Monotonicity Criterion

Electoral system criteria

In social choice, the negative response, perversity, or additional support paradox is a pathological behavior of some voting rules where a candidate loses as a result of having too much support (or wins because of increased opposition). In other words, increasing (decreasing) a candidate's ranking or rating causes that candidate to lose (win), respectively. Electoral systems that do not exhibit perversity are sometimes said to satisfy the monotonicity criterion.D R Woodall
"Monotonicity and Single-Seat Election Rules"
'' Voting matters'', Issue 6, 1996

Perversity is often described by social choice theorists as an exceptionally severe kind of electoral pathology, as such rules can have "backwards" responses to voters' opinions, where popularity causes defeat while unpopularity leads to a win. Similar rules treat the well-being of some voters as "less than worthless". These issues have led to constitutional prohibitions on such systems as violating the right to equal and direct suffrage. Negative response is often cited as an example of a perverse incentive, as rules with negative response can incentivize politicians to take extreme or unpopular positions in an attempt to shed excess votes.

Most ranked methods (including Borda and all common round-robin rules) satisfy positive response, as do all common rated voting methods (including approval, highest medians, and score).

Negative responsiveness occurs in instant-runoff voting (IRV), the single transferable vote, and the two-round system. Some quota-based apportionment methods also violate the rule, as can the randomized Condorcet method in cases of cyclic ties.

The participation criterion is closely-related, but different. While positive responsiveness deals with a voter changing their opinion (or vote), participation deals with situations where a voter choosing to cast a ballot at all has a backwards effect on the election.

Definition

Miller defined two main classes of monotonicity failure in 2012, which have been repeated in later papers:

In simpler terms, an upward failure occurs when a winner loses from more support, and a downward failure occurs when a loser wins with less support.

Views

Social choice theorists generally agree that negative responsiveness is an especially severe issue for a voting rule. Some have argued the mere possibility should be enough to disqualify runoff-based electoral methods, while others argue this is only true if it occurs in "easy" or "common" cases, generally meaning those without a Condorcet cycle.

Gallagher notes some political scientists are less concerned about negative response, arguing voters will not notice or understand it, making it appear random from their perspective and preventing exploitation by strategic voters. By contrast, other researchers have argued voters will predict negative response and respond by strategically down-ranking their preferred candidates, keeping it from affecting the results.

By method

Runoff voting

Runoff-based voting systems such as ranked choice voting (RCV) are typically vulnerable to perverse response. A notable example is 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. Another example is given by the 2022 Alaska at-large special election.

An example with three parties (Top, Center, Bottom) is shown below. In this scenario, the Bottom party initially loses. However, they are ''elected'' after running an ''un''successful campaign and adopting an ''un''popular platform, which pushes their supporters away from the party and into the Top party.

This election is an example of a center-squeeze, a class of elections where instant-runoff and plurality have difficulties electing the majority-preferred candidate. Here, the loss of support for Bottom policies makes the Top party more popular, allowing it to defeat the Center party in the first round.

Proportional rules

Some proportional representation systems can exhibit negative responsiveness. These include the single transferable vote and some implementations of mixed-member proportional representation, generally as a result of poorly-designed overhang rules. An example can be found in the 2005 German federal election.

Frequency of violations

The frequency of negative response will depend on the electoral method, the candidates, and the distribution of outcomes.

Empirical analysis

In the US, a 2021 analysis of instant-runoff elections in California between 2008 and 2016, as well as the 2009 Burlington, Vermont mayoral election, found an upward monotonicity anomaly rate of 0.74% (1/135) in all elections, 2.71% (1/37) when limited to elections going to a second round of counting and 7.7% (1/13) of elections with three competitive candidates. A more comprehensive 2023 survey of 182 American IRV elections where no candidate was ranked first by a majority of voters found seven total examples of non-monotonicity (3.8%), broken down into 2.2% (4/182) examples of upward monotonicity, 1.6% (3/182) of downward montonicity and 0.5% (1/182) of no-show or truncation (one example was both an upward and downward monotonicity failure). Two of those elections are also noted as specific examples below.

Semi-empirical

Some empirical research do not have access to full ballot preference data, and thus make probabilistic estimates of transfer patterns. A 2013 survey of Irish elections using IRV and PR-STV found plausible non-monotonicity in 20 out of 1326 elections between 1922 and 2011.

Data from the five UK general elections between 1992 and 2010 showed 2642 three candidate elections in English constituencies. With second preferences imputed from survey data, 1.7% of all elections appeared vulnerable to monotonicity anomalies (1.4% upward, 0.3% downward), significantly lower than simulated datasets from the same paper. However, when limited to the 4.2% of elections considered three-way competitive, 40.2% appeared vulnerable (33% upward, 7.1% downward), and further increasing with closer competition, a result closer to the simulations.

A 2022 analysis out of the 10 French presidential elections (conducted under the two-round system) 2 had results where monotonicity violations were not mathematically possible, another 6 where violations were unlikely given the evidence, leaving 2 elections ( 2002 and 2007) where an upward monotonicity violation was probable and likely respectively.

Theoretical models

Results using the impartial culture model estimate about 15% of elections with 3 candidates; however, the true probability may be much higher, especially when restricting observation to close elections.

A 2013 study using a two-dimensional spatial model of voting estimated at least 15% of IRV elections would be nonmonotonic in the best-case scenario (with only three equally-competitive candidates). The researchers concluded that "three-way competitive races will exhibit unacceptably frequent monotonicity failures" and "In light of these results, those seeking to implement a fairer multi-candidate election system should be wary of adopting IRV."

Specific examples

2022 in Alaska

Alaska's first-ever instant-runoff election resulted in a victory for Democrat Mary Peltola, but had many voters for Republican Sarah Palin instead ranked Peltola first, Peltola would have lost.

Burlington, Vermont

In Burlington's second IRV election, incumbent Bob Kiss was re-elected, despite losing in a head-to-head matchup with Democrat Andy Montroll (the Condorcet winner). However, if Kiss had gained more support from Wright voters, Kiss would have lost.

2005 German election in Dresden

In the 2005 German federal election, CDU supporters in Dresden were instructed to vote for the FDP, a strategy that allowed the CDU to win an additional seat. This led the Federal Constitutional Court to rule that negative responsiveness violates the German constitution's guarantee of equal and direct suffrage.

See also

* Participation criterion, a closely-related concept
* Voting system
* Voting system criterion
* Monotone preferences in consumer theory
* Monotonicity (mechanism design)
* Maskin monotonicity

Notes

References

{{voting systems

Electoral system criteria