A LevelPoliticsEdexcelRevision Notes1. UK Politics & Core Political Ideas1.3 Electoral SystemsElectoral Systems: Proportional Systems

Electoral Systems: Proportional Systems (Edexcel A Level Politics): Revision Note

Exam code: 9PL0

Sarra Jenkins

Written by: Sarra Jenkins

Reviewed by: Lisa Eades

Updated on

Additional Member System (AMS)

  • The Additional Member System (AMS) is a mixed electoral system that combines First-Past-the-Post (FPTP) with proportional representation to produce a balanced and representative outcome

Features of AMS

1. Single-member and multi-member constituencies

  • AMS uses constituency representatives elected using FPTP and regional representatives elected from party lists

2. Proportional system

  • The FPTP element is balanced by regional list seats, making the overall result more proportional

3. Each voter has two votes

  • Voters cast one vote for a constituency candidate and one vote for a regional party list

How AMS works

  • Voters cast two votes: one for a constituency representative and one for a regional party

  • Constituency seats are allocated using FPTP

  • Regional seats are allocated using the d’Hondt formula, which compensates parties harmed by FPTP

  • A party’s total seats equal its constituency seats plus its regional seats

  • The largest party is invited to form a government, or parties may form a coalition

An evaluation of AMS

Advantages

Disadvantages

  • AMS is more proportional than FPTP, meaning fewer votes are wasted and outcomes better match voter preferences

  • AMS is not fully proportional because the constituency element still favours larger parties

  • AMS maintains the constituency link by ensuring voters have a directly elected local representative

  • AMS creates two types of representative - constituency and regional - which can lead to perceptions of unequal legitimacy

  • AMS increases representation, giving voters both local and regional representatives to contact

  • AMS is more complex than simple plurality systems, which may discourage some voters

  • AMS helps smaller parties succeed, particularly in regional list seats

  • AMS gives party leaders considerable control over regional lists, increasing leadership influence

  • AMS often results in coalition or minority governments, encouraging compromise and cross-party working

  • AMS often produces coalition or minority governments that may struggle to pass legislation effectively

Case Study

AMS in Scotland, 2011

Wide view of a parliamentary chamber with delegates seated in curved rows, wooden interior, and large windows with natural light streaming in.
The chamber of the Scottish Parliament

  • The 2021 Holyrood election demonstrated AMS’s proportional nature.

    • The SNP won 64 of 129 MSPs, one short of a majority, reflecting broad but not overwhelming support

    • The Greens won 8 MSPs, enabling a pro-independence majority of 72 through the Bute House Agreement

  • Turnout reached 63.5%, the highest in a Scottish Parliament election

    • Constituency seats overwhelmingly favoured the SNP (62 of 73), while list seats increased representation for Labour, Conservatives, Liberal Democrats and Greens

  • AMS enabled the Greens to join government for the first time, giving them influence over climate and social policy

Single Transferable Vote (STV)

  • The Single Transferable Vote (STV) is a highly proportional electoral system in which voters rank candidates in order of preference in multi-member constituencies

Features of STV

1. Multi-member constituencies

  • Each constituency elects several representatives, allowing results to reflect a wider range of political views

2. Highly proportional system

  • STV redistributes surplus and unused votes, ensuring more votes contribute to electing a representative

3. Ordinal voting

  • Voters rank candidates 1, 2, 3, and so on, giving them control over both party and candidate choice

How STV works

  • Multi-member constituencies are established (e.g., Northern Ireland has 18 constituencies, each electing six MLAs)

  • The Droop quota is calculated using the formula

Droop space formula space equals space fraction numerator Votes over denominator seats space plus space 1 end fraction space plus space 1

  • First-preference votes are counted, and any candidate meeting the quota is elected

  • Surplus votes from elected candidates are transferred based on voters’ next preferences

    • If seats remain, the lowest-ranked candidate is eliminated and their votes are transferred

  • Steps continue until all seats are filled

    • The largest party forms a government unless a coalition is required

    • Northern Ireland must form a power-sharing government between unionist and republican parties

An evaluation of STV

Advantages

Disadvantages

  • STV is highly proportional, allowing seat shares to closely match vote shares

  • STV is complex and may reduce turnout or increase the number of spoilt ballots

  • STV gives voters more choice by allowing them to rank multiple candidates, including several from the same party

  • STV weakens the local constituency link because representatives cover larger areas

  • It minimises wasted votes because votes can be transferred to help elect another preferred candidate

  • STV often leads to coalition governments, which can make decision-making slower

  • STV reduces the number of safe seats, making elections more competitive

  • STV counting takes longer due to multiple rounds of transfers

  • STV improves smaller parties’ chances because votes for them are less likely to be wasted

  • STV requires voters to understand a wide range of candidates, which can make informed ranking difficult

  • STV encourages intra-party competition, reducing party leadership’s control

  • STV’s complexity means voters may need more education to feel confident using the system

Case Study

STV in Northern Ireland, 2022

Illustration of five adults in formal attire, showing diverse individuals with varied expressions and hairstyles against a plain white background.
The Northern Ireland political landscape changed significantly in 2022

  • The 2022 Assembly election chose 90 representatives across 18 five-member constituencies, with a 64% turnout

    • Sinn Féin won 27 seats, becoming the largest party in Stormont for the first time

    • The DUP won 25 seats, losing ground due to internal splits and the Protocol dispute

    • STV benefited Alliance, which grew to 17 seats, becoming the third-largest party

  • Despite a proportional outcome, no government formed due to the DUP’s refusal to participate in power-sharing, showing proportional systems cannot guarantee stability

Supplementary Vote (SV)

  • The Supplementary Vote (SV) is a majoritarian system in which voters mark a first and second preference

  • It is used for single-office elections but was abolished for the London Mayor by the Elections Act 2022

Features of SV

  • Used in single-member constituencies

  • Requires candidates to secure over 50% of the vote after redistributions

  • Voters mark a first and second preference

How SV works

  • Voters mark their first and second choices

    • First-preference votes are counted; a candidate with more than 50% wins outright

    • If not, only the top two remain.

    • Second preferences from eliminated candidates’ ballots are added to the totals of the top two candidates

  • The candidate with over 50% after redistribution wins

An evaluation of SV

Advantages

Disadvantages

  • SV encourages broad appeal because candidates must attract enough second-preference support to win

  • SV can produce disproportionate results, reducing the perceived legitimacy of the winner

  • SV is relatively simple because voters only need to make two choices

  • SV still leads to wasted votes, particularly if voters’ preferred candidates are eliminated early

  • SV results are counted faster than those under proportional systems

  • SV encourages tactical voting, particularly in deciding which candidate to rank second

  • SV gives smaller parties some opportunity to gain support if they can appeal widely

  • SV often reinforces a two-party system because only the top two candidates remain in the final round

  • SV suits single-office elections such as those for regional Mayors

  • SV remains disproportionate because only one candidate can win the office

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    Author
    Reviewer
    Sarra Jenkins

    Author: Sarra Jenkins

    Expertise: Content Writer

    Sarra is a highly experienced A-Level Politics educator with over two decades of teaching and examining experience. She was part of the team that wrote the Edexcel 2017 Politics Specification and currently works as a Senior Examiner. A published author of 14 textbooks and revision guides, her expertise lies in UK and US politics, exam skills, and career guidance. She continues to teach, driven by her passion for this "evolving and dynamic subject".

    Lisa Eades

    Reviewer: Lisa Eades

    Expertise: Business Content Creator

    Lisa has taught A Level, GCSE, BTEC and IBDP Business for over 20 years and is a senior Examiner for Edexcel. Lisa has been a successful Head of Department in Kent and has offered private Business tuition to students across the UK. Lisa loves to create imaginative and accessible resources which engage learners and build their passion for the subject.