AP® Comparative Government and Politics review sheet from Aim for Five (aimforfive.com/comp-gov/units/4/4-1)
Unit 4 · Topic 4.1
4.1 Electoral Systems and Rules
Electoral systems turn votes into seats, and the rules shape which parties win and how many survive. Single-member district plurality rewards big parties; proportional representation spreads seats by vote share; mixed systems combine the two. The course countries use all three, plus special rules for presidents.
Key terms
- single-member district plurality
- first-past-the-post
- proportional representation
- mixed electoral system
- runoff election
- threshold
Recent elections and rules (as of October 2026)
- Russia held State Duma elections on 18–20 September 2026 under its mixed system. United Russia won 349 of the 450 seats.
- After Nigeria's Electoral Act 2026 became law in February 2026, the electoral commission (INEC) moved the presidential and National Assembly elections to 16 January 2027, the first January presidential vote since civilian rule returned in 1999. Governorship elections follow on 6 February 2027. Check the results after those votes.
- In May 2026 Wales elected its Senedd under a new closed-list proportional system for the first time, with 96 members. Elections for the UK House of Commons still use single-member district plurality.
The main systems
- Single-member district plurality (SMDP), or first-past-the-post: the country is split into districts, each electing one member. The candidate with the most votes wins, even without a majority. Used for the UK House of Commons and Nigeria's National Assembly.
- Proportional representation (PR): each district elects several members (a multimember district), voters choose a party list, and parties get seats in proportion to their vote share. A threshold (a minimum vote share, like 5%) keeps very small parties out.
- Mixed system: some seats are filled by districts and some by PR. Mexico's Chamber of Deputies has 300 district and 200 PR seats; Russia's Duma has 225 of each, with a 5% threshold for list seats. Mexico caps how overrepresented any party can be; Russia counts the two halves separately. Russia used all-PR elections with a 7% threshold in 2007 and 2011, then went back to the mixed system in 2016.
- Two-round (runoff) system: if no candidate wins a majority, the top two face a second round. Used for presidents in Russia and Iran; Iran held a runoff in 2024.
- Other legislatures: Iran's Majles is elected in single- and multi-member districts, and candidates who fall short of a minimum vote share face a second round. Candidates run as individuals or on loose factional lists, not formal party lists. China's National People's Congress is chosen indirectly, each level of people's congress electing the one above it.
Presidential election rules
Mexico's president needs only a plurality, not a majority. Russia and Iran require a majority, using a runoff if needed. Nigeria's 25% rule is designed to make the winner build support across regions, not just in one ethnic or religious bloc.
| Country | How the president is chosen |
|---|---|
| Mexico | Plurality: most votes wins, in one round |
| Nigeria | Most votes plus at least 25% in at least two-thirds of the 36 states, or a runoff |
| Russia | Majority, or a runoff between the top two |
| Iran | Majority, or a runoff between the top two; candidates vetted by the Guardian Council |
| UK | No president; the prime minister is the leader of the Commons majority |
| China | State president chosen by the National People's Congress, on the party's nomination |
Effects of the rules
SMDP tends to produce two large parties and big majorities for the winner, because votes for losing candidates in each district win nothing. Duverger's law is the name for this pattern. Smaller parties do well only when their votes are concentrated in certain regions, like the Scottish National Party. PR tends to produce many parties and coalition governments, and seat shares closely match vote shares.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading election data (UK 2024 general election)
In the July 2024 UK general election, Labour won about 33.7% of the vote and 411 of 650 seats; Reform UK won about 14.3% of the vote and 5 seats; the Liberal Democrats won about 12.2% of the vote and 72 seats. (a) Calculate Labour's share of seats. (b) Describe the pattern for Reform UK and the Liberal Democrats. (c) Explain how the electoral system produced this pattern.
Show the solutionHide the solution
- Step 1: (a) Seat share = 411 ÷ 650 ≈ 0.632, or about 63%. Labour won nearly two-thirds of seats with about a third of the vote.
- Step 2: (b) Reform UK got more votes than the Liberal Democrats (14.3% vs. 12.2%) but far fewer seats (5 vs. 72).
- Step 3: (c) Under single-member district plurality, only first place in each district wins a seat. Reform's votes were spread across many districts where it came second or third, so they won little. Liberal Democrat votes were concentrated in districts where the party could win.
- Step 4: Link to the concept: SMDP over-rewards the largest party and parties with geographically concentrated support, and punishes parties with spread-out support.
Answer: (a) About 63% of seats. (b) Reform had a larger vote share than the Liberal Democrats but won far fewer seats. (c) Plurality voting in single-member districts rewards concentrated support and first-place finishes, not total votes.
Common mistakes
- Saying first-past-the-post needs a majority. The winner needs only the most votes (a plurality).
- Forgetting that Mexico and Russia both use mixed systems for their lower houses.
- Stating Nigeria's rule as “25% in every state.” It's at least 25% in at least two-thirds of the states.
On the exam
- Quantitative analysis questions often show vote and seat shares. Compute or compare shares, describe the gap and explain it with the electoral system.
- Learn the effects of each system (number of parties, majorities, representation of minorities), not just the definitions.
Connected topics
Videos
Check yourself
4 questions on 4.1 Electoral Systems and Rules. Pick an answer to see if you got it, and why.
In Russia's State Duma election of 18–20 September 2026, United Russia won 349 of the 450 seats. Which statement best describes the rules under which the Duma was elected?
Which feature defines a single-member district plurality system?
Compared with single-member district plurality, proportional representation tends to
Which pair of course countries elects its president by an absolute majority, holding a runoff if no candidate wins one in the first round?
0 of 4 answered