Participation criterion
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The participation criterion, also called vote or population monotonicity, is a voting system criterion that says that a candidate should never lose an election as a result of receiving too many votes in support.[1][2] More formally, it says that adding more voters who prefer Alice to Bob should not cause Alice to lose the election to Bob.[3]
Voting systems that fail the participation criterion exhibit the no-show paradox,[4] where a voter is effectively disenfranchised by the electoral system because turning out to vote would make the outcome worse. In such a scenario, these voters' ballots are treated as less than worthless, actively harming their own interests by reversing an otherwise-favorable outcome.[5]
The criterion can also be thought of as a weak kind of strategyproofness: while it is impossible for honesty to always be the best strategy (by Gibbard's theorem), the participation criterion guarantees honesty will always "work" as a strategy (i.e. an honest vote will make the outcome better).
Positional methods and score voting satisfy the participation criterion. All methods satisfying paired majority-rule[4][6] can fail in situations involving four-way cyclic ties, though such scenarios are empirically rare. Most notably, instant-runoff voting and the two-round system often fail the participation criterion in competitive elections, typically as a result of center squeeze.[1][2][7]
Noncompliant methods
[edit]The most common cause of no-show paradoxes is the use of instant-runoff (often called ranked-choice voting in the United States). In instant-runoff voting, a no-show paradox can occur even in elections with only three candidates, and occur in 50%-60% of all 3-candidate elections where the results of IRV disagree with those of plurality.[7][2]
A notable example is given in the 2009 Burlington mayoral election, the United States' second instant-runoff election in the modern era, where Bob Kiss won the election as a result of 750 ballots ranking him in last place.[8]
An example with three parties (Top, Center, Bottom) is shown below. In this scenario, the Bottom party initially loses. However, say that a group of pro-Top voters joins the election, making the electorate more supportive of the Top party, and more strongly opposed to the Bottom party. This increase in the number of voters who rank Bottom last causes the Center candidate to lose to the Bottom party:
| More-popular Bottom | Less-popular Bottom | |||||
|---|---|---|---|---|---|---|
| Round 1 | Round 2 | Round 1 | Round 2 | |||
| Top | 
|
+6 | Top | 31 | 46 | |
| Center | 30 | 55 
|
Center | |
||
| Bottom | 39 | 39 | Bottom | 39 | 54
| |
Here, the increase in support for the Top party allowed it to defeat the Center party in the first round. This makes the election an example of a center-squeeze, a class of elections where instant-runoff and plurality have difficulty electing the majority-preferred candidate because of vote-splitting in the first round.[9]
Condorcet methods
[edit]When there are at most 3 major candidates, Minimax Condorcet and its variants (such as Ranked Pairs and Schulze's method) satisfy the participation criterion.[10] However, with more than 3 candidates, every resolute and deterministic Condorcet method can sometimes fail participation.[10][11] Similar incompatibilities have also been proven for set-valued voting rules.[11][12][13]
Studies suggest such failures may be empirically rare, however. One study surveying 306 publicly-available election datasets found no participation failures for methods in the ranked pairs-minimax family.[14]
Certain conditions weaker than the participation criterion are also incompatible with the Condorcet criterion. For example, weak positive involvement requires that adding a ballot in which candidate A is one of the voter's most-preferred candidates does not change the winner away from A. Similarly, weak negative involvement requires that adding a ballot in which A is one of the voter's least-preferred does not make A the winner if it was not the winner before. Both conditions are incompatible with the Condorcet criterion.[15]
In fact, an even weaker property can be shown to be incompatible with the Condorcet criterion: it can be better for a voter to submit a completely reversed ballot than to submit a ballot that ranks all candidates honestly.[16]
Quota rules
[edit]Proportional representation systems using largest remainders for apportionment (such as STV or Hamilton's method) do not pass the participation criterion. This happened in the 2005 German federal election, when CDU voters in Dresden were instructed to vote for the FDP, a strategy that allowed the party an additional seat.[17] As a result, the Federal Constitutional Court ruled that negative voting weights violate the German constitution's guarantee of the one man, one vote principle.[18]
Quorum requirements
[edit]One common failure of the participation criterion in elections is not in the use of particular voting systems to elect candidates to office, but in simple yes or no measures that place quorum requirements. A public referendum, for example, if it required majority approval and a certain number of voters to participate in order to pass, would fail the participation criterion, as a minority of voters preferring the "no" option could cause the measure to fail by simply not voting rather than voting no. In other words, the addition of a "no" vote may make the measure more likely to pass. A referendum that required a minimum number of yes votes (not counting no votes), by contrast, would pass the participation criterion.[19] Many representative bodies have quorum requirements where the same dynamic can be at play.
Relationship to vote positivity
[edit]Negative vote weight refers to an effect that occurs in certain elections where votes can have the opposite effect of what the voter intended. A vote for a party might result in the loss of seats in parliament, or the party might gain extra seats by not receiving votes. This runs counter to the intuition that an individual voter voting for an option in a democratic election should only increase the chances of that option winning the election overall, compared to not voting (a no-show pathology) or voting against it (a monotonicity or negative response pathology).[citation needed]
Examples
[edit]Majority judgment
[edit]This example shows that majority judgment violates the participation criterion. Assume two candidates A and B with 5 potential voters and the following ratings:
| Candidates | # of
voters | |
|---|---|---|
| A | B | |
| Excellent | Good | 2 |
| Fair | Poor | 2 |
| Poor | Good | 1 |
The two voters rating A "Excellent" are unsure whether to participate in the election.
Voters not participating
[edit]Assume the 2 voters would not show up at the polling place.
The ratings of the remaining 3 voters would be:
| Candidates | # of
voters | |
|---|---|---|
| A | B | |
| Fair | Poor | 2 |
| Poor | Good | 1 |
The sorted ratings would be as follows:
| Candidate |
| |||||||||
| A |
| |||||||||
| B |
| |||||||||
|
Result: A has the median rating of "Fair" and B has the median rating of "Poor". Thus, A is elected majority judgment winner.
Voters participating
[edit]Now, consider the 2 voters decide to participate:
| Candidates | # of
voters | |
|---|---|---|
| A | B | |
| Excellent | Good | 2 |
| Fair | Poor | 2 |
| Poor | Good | 1 |
The sorted ratings would be as follows:
| Candidate |
| |||||||||
| A |
| |||||||||
| B |
| |||||||||
|
Result: A has the median rating of "Fair" and B has the median rating of "Good". Thus, B is the majority judgment winner.
Condorcet methods
[edit]This example shows how Condorcet methods can violate the participation criterion when there is a preference paradox. Assume four candidates A, B, C and D with 26 potential voters and the following preferences:
| Preferences | # of voters |
|---|---|
| A > D > B > C | 8 |
| B > C > A > D | 7 |
| C > D > B > A | 7 |
This gives the pairwise counting method:
| X | |||||
|---|---|---|---|---|---|
| A | B | C | D | ||
| Y | A | [X] 14
[Y] 8 |
[X] 14
[Y] 8 |
[X] 7
[Y] 15 | |
| B | [X] 8
[Y] 14 |
[X] 7
[Y] 15 |
[X] 15
[Y] 7 | ||
| C | [X] 8
[Y] 14 |
[X] 15
[Y] 7 |
[X] 8
[Y] 14 | ||
| D | [X] 15
[Y] 7 |
[X] 7
[Y] 15 |
[X] 14
[Y] 8 |
||
| Pairwise results for X,
won-tied-lost |
1-0-2 | 2-0-1 | 2-0-1 | 1-0-2 | |
The sorted list of victories would be:
| Pair | Winner |
|---|---|
| A (15) vs. D (7) | A 15 |
| B (15) vs. C (7) | B 15 |
| B (7) vs. D (15) | D 15 |
| A (8) vs. B (14) | B 14 |
| A (8) vs. C (14) | C 14 |
| C (14) vs. D (8) | C 14 |
Result: A > D, B > C and D > B are locked in (and the other three can't be locked in after that), so the full ranking is A > D > B > C. Thus, A is elected ranked pairs winner.
Voters participating
[edit]Now, assume an extra 4 voters, in the top row, decide to participate:
| Preferences | # of voters |
|---|---|
| A > B > C > D | 4 |
| A > D > B > C | 8 |
| B > C > A > D | 7 |
| C > D > B > A | 7 |
The results would be tabulated as follows:
| X | |||||
|---|---|---|---|---|---|
| A | B | C | D | ||
| Y | A | [X] 14
[Y] 12 |
[X] 14
[Y] 12 |
[X] 7
[Y] 19 | |
| B | [X] 12
[Y] 14 |
[X] 7
[Y] 19 |
[X] 15
[Y] 11 | ||
| C | [X] 12
[Y] 14 |
[X] 19
[Y] 7 |
[X] 8
[Y] 18 | ||
| D | [X] 19
[Y] 7 |
[X] 11
[Y] 15 |
[X] 18
[Y] 8 |
||
| Pairwise results for X,
won-tied-lost |
1-0-2 | 2-0-1 | 2-0-1 | 1-0-2 | |
The sorted list of victories would be:
| Pair | Winner |
|---|---|
| A (19) vs. D (7) | A 19 |
| B (19) vs. C (7) | B 19 |
| C (18) vs. D (8) | C 18 |
| B (11) vs. D (15) | D 15 |
| A (12) vs. B (14) | B 14 |
| A (12) vs. C (14) | C 14 |
Result: A > D, B > C and C > D are locked in first. Now, D > B can't be locked in since it would create a cycle B > C > D > B. Finally, B > A and C > A are locked in. Hence, the full ranking is B > C > A > D. Thus, B is elected ranked pairs winner by adding a set of voters who prefer A to B.
See also
[edit]References
[edit]- ^ a b Doron, Gideon; Kronick, Richard (1977). "Single Transferrable Vote: An Example of a Perverse Social Choice Function". American Journal of Political Science. 21 (2): 303–311. doi:10.2307/2110496. ISSN 0092-5853. JSTOR 2110496.
- ^ a b c Ray, Depankar (1986-04-01). "On the practical possibility of a 'no show paradox' under the single transferable vote". Mathematical Social Sciences. 11 (2): 183–189. doi:10.1016/0165-4896(86)90024-7. ISSN 0165-4896.
- ^ Woodall, Douglas (December 1994). "Properties of Preferential Election Rules, Voting matters - Issue 3, December 1994".
- ^ a b Moulin, Hervé (1988-06-01). "Condorcet's principle implies the no show paradox". Journal of Economic Theory. 45 (1): 53–64. doi:10.1016/0022-0531(88)90253-0.
- ^ Fishburn, Peter C.; Brams, Steven J. (1983-01-01). "Paradoxes of Preferential Voting". Mathematics Magazine. 56 (4): 207–214. doi:10.2307/2689808. JSTOR 2689808.
- ^ Brandt, Felix; Geist, Christian; Peters, Dominik (2016-01-01). "Optimal Bounds for the No-Show Paradox via SAT Solving". Proceedings of the 2016 International Conference on Autonomous Agents & Multiagent Systems. AAMAS '16. Richland, SC: International Foundation for Autonomous Agents and Multiagent Systems: 314–322. arXiv:1602.08063. ISBN 9781450342391.
- ^ a b McCune, David; Wilson, Jennifer (2024-04-07). "The Negative Participation Paradox in Three-Candidate Instant Runoff Elections". arXiv:2403.18857 [physics.soc-ph].
- ^ Graham-Squire, Adam T.; McCune, David (2023-06-12). "An Examination of Ranked-Choice Voting in the United States, 2004–2022". Representation: 1–19. arXiv:2301.12075. doi:10.1080/00344893.2023.2221689.
- ^ Laslier, Jean-François; Sanver, M. Remzi, eds. (2010). Handbook on Approval Voting. Studies in Choice and Welfare. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 2. doi:10.1007/978-3-642-02839-7. ISBN 978-3-642-02838-0.
By eliminating the squeezing effect, Approval Voting would encourage the election of consensual candidates. The squeezing effect is typically observed in multiparty elections with a runoff. The runoff tends to prevent extremist candidates from winning, but a centrist candidate who would win any pairwise runoff (the "Condorcet winner") is also often "squeezed" between the left-wing and the right-wing candidates and so eliminated in the first round.
- ^ a b Moulin, Hervé (1988-06-01). "Condorcet's principle implies the no show paradox". Journal of Economic Theory. 45 (1): 53–64. doi:10.1016/0022-0531(88)90253-0.
- ^ a b Brandt, Felix; Geist, Christian; Peters, Dominik (2016-01-01). "Optimal Bounds for the No-Show Paradox via SAT Solving". Proceedings of the 2016 International Conference on Autonomous Agents & Multiagent Systems. AAMAS '16. Richland, SC: International Foundation for Autonomous Agents and Multiagent Systems: 314–322. arXiv:1602.08063. ISBN 9781450342391.
- ^ Pérez, Joaquín (2001-07-01). "The Strong No Show Paradoxes are a common flaw in Condorcet voting correspondences". Social Choice and Welfare. 18 (3): 601–616. CiteSeerX 10.1.1.200.6444. doi:10.1007/s003550000079. ISSN 0176-1714. S2CID 153489135.
- ^ Jimeno, José L.; Pérez, Joaquín; García, Estefanía (2009-01-09). "An extension of the Moulin No Show Paradox for voting correspondences". Social Choice and Welfare. 33 (3): 343–359. doi:10.1007/s00355-008-0360-6. ISSN 0176-1714. S2CID 30549097.
- ^ Mohsin, F., Han, Q., Ruan, S., Chen, P. Y., Rossi, F., & Xia, L. (2023, May). Computational Complexity of Verifying the Group No-show Paradox. In Proceedings of the 2023 International Conference on Autonomous Agents and Multiagent Systems (pp. 2877-2879).
- ^ Duddy, Conal (2013-11-29). "Condorcet's principle and the strong no-show paradoxes". Theory and Decision. 77 (2): 275–285. doi:10.1007/s11238-013-9401-4. hdl:10379/11267. ISSN 0040-5833.
- ^ Sanver, M. Remzi; Zwicker, William S. (2009-08-20). "One-way monotonicity as a form of strategy-proofness". International Journal of Game Theory. 38 (4): 553–574. doi:10.1007/s00182-009-0170-9. ISSN 0020-7276. S2CID 29563457.
- ^ Pukelsheim, Friedrich (2014). Proportional representation : apportionment methods and their applications. Internet Archive. Cham; New York : Springer. ISBN 978-3-319-03855-1.
- ^ dpa (2013-02-22). "Bundestag beschließt neues Wahlrecht". Die Zeit (in German). ISSN 0044-2070. Retrieved 2024-05-02.
- ^ Aguiar-Conraria, Luis & Magalhães, Pedro. (2010). "Referendum design, quorum rules and turnout." Downloaded on July 1, 2024 from https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1451131 .
Further reading
[edit]- Woodall, Douglas R, "Monotonicity and Single-Seat Election Rules" Voting matters, Issue 6, 1996