The Gibbard-Satterthwaite theorem implies that, in every non-dictatorial voting system with 3 or more candidates, there is a preference profile in which some of the voters can gain by lying about their preferences. However, usually this gain requires that the voters know the voting rule.
For example, consider a voting system with 3 candidates, A B and C, and 12 voters with the following preferences:
- 3 voters prefer A>B>C;
- 4 voters prefer B>C>A;
- 5 voters prefer C>A>B.
Consider the following two voting rules:
- plurality - in which C is elected;
- Instant-runoff - in which B is elected (after A is eliminated in the first round).
Consider the 5 C>A>B voters, and assume all other voters are truthful:
- In plurality voting, they should of course be truthful so that their favorite candidate C wins.
- But in instant-runoff voting, they can gain by lying and saying that their best candidate is A, since this will prevent the election of their worst option B.
So without knowing the voting rule, these voters cannot know whether lying about their preferences will help them or harm them.
The situation is different with the 3 A>B>C voters:
- In plurality voting, they can gain by lying and saying that they prefer B, as this will prevent the election of their worst option C;
- In instant-runoff voting, although they won't gain from lying, they also won't lose.
So even without knowing the voting rule, these voters can know that voting for B will either help them or at least not harm them.
For the sake of this question, assume that all voters are truth-biased. This means that, if they are not sure whether lying will help them or harm them, they will be truthful. They will only lie if lying may help them and may not harm them. So in the above example, the C>A>B voters will be truthful and the A>B>C voters will lie (assuming the B>C>A voters are truthful).
MY QUESTION IS: Is there a collection of reasonable (non-dictatorial, monotone, etc.) voting rules, such that, in all preference profiles, no truth-biased voter will want to lie?
In other words: we look for a collection of voting rules so that, for every preference profile and every group of voters, there exists a voting rule in our collection where their best response (when all other voters are truthful) is to be truthful.