This article is about the voting system criterion. For the mathematical notion of an order preserving mapping, see monotonic function. For the concept of population or voter monotonicity, see Participation criterion.
The positive response,[1][2]monotonicity, or nonperversitycriterion[3] is a principle of social choice theory that says that increasing a candidate's ranking or rating should not cause them to lose.[4] Positive response rules out cases where a candidate loses an election as a result of receiving too much support from voters (i.e. being "too popular to win"). Rules that violate positive response are also called perverse[5] and are said to suffer the more-is-less paradox. Such paradoxes are especially common in ranked-choice voting (RCV-IRV),[6] a behavior which often leads to the elimination of moderate candidates and the election of extremists.[7]
Systems that violate positive response can create situations where a voter's ballot has a reversed effect on the election, making it "less than worthless". This runs counter to the basic democratic principle that increasing an option's popularity in a democratic election should only improve their chances of winning; as a result, German courts have previously struck down nonmonotonic systems for violating the right to equal and direct suffrage.[2][8]
The participation criterion is a closely-related, but different, concept. While positive responsiveness deals with a voter changing their opinion (or vote), participation deals with situations where a voter choosing to cast a ballot can have a reversed effect on the election.
^Ornstein, Joseph T.; Norman, Robert Z. (2014-10-01). "Frequency of monotonicity failure under Instant Runoff Voting: estimates based on a spatial model of elections". Public Choice. 161 (1–2): 1–9. doi:10.1007/s11127-013-0118-2. ISSN0048-5829. S2CID30833409.
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