Chicken dilemma

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The "chicken dilemma" refers to a situation where two similar candidates share a majority, but are opposed by one candidate which has a plurality against either of the two. This can happen when there is a majority split into two subfactions (below called A and B), competing against a united minority (below called C) that is bigger than either of the subfactions.

This scenario has been called the "chicken dilemma" because in many election systems, the two majority subfactions are in a situation that resembles the classic "chicken" or "snowdrift" game (especially if voters are not sure which of the two subfactions is larger). A method that encourages cooperation by threatening to punish defectors is said to pass the chicken dilemma criterion. See Analysis for more information.

Definition[edit | edit source]

Below are two definitions of the Chicken Dilemma criterion; "CD" and "CD2".

CD[edit | edit source]

The A voters are the voters who vote A over everyone else. The B voters are the voters who vote B over everyone else. The C voters are the voters who vote C over everyone else.

Premise[edit | edit source]

  1. There are 3 candidates: A, B, and C.
  2. The A voters and the B voters, combined, add up to more than half of the voters in the election.
  3. The A voters are more numerous than the B voters. The C voters are more numerous than the A voters, and more numerous than the B voters.
  4. The A voters vote B over C. The B voters refuse to vote A over anyone.
  5. None of the C voters vote A or B over the other.

Requirement[edit | edit source]

B doesn't win.

CD2[edit | edit source]

CD is sufficient, as-is, but here is a non-numerical definition:

The A voters are the voters who vote A over everyone else. The B voters are the voters who vote B over everyone else. The C voters are the voters who vote C over everyone else.

Premise:

1. There are 3 candidate: A, B, and C.

2. If the A voters and B voters all voted both A and B over C, then C couldn't win.

3. The ballot set is such that if C withdrew from the election and the count, A would win.

4. The A voters vote B over C.

5. The B voters don't vote A over anyone.

Requirement:

B doesn't win.

Analysis[edit | edit source]

If we assume each faction has a single, coordinated strategy defined as "cooperate" (vote both candidates A and B above bottom) or "defect" (bullet vote, with only the favorite above bottom); and that each faction values its preferred choice at 10, its less-preferred choice at 8, and candidate C at 0, many voting systems lead to the following payoff matrix:

cooperate defect
cooperate 9, 9 8, 10
defect 10, 9 0, 0
Fig. 2: Chicken with numerical payoffs

There are various ways to deal with this situation. For instance:

  1. Some voting systems, such as approval voting, ignore the problem. Perhaps the assumption here is that it will be impossible to organize a defection without prompting a retaliation, and thus that both sides will prefer to cooperate. ("Mutual assured destruction"?)
  2. Some voting systems, such as Majority Choice Approval, try to exploit the fact that each faction is not a single coordinated entity, but a group of individual voters. The idea is that if a small number of voters defect, they should be ignored; hopefully, in that situation, majority cooperation will be a stable strategy.
  3. Other voting systems, such as ICT, try to exploit the fact that in a real-world election, A and B are never perfectly balanced; one subfaction is always larger. In this case, a voting system can encourage the smaller group to cooperate by threatening to elect C (punishing both groups) if the smaller group defects. The chicken dilemma criterion is passed only by this kind of voting system.

The chicken dilemma happens when there is a Condorcet winner and a majority Condorcet loser, but not a majority Condorcet winner. In many voting systems, supporters of one of the two similar candidates have a dilemma, like a game of "chicken": they can either "cooperate" and support both similar candidates, helping to ensure the opposing plurality candidate loses but risking a win by the less-preferred of the similar ones; or they can "betray" and support only their favorite candidate, trying to take advantage of cooperation by the other side.

An example of a chicken dilemma scenario, in the format of "#voters:true preferences":

  • 33: A>B>>C
  • 22: B>A>>C
  • 45: C>>A=B

In the chicken dilemma scenario described in the premise of the Chicken Dilemma Criterion (CD), defined above, if B won, then the B voters would have successfully taken advantage of the A voters' cooperativeness. The A voters wanted to vote both A and B over the candidate disliked by both the A voters and B voters. Thereby they helped {A,B} against the worse candidate. But, with methods that fail CD, the message is "You help, you lose".

Methods passing criterion[edit | edit source]

Some methods that pass the Chicken Dilemma Criterion:

ICT, Symmetrical ICT, MMPO, MDDTR, IRV, Benham's method, Woodall's method

Notes[edit | edit source]

One direct implication of the chicken dilemma criterion is a spoiler effect. This is because if A drops out of the race, then B becomes a majority's 1st choice and wins in any majority criterion-passing method. Further, there is also usually incentive for Favorite Betrayal, since A-top voters generally benefit from putting B 1st. That would be the case in IRV for:

26 A>B

25 B

49 C

If two A-top voters instead vote B-top, B wins instead of C.


Most advocates of the CD criterion would likely counter that it's a more common issue to encounter CD-type scenarios because of strategic voting rather than honest voting, therefore these are worthy prices to pay for the strategic resistance.