Majority_criterion

Majority criterion

Majority criterion

Property of electoral systems

The majority criterion is a voting system criterion. The criterion states that "if only one candidate is ranked first by a majority (more than 50%) of voters, then that candidate must win."[1][2][3]

Some methods that comply with this criterion include any Condorcet method, instant-runoff voting, Bucklin voting, plurality voting, and approval voting.

The criterion was originally defined in relation to methods which rely only on ranked ballots (voted preference orders of the candidates), so while ranked methods such as the Borda count fail the criterion under any definition, its application to methods which give weight to preference strength is disputed. For these methods, such as Score (Range) voting, the system may pass or fail depending on the definition of the criterion which is used.

Advocates of other voting systems contend that the majority criterion is actually a flaw of a voting system and not a feature, since it can lead to a tyranny of the majority where a polarizing candidate is elected who is loved by a little over half of the population and hated by everyone else.[4][5][6][7] Other systems may be better at electing consensus candidates who have broader appeal, which is claimed to make them better representatives of the population as a whole.[8][9] These are described as consensus-seeking rather than majoritarian.[10][11][12]

The mutual majority criterion is a generalized form of the majority criterion meant to account for when the majority prefers multiple candidates above all others; voting methods which pass majority but fail mutual majority can encourage all but one of the majority's preferred candidates to drop out in order to ensure one of the majority-preferred candidates wins, creating a spoiler effect.[13] The common choose-one first-past-the-post voting method is notable for this, as major parties vying to be preferred by a majority often attempt to prevent more than one of their candidates from running and splitting the vote by using primaries.

Difference from the Condorcet criterion

By the majority criterion, a candidate C should win if a majority of voters answers affirmatively to the question "Do you (strictly) prefer C to every other candidate?"

The Condorcet criterion is stronger. According to it, a candidate C should win if for every other candidate Y there is a majority of voters that answers affirmatively to the question "Do you prefer C to Y?". A Condorcet system necessarily satisfies the majority criterion, but not vice versa.

A Condorcet winner C only has to defeat every other candidate "one-on-one"--in other words, when comparing C to any specific alternative. To be the majority choice of the electorate, a candidate C must be able to defeat every other candidate simulataneously--i.e. voters who are asked to choose between C and "anyone else" must pick "C" instead of any other candidate.

Equivalently, a Condorcet winner can have several different majority coalitions supporting them in each one-on-one matchup. A majority winner must instead have a single (consistent) majority that supports them across all one-on-one matchups.

Application to cardinal voting methods

The majority criterion was initially defined with respect to voting systems based only on preference order. In systems with absolute rating categories such as score and highest median methods, it is not clear how the majority criterion should be defined. There are three notable definitions of for a candidate A:

  1. If a majority of voters have (only) A receiving a higher score than any other candidate (even if this is not the highest possible score), this candidate will be elected.
  2. If (only) A receives a perfect score from more than half of all voters, this candidate will be elected.
  3. If a majority of voters prefer (only) A to any other candidate, they can choose to elect candidate A by strategizing.

The first criterion is not satisfied by any common cardinal voting method, but arguably lacks the persuasive force it has when comparing ordinal systems. Ordinal ballots can only tell us whether A is preferred to B (not by how much A is preferred to B), and so if we only know most voters prefer A to B, it is reasonable to say the majority should win.

However, with cardinal voting systems, there is more information available, as voters also state the strength of their preferences. Thus cardinal voting systems do not disregard minority voices when making decisions; a sufficiently-motivated minority can sometimes outweigh the voices of a majority, if they would be strongly harmed by a policy or candidate.

Examples

Systems that meet the majority criterion (plurality, Condorcet, and IRV) elect the Red candidate when they receive a majority of the vote. Borda count does not meet the majority criterion and does not select Red.[14][self-published source?]

Approval voting

Main article: Approval voting

Approval voting trivially satisfies the majority criterion: if a majority of voters approve of A, but a majority do not approve of any other candidate, then A will have an average approval above 50%, while all other candidates will have an average approval below 50%, and A will be elected.

Plurality voting

Any candidate receiving more than 50% of the vote will be elected by plurality.

Instant runoff

Instant-runoff voting satisfies majority--if a candidate is rated first by 50% of the electorate, they will win in the first round.

Borda count

Main article: Borda count

For example 100 voters cast the following votes:

More information Preference, Voters ...

A has 110 Borda points (55 × 2 + 35 × 0 + 10 × 0). B has 135 Borda points (55 × 1 + 35 × 2 + 10 × 1). C has 55 Borda points (55 × 0 + 35 × 1 + 10 × 2).

More information Preference, Points ...

Candidate A is the first choice of a majority of voters but candidate B wins the election.

Condorcet methods

Any Condorcet method will automatically satisfy the majority criterion, as a majority choice of the electorate is always a Condorcet winner.

Cardinal methods

Score voting

Main article: Score voting

For example 100 voters cast the following votes:

More information Ballot, Voters ...

Candidate B would win with a total of 80 × 9 + 20 × 10 = 720 + 200 = 920 rating points, versus 800 for candidate A.

Because candidate A is rated higher than candidate B by a (substantial) majority of the voters, but B is declared winner, this voting system fails to satisfy the criterion due to using additional information about the voters' opinion. Conversely, if the bloc of voters who rate A highest know they are in the majority, such as from pre-election polls, they can strategically give a maximal rating to A, a minimal rating to all others, and thereby guarantee the election of their favorite candidate. In this regard, score voting gives a majority the power to elect their favorite, but just as with approval voting, it does not force them to.

STAR voting

Main article: STAR voting

STAR voting fails majority, but satisfies the majority loser criterion.

Highest medians

Main article: Highest median voting rules

It is controversial how to interpret the term "prefer" in the definition of the criterion. If majority support is interpreted in a relative sense, with a majority rating a preferred candidate above any other, the method does not pass, even with only two candidates. If the word "prefer" is interpreted in an absolute sense, as rating the preferred candidate with the highest available rating, then it does.

Criterion 1

If "A is preferred" means that the voter gives a better grade to A than to every other candidate, majority judgment can fail catastrophically. Consider the case below when n is large:

More information # ballots, A's score ...

A is preferred by a majority, but B's median is Good and A's median is only Fair, so B would win. In fact, A can be preferred by up to (but not including) 100% of all voters, an exceptionally severe violation of the criterion.

Criterion 2

If we define the majority criterion as requiring a voter to uniquely top-rate candidate A, then this system passes the criterion; any candidate who receives the highest grade from a majority of voters receives the highest grade (and so can only be defeated by another candidate who has majority support).

See also

References

  1. [1]
    Rothe, Jörg (2015-08-18). Economics and Computation: An Introduction to Algorithmic Game Theory, Computational Social Choice, and Fair Division. Springer. p. 231. ISBN 9783662479049. A voting system satisfies the majority criterion if a candidate who is placed on top in more than half of the votes always is a winner of the election.
  2. [2]
    Pennock, Ronald; Chapman, John W. (1977). Due Process: Nomos XVIII. NYU Press. p. 266. ISBN 9780814765692. if there is some single alternative which is ranked first by a majority of voters, we shall say there exists a majority will in favor of that alternative, according to the absolute majority (AM) criterion.
  3. [3]
    "Single-winner Voting Method Comparison Chart". Archived from the original on 2011-02-28. Majority Favorite Criterion: If a majority (more than 50%) of voters prefer candidate A to all other candidates, then A should win.
  4. [4]
    Beatty, Harry (1973). "Voting Rules and Coordination Problems". The Methodological Unity of Science. Theory and Decision Library. Springer, Dordrecht. pp. 155–189. doi:10.1007/978-94-010-2667-3_9. ISBN 9789027704047. This is true even if the members of the majority are relatively indifferent among a, b and c while the members of the minority have an intense preference for b over a. So the objection can be made that plurality or majority voting allows a diffident majority to have its way against an intense minority.
  5. [5]
    "Score Voting, Approval Voting, and Majority Rule". The Center for Election Science. 2015-05-21. Retrieved 2016-12-03. Score voting [and] approval voting, are sometimes attacked for not abiding by the majority criterion in all cases. ... This page shows that such an event with these methods is not catastrophic and may even be desirable.
  6. [6]
    Emerson, Peter (2016). From Majority Rule to Inclusive Politics (1st ed.). Cham: Springer. ISBN 9783319235004. OCLC 948558369. Unfortunately, one of the worst democratic structures is the most ubiquitous: majority rule based on majority voting. It must be emphasised, furthermore, that these two practices are often the catalysts of division and bitterness, if not indeed violence and war.
  7. [7]
    Emerson, Peter (2016-03-23). "Majority Rule - A Cause of War?". In Gardner, Hall; Kobtzeff, Oleg (eds.). The Ashgate Research Companion to War: Origins and Prevention. Routledge. ISBN 9781317041108.
  8. [8]
    "Majority Criterion". The Center for Election Science. 2015-05-21. Retrieved 2016-12-03. Sometimes a candidate who is the Condorcet winner, or even the majority winner, isn't the favored or "most representative" candidate of the electorate.
  9. [9]
    Lippman, David. "Voting Theory" (PDF). Math in Society. Borda count is sometimes described as a consensus-based voting system, since it can sometimes choose a more broadly acceptable option over the one with majority support.
  10. [10]
    "Utilitarian vs. Majoritarian Election Methods - The Center for Election Science". sites.google.com. Retrieved 2017-01-07.
  11. [11]
    "Vote Aggregation Methods". lorrie.cranor.org. Retrieved 2017-01-12.
  12. [12]
    Hillinger, Claude (2006-05-15). "The Case for Utilitarian Voting". Rochester, NY: Social Science Research Network. SSRN 878008. {{cite journal}}: Cite journal requires |journal= (help)
  13. [13]
    Kondratev, Aleksei Y.; Nesterov, Alexander S. (2020). "Measuring Majority Power and Veto Power of Voting Rules". Public Choice. 183 (1–2): 187–210. arXiv:1811.06739. doi:10.1007/s11127-019-00697-1. S2CID 53670198.
  14. [14]
    Yee, Ka-Ping (2010-03-13). "Election Methods in Pictures". zesty.ca. Retrieved 2016-12-03.
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