[EM] Strategy-free criterion

[Kristofer Munsterhjelm]

On 2024-08-02 19:35, Chris Benham wrote:
> Now to talk about a problem of all preferential methods.  Say there are 
> three factions of voters, each with its own candidate. Each faction 
> strongly wants its own candidate to win and has no real preference among 
> the other two, i.e. is about equally hostile to or disinterested in both 
> of them.  Say it is known from polling that candidate A has about 50% 
> support and candidates B and C each have about 25%.
> 
> A is obviously the highest SU candidate, and with normal natural sincere 
> say Approval voting all the voters will truncate two of the candidates 
> and A easily wins.  But with a preferential system (especially one that 
> meets Clone-Winner, as they all should) the B and C factions
> have a huge incentive to do a preference-swap deal. That way each of 
> them raise their chance of winning from zero to about 25%.
> 
> In light of that, to some even a result like
> 
> 49 A
> 26 B>C
> 25 C>B
> 
> can look suspicious. Yes B and C are a perfect set of voted clones and a 
> Mutual Majority and B is a voted Condorcet winner, but in the real world 
> some might say that the B and C factions "ganged up" on A and 
> "manipulated the election".

Just to be sure I got what you were saying, this is in an imperfect 
information setting, right? The B and C factions don't know which of 
them have more first preferences than the other.

I'm assuming that's the case because if they knew, then while the B 
faction would be interested in doing a swap to flip the winner from A to 
B, the C faction is indifferent to who wins out of A and B, and thus 
wouldn't bother.

> In Australia voting is compulsory and in most elections truncation is 
> not allowed. This would be a big burden on voters who take it seriously 
> if it wasn't for the regular custom of parties and sometimes independent 
> candidates advising their supporters on how to complete their ballots. 
> So a lot of voters are only thinking about which party should get their 
> "primary vote", and the lower preferences are just some relatively 
> arbitrary formality to ensure that their vote is counted.

IMHO, that seems like a significant flaw, because it potentially turns 
party-neutral methods (IRV and STV) into something considerably closer 
to party list.

Given that it does, it would make sense for the voters to treat higher 
ranks as more important than lower ones - because if filling out lower 
ranks is a chore, then voters would focus on higher ranks.

Say a voter "independently ranks" those candidates where he neither 
copies some other advice or ranks randomly, and "fills" the other 
candidates (for lack of better terms).

Then it's possible that the connotation of lower ranks as something that 
you fill, not independently rank, carries through to the point that 
voters independently rank fewer candidates than they would explicitly 
rank if they could truncate.

IRV and STV's focus on higher preferences might also contribute to the 
voters' focus, but it's hard to say what the direction of causality is 
there: whether these methods were adopted because the public was used to 
Plurality-style reasoning, or if their (particularly AV's) compromise 
incentive leads the voters to focus further on first preferences.

-km