The participation criterion is a voting system criterion that says candidates should never lose an election as a result of receiving too many votes in support. More formally, it says that adding more voters who prefer Alice to Bob should not cause Alice to lose the election to Bob.
Voting systems that fail the participation criterion exhibit the no-show paradox, where a voter is effectively disenfranchised by the electoral system because turning out to vote could make the result worse for them; such voters are sometimes referred to as having negative vote weights, particularly in the context of German constitutional law, where courts have ruled such a possibility violates the principle of one man, one vote.
Positional methods and score voting satisfy the participation criterion. All deterministic voting rules that satisfy pairwise majority-rule can fail in situations involving four-way cyclic ties, though such scenarios are empirically rare, and the randomized Condorcet rule is not affected by the pathology. The majority judgment rule fails as well. Instant-runoff voting and the two-round system both fail the participation criterion with high frequency in competitive elections, typically as a result of a center squeeze.
The no-show paradox is similar to, but not the same as, the perverse response paradox. Perverse response happens when an existing voter can make a candidate win by decreasing their rating of that candidate (or vice-versa). For example, under instant-runoff voting, moving a candidate from first-place to last-place on a ballot can cause them to win.
Noncompliant methods
Instant-runoff (ranked-choice) voting
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,
An example with three parties (Top, Center, Bottom) is shown below. In this scenario, the Center party initially wins. However, say that a group of pro-Top voters joins the election, all of whom cast their ballots with the ranking Top > Center > Bottom:
{| class="wikitable"
|+
! colspan="3" |Baseline
!
! colspan="3" |More votes ranking Center > Bottom
|-
!
!Round 1
!Round 2
!
!
!Round 1
!Round 2
|-
!Top
|<s>25</s>
|
!+6
!Top
|31
|46
|-
!Center
|30
|55
!
!Center
|<s>30</s>
|
|-
!Bottom
|39
|39
!
!Bottom
|39
|54
|}
The participation criterion says that the addition of voters who prefer Center over Bottom should not cause Bottom to win. Yet the increase in support for the Top party allows it to defeat the Center party in the first round. As a result, 15 votes that previously counted toward Center now count toward their second preference, Bottom, which is enough to give Bottom the victory. 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.
Condorcet methods
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. The randomized Condorcet rule satisfies the criterion, but fails the closely related monotonicity criterion in situations with Condorcet cycles.
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.
In addition to participation, an even more severe paradox property can occur with Condorcet's principle: it may be better for a voter to submit a completely reversed ballot than to submit a ballot that ranks all candidates honestly.
Implementations of proportional representation
Some proportional representation systems allow for no-show paradoxes. Rules based on the Droop quota (including all current implementations of the single transferable vote) allow for no-show paradoxes.
Hamilton's method exhibits a milder form of the paradox called the population paradox, but does not have a true no-show paradox. However, adding an electoral threshold produces a no-show paradox.
In Germany
In Germany, situations where a voter's ballot has the opposite of its intended effect (e.g. a vote for a party or candidate causes them to lose) are called negatives Stimmgewicht (). An infamous example occurred in the 2005 German federal election, when an article in Der Spiegel laid out how CDU voters in Dresden I would have to vote against their own party if they wished to avoid losing a seat in the Bundestag.
The Federal Constitutional Court agreed with the plaintiffs, ruling that negative vote weights violate the German constitution's guarantee of equal and direct suffrage. The majority wrote that:
Many representative bodies have quorum requirements where the same dynamic can be at play. For example, the requirement for a two-thirds quorum in the Oregon Legislative Assembly effectively creates an unofficial two-thirds supermajority requirement for passing bills, and can result in a law passing if too many senators turn out to oppose it. Deliberate ballot-spoiling strategies have been successful in ensuring referendums remain non-binding, as in the 2023 Polish referendum.
Manipulation
The participation criterion can also be justified as a weaker form 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 be an effective, rather than actively counterproductive, strategy (i.e. a voter can always safely cast a sincere vote).
Quorum-busting
Examples
Majority judgment
This example shows that majority judgment violates the participation criterion. Assume two candidates A and B with 5 potential voters and the following ratings:
{| class="wikitable"
! colspan="2" |Candidates
! rowspan="2" |# of
voters
|-
!A
!B
|-
| bgcolor="green" |Excellent
| bgcolor="YellowGreen" |Good
|2
|-
| bgcolor="yellow" |Fair
| bgcolor="orangered" |Poor
|2
|-
| bgcolor="orangered" |Poor
| bgcolor="YellowGreen" |Good
|1
|}
The two voters rating A "Excellent" are unsure whether to participate in the election.
Voters not participating
Assume the 2 voters would not show up at the polling place.
The ratings of the remaining 3 voters would be:
{| class="wikitable"
! colspan="2" |Candidates
! rowspan="2" |# of
voters
|-
!A
!B
|-
| bgcolor="yellow" |Fair
| bgcolor="orangered" |Poor
|2
|-
| bgcolor="orangered" |Poor
| bgcolor="YellowGreen" |Good
|1
|}
The sorted ratings would be as follows:
{|
| align="right" |Candidate
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="49%" |
| width="2%" |↓
| width="49%" |Median point
|}
|-
| align="right" |A
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="67%" bgcolor="yellow" |
| width="33%" bgcolor="orangered" |
|}
|-
| align="right" |B
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="33%" bgcolor="yellowgreen" |
| width="67%" bgcolor="orangered" |
|}
|-
|
|
|-
|
|
{| cellspacing="1" border="0" cellpadding="1"
|
| bgcolor="green" |
| Excellent
| bgcolor="YellowGreen" |
| Good
| bgcolor="Yellow" |
| Fair
| bgcolor="Orangered" |
| Poor
|}
|}
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
Now, consider the 2 voters decide to participate:
{| class="wikitable"
! colspan="2" |Candidates
! rowspan="2" |# of
voters
|-
!A
!B
|-
| bgcolor="green" |Excellent
| bgcolor="YellowGreen" |Good
|2
|-
| bgcolor="yellow" |Fair
| bgcolor="orangered" |Poor
|2
|-
| bgcolor="orangered" |Poor
| bgcolor="YellowGreen" |Good
|1
|}
The sorted ratings would be as follows:
{|
| align="right" |Candidate
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="49%" |
| width="2%" |↓
| width="49%" |Median point
|}
|-
| align="right" |A
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="40%" bgcolor="green" |
| width="40%" bgcolor="yellow" |
| width="20%" bgcolor="orangered" |
|}
|-
| align="right" |B
|
{| cellspacing="0" border="0" width="500" cellpadding="0"
| width="60%" bgcolor="yellowgreen" |
| width="40%" bgcolor="orangered" |
|}
|-
|
|
|-
|
|
{| cellspacing="1" border="0" cellpadding="1"
|
| bgcolor="green" |
| Excellent
| bgcolor="YellowGreen" |
| Good
| bgcolor="Yellow" |
| Fair
| bgcolor="Orangered" |
| Poor
|}
|}
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
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:
{| class="wikitable"
!Preferences
!# of voters
|-
|A > D > B > C
|8
|-
|B > C > A > D
|7
|-
|C > D > B > A
|7
|}
This gives the pairwise counting method:
{| class="wikitable" border="1"
|+Pairwise election results
!
!A
!B
!C
!D
|-
! scope="row" |A
|
|
|
|
|-
! scope="row" |B
|
|
|
|
|-
! scope="row" |C
|
|
|
|
|-
! scope="row" |D
|
|
|
|
|-
! Pairwise results<br />for X,<br />won-tied-lost
|1-0-2
|2-0-1
|2-0-1
|1-0-2
|}
The sorted list of victories would be:
{| class="wikitable"
!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
Now, assume an extra 4 voters, in the top row, decide to participate:
{| class="wikitable"
!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:
{| class="wikitable" border="1"
|+Pairwise election results
!
!A
!B
!C
!D
|-
! scope="row" |A
|
|
|
|
|-
! scope="row" |B
|
|
|
|
|-
! scope="row" |C
|
|
|
|
|-
! scope="row" |D
|
|
|
|
|-
! Pairwise results<br />for X,<br />won-tied-lost
|1-0-2
|2-0-1
|2-0-1
|1-0-2
|}
The sorted list of victories would be:
{| class="wikitable"
!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.