of Deputies, blending 618% constituency seats with list top-ups, though with less stringent compensation, reflecting influences from German advisors in European reconstruction efforts. These origins underscore causal trade-offs: additional members mitigate disproportionality (e.g., Germany's Gallagher index averaged 2.1 from 1949–1990, far below pure majoritarian benchmarks) but introduce complexities like overhang seats when constituency wins exceed proportional entitlements, prompting refinements in later adoptions.[13]Post-War Adoption and Evolution in Europe
The mixed-member electoral system, incorporating elements later refined into the additional-member system (AMS), was first adopted in post-war Europe by West Germany in the Federal Elections of 1949. This system allocated half of the Bundestag seats through single-member districts using plurality voting and the other half via party lists to achieve approximate proportionality, serving as a deliberate compromise to mitigate the perceived instabilities of pure proportional representation under the Weimar Republic while preserving local constituency representation.[14] The design emerged from negotiations among Allied powers and German political actors, prioritizing a 5% national threshold for list seats to exclude fringe parties and prevent fragmentation, with initial overhang seats addressed ad hoc until formalized in later reforms.[12] During the Cold War era, adoption remained limited in Western Europe, where countries like Italy (pure list PR from 1948) and France (two-round majoritarian) favored other systems amid reconstruction and polarization concerns.[15] However, Germany's model influenced theoretical discussions on hybrid systems, emphasizing dual votes for candidates and parties to balance majoritarian accountability with broader representation. Evolution accelerated in the 1990s following the collapse of communist regimes, as Central and Eastern European states sought institutions promoting democratic consolidation without the extremes of winner-take-all or unmitigated PR. Hungary pioneered a variant in its 1990 parliamentary elections, combining 176 single-member districts (46% of seats) with compensatory national and regional lists (54%), where overrepresentation in districts triggered additional "winner compensation" mandates to approximate proportionality, though without full linkage.[16] [17] Italy transitioned to a mixed system via the 1993 Mattarellum law, enacted after a referendum amid corruption scandals and demands for reform; it assigned 75% of Chamber seats via single-member plurality districts and 25% through uninominal PR lists, with a partial compensatory mechanism deducting votes for district winners from party list allocations to curb disproportionality.[18] This parallel-oriented approach, lacking full compensation, amplified majoritarian outcomes, yielding coalition governments but favoring larger parties, as evidenced by the 1994 elections where Forza Italia secured 21% of votes but 30% of seats.[15] Similar adoptions occurred in Albania (1991, mixed with PR topping up), Bulgaria (post-1990 reforms blending SMD and lists), and Russia (1993, strict parallel 50-50 split without compensation), reflecting a regional preference for hybrids to stabilize transitions by rewarding district-level organization while allocating list seats independently or semi-proportionally.[19] By the early 2000s, refinements addressed shortcomings like dual-candidate incentives and disproportionality; Hungary's 2011 overhaul reduced districts to 106 seats (46%) and introduced a single national list with stronger majoritarian bias via winner bonuses, prioritizing governability over strict proportionality amid Fidesz's dominance.[20] Italy iterated further, replacing Mattarellum with the 2005 Porcellum (largely majoritarian with PR scraps) and 2017 Rosatellum (37% SMD, 63% PR with compensation), driven by judicial interventions and instability from fragmented coalitions.[21] These evolutions underscore AMS variants' appeal in Europe for empirical adaptability—offering 10-20% deviation from proportionality in practice versus pure systems' extremes—yet revealing causal risks of entrenching incumbents when thresholds and formulas favor established parties over voter-driven equity.[22]Introduction in UK Devolved Institutions
The additional member system (AMS) was adopted for elections to the devolved legislatures of Scotland and Wales as part of the United Kingdom's devolution framework established in the late 1990s. Following affirmative referendums in September 1997, the Scotland Act 1998 created the Scottish Parliament with 129 members: 73 elected by first-past-the-post in single-member constituencies mirroring Westminster boundaries, and 56 additional members allocated across eight regions using party lists to compensate for disproportionality in constituency results.[7] The system's design aimed to combine local constituency representation with broader proportionality, drawing on mixed systems trialed elsewhere in Europe. The inaugural election occurred on 6 May 1999, yielding a Scottish National Party-Labour coalition government and enabling smaller parties like the Scottish Greens to secure regional seats despite limited constituency success.[1] In Wales, the Government of Wales Act 1998 similarly instituted AMS for the National Assembly for Wales (renamed Senedd Cymru in 2020), comprising 60 members: 40 from single-member constituencies and 20 additional members from five regional electoral areas.[4] Voters cast two ballots—one for a constituency candidate and one for a regional party list—with additional seats distributed via the d'Hondt method to align overall party representation more closely with vote shares.[23] The first election, also on 6 May 1999, resulted in a Labour minority administration, with Plaid Cymru gaining regional top-ups to offset constituency losses. This structure persisted through subsequent reforms, including expansions considered in the Senedd Cymru (Members and Elections) Act 2024, which maintained AMS while increasing seats to 96 from 2026.[24] The Greater London Authority, established by the Greater London Authority Act 1999 after a 1998 referendum, extended AMS to the 25-member London Assembly from its inaugural election on 4 May 2000: 14 constituency members elected by plurality and 11 London-wide additional members via party lists.[1] In contrast, the Northern Ireland Assembly, restored under the 1998 Good Friday Agreement, employs the single transferable vote system across 90 members in 18 multi-member constituencies, eschewing AMS to prioritize cross-community consociational representation.[25] These implementations marked the UK's initial domestic application of AMS, prioritizing empirical proportionality over pure majoritarianism to foster inclusive devolved governance.[26]Operational Mechanics
Two-Vote Process and Constituency Seats
In the additional-member system, voters cast two separate votes to elect representatives. The first vote selects a candidate to represent a specific geographic constituency, typically through single-member districts using the first-past-the-post (FPTP) method, where the candidate with the most votes wins the seat outright.[1] This process mirrors traditional majoritarian systems and ensures direct local representation, with constituency boundaries drawn to reflect population distributions.[7] Constituency seats form the foundational layer of AMS legislatures, often comprising a majority of total seats to maintain a balance between local accountability and proportionality. For instance, in the Scottish Parliament, 73 out of 129 members are elected as constituency members of the Scottish Parliament (MSPs) via FPTP across 73 districts aligned with Westminster constituencies.[7] Similarly, the Senedd in Wales allocates 40 of its 60 seats through FPTP in 40 constituencies.[27] These seats are filled independently of the second vote, allowing candidates—often affiliated with parties but nominated individually—to compete based on personal appeal and local issues.[8] The first vote's FPTP allocation can lead to disproportional outcomes at the constituency level, such as winner-take-all results that favor larger parties, but this is offset by additional seats derived from the second vote.[1] Voters mark their first ballot by placing an "X" next to their preferred candidate's name, with invalid votes (e.g., multiple marks) discarded during counting at local polling stations. This direct linkage between voter preference and a specific representative fosters accountability, as constituency members must address constituents' concerns without dilution from list-based selection.[28]Allocation of Additional Seats
In the additional-member system, additional seats are allocated to parties using the results from the second vote (party list vote), with the aim of compensating for disproportionalities in the constituency tier results. This compensatory mechanism typically employs a highest averages formula, most commonly the d'Hondt method, applied at the regional or sub-national level. Under this approach, each party's regional list vote total is divided by one plus the number of seats (constituency and any already allocated regional seats) it has secured in that region, producing a quotient; the party with the highest quotient receives the next additional seat, after which the process repeats with updated divisors until all additional seats in the region are filled.[7][29] The d'Hondt method favors larger parties due to its structure, which progressively dampens the influence of smaller parties' votes as seats are allocated, but in the AMS context, it adjusts for constituency over- or under-representation by effectively penalizing parties that have already won disproportionate constituency seats. For instance, a party winning multiple constituency seats in a region starts with a higher initial divisor (e.g., votes divided by constituency seats + 1), reducing its quotients for additional seats compared to parties with fewer or no constituency wins. This results in additional seats flowing primarily to parties underperforming in the first tier relative to their list vote share, though the fixed ratio of constituency to additional seats (often around 2:1 or 3:2) limits full proportionality.[7][30] Variations exist across implementations; for example, some systems use the Sainte-Laguë method, which employs odd-numbered divisors (1, 3, 5, etc.) to provide slightly more favorable treatment to smaller parties, though d'Hondt remains predominant in regions like Scotland and Wales. Candidates who win constituency seats are ineligible for additional seats from the same region's list, ensuring no double representation, and parties must typically meet a vote threshold (e.g., 5% of the regional list vote in some cases) to qualify for additional allocations, though this is not universal. In non-compensatory parallel variants of AMS, such as Japan's House of Representatives, additional seats are allocated directly proportional to list votes without adjusting for constituency results, yielding less overall proportionality.[1][29]Thresholds and Eligibility Rules
In implementations of the additional-member system (AMS), electoral thresholds represent the minimum percentage of votes a party must secure to qualify for allocation of additional seats, aimed at limiting parliamentary fragmentation by excluding minor parties from proportionality calculations. Such thresholds are not inherent to AMS but are adopted in certain variants, particularly those emphasizing strict proportionality like mixed-member proportional (MMP) systems. For instance, New Zealand's MMP requires parties to obtain at least 5% of the national party vote or win at least one electorate seat to be eligible for list seats, a rule introduced in the 1996 Electoral Act to balance representation with governability.[31] Similarly, Germany's federal elections under its MMP variant mandate a 5% threshold of second (party list) votes nationwide or securing three direct constituency mandates, as codified in the Federal Electoral Act, with exceptions for ethnic minorities. In contrast, AMS as applied in United Kingdom devolved institutions lacks formal vote thresholds for additional (regional list) seats, allowing even small parties to receive allocations if their regional vote shares yield quotients under the d'Hondt method. For the Scottish Parliament, comprising 73 constituency seats and 56 regional seats, no minimum percentage is required for regional eligibility; parties nominate lists per region, and seats are distributed proportionally among all contesting registered parties based on valid regional votes, as outlined in the Scotland Act 1998.[7] The Senedd (Welsh Parliament) follows a parallel structure with 40 constituency and 20 additional seats across four regions, applying d'Hondt without a threshold since its establishment under the Government of Wales Act 1998, enabling parties like Plaid Cymru to gain seats from modest regional support.[32] The Greater London Assembly employs AMS for its 14 constituency and 11 London-wide additional seats, again without thresholds, per the Greater London Authority Act 1999, where independents and minor parties have occasionally secured additional member spots through list votes alone. Eligibility rules beyond thresholds typically require parties to register with national electoral commissions, submit complete candidate lists by statutory deadlines, and comply with nomination deposit requirements—such as £500 per candidate in Scottish regional contests—to deter frivolous entries. These provisions ensure administrative feasibility while maintaining openness, though effective barriers arise from the mathematical nature of seat allocation, where parties below roughly the reciprocal of total seats (e.g., about 1.4% in Scotland's eight-region setup) rarely secure additional members without concentrated regional support. In threshold-free AMS, this de facto hurdle promotes broader contestation compared to high-bar systems, though critics argue it risks diluting proportionality by admitting fringe voices without compensatory exclusions.[33]Comparative Allocation Examples
In systems employing the additional-member system (AMS), additional seats are typically allocated at the regional or national level using a highest averages formula, such as d'Hondt, applied to party list votes while accounting for seats already won in single-member constituencies; this provides partial compensation for disproportionality but maintains a fixed total number of seats, unlike fully proportional variants.[10] For instance, in Scotland's implementation, 73 constituency members of the Scottish Parliament (MSPs) are elected via first-past-the-post, followed by allocation of 56 regional additional members using d'Hondt on regional list votes, with each constituency seat won by a party treated as an initial allocation in the formula to adjust subsequent divisors.[7] Comparatively, pure parallel voting systems allocate list seats independently of constituency results, often exacerbating disproportionality by rewarding parties that dominate single-member districts without compensation. In contrast, mixed-member proportional (MMP) systems, such as those in Germany or New Zealand, prioritize overall proportionality by using list seats to fully offset constituency imbalances, potentially introducing overhang seats (where parties retain excess constituency wins) or expanding the legislature to achieve exact vote-seat matches.[10] A simulated comparison based on Mongolia's 2020 election results illustrates these differences across 76 total seats (47 constituency, 29 list): under parallel voting with largest remainder Hare quota for list seats, the leading Mongolian People's Party (MPP) alliance secured 55 seats despite approximately 51% of list votes, yielding high disproportionality; AMS with d'Hondt for list seats reduced this to 40 seats for MPP; MMP with compensation further aligned outcomes closer to vote shares, distributing seats as 40 for MPP but requiring overhang adjustments in practice.[10]| System Variant | MPP Seats | DP Seats | Other Parties | Total Disproportionality Note |
|---|---|---|---|---|
| Parallel (LR Hare) | 55 | 14 | 7 combined | High overrepresentation of leading party (72.4% seats vs. ~51% votes)[10] |
| AMS (d'Hondt) | 40 | 21 | 15 combined | Partial compensation, fixed size, moderate proportionality[10] |
| MMP (with overhang) | 40 | 21 | 17 combined (78 seats total) | Full compensation, potential seat expansion for proportionality[10] |
Variations and Classifications
AMS Versus MMP: Key Differences
The Additional Member System (AMS) and Mixed Member Proportional (MMP) representation differ primarily in their mechanisms for linking constituency and list seat outcomes to achieve proportionality. AMS operates as a parallel or semi-compensatory system, where additional seats are allocated regionally based on party list votes, subtracting constituency wins from each party's proportional entitlement without allowing negative adjustments or seat reductions. This fixed-total approach can result in overrepresentation for parties excelling in single-member districts, as seen in systems like Scotland's Parliament, where the total seats remain constant regardless of imbalances.[10][1] MMP, conversely, uses a fully compensatory national or statewide formula to align total seats with party list vote shares, treating constituency results as a baseline that list seats adjust. Parties retain any "overhang" seats—constituency wins exceeding their proportional share—and legislatures may expand via leveling seats allocated to underrepresented parties, ensuring closer overall proportionality at the expense of fixed chamber sizes. For example, Germany's Bundestag has grown beyond its nominal 598 seats due to overhang, reaching 736 in the 2021 election to balance outcomes.[10][8] These structural variances yield distinct proportionality levels: AMS moderates first-past-the-post disproportionality but retains majoritarian biases, often yielding Gallagher indices (a measure of disproportionality) around 5-10 in UK applications, while MMP targets near-perfect alignment, with indices typically under 3 in implementations like New Zealand's, where overhang and leveling prevent persistent overrepresentation. AMS emphasizes regional balance and local linkage without national overhang resolution, potentially fragmenting compensation, whereas MMP prioritizes holistic proportionality, sometimes at the cost of larger assemblies or diluted constituency representation ratios.[10][8]| Aspect | AMS (e.g., Scotland, Wales) | MMP (e.g., Germany, New Zealand) |
|---|---|---|
| Vote Linkage | Regional; semi-independent with subtraction for constituency wins | National; fully compensatory across all seats |
| Overhang Handling | No adjustment; overrepresented parties keep seats, fixed totals | Retained overhang; leveling seats added to others, variable totals |
| Proportionality Goal | Moderate; retains some majoritarian bias | High; overall seats mirror list votes closely |
| Total Seats | Fixed (e.g., 129 in Scotland) | Base fixed but expandable (e.g., up to 20% overhang in Germany) |