[EM] serious strategy problem in Condorcet, but not in IRV?

[Markus Schulze]

Dear James Green-Armytage,

you wrote (17 Aug 2003):
> Without further ado, let me try to get into an example. This illustrates
> my interpretation of the implications of the Monroe paper, which is not
> necessarily the same as his intention, but which should be close enough
> for starters.
>
> Sincere preferences
> 46: A>B
> 44: B>A
> 5: C>A
> 5: C>B
>
> It is extremely clear here that C seriously does not deserve to win, as he
> is ranked last by 90% of the voters. Also, it is clear that A deserves to
> win, albeit by a narrow margin.
> Now, if the method is Condorcet (minimax, Schwartz / minimax, ranked
> pairs, or beatpath), and if everyone voted sincerely, A would win.
> However, if the 44 B>A voters strategically vote B>C (offensive order
> reversal), a cycle is formed, in which the defeat of B is now the defeat
> of least magnitude, and so B wins.
>
> 46: A>B
> 44: B>C
> 5: C>A
> 5: C>B
>
> A:B = 51:49
> A:C = 46:54
> B:C = 90:10
>
> This is already very unfair, and a clear subversion of the democratic
> process.

According to Blake Cretney's terminology, the 44 BAC voters are "burying"
(i.e. ranking a candidate insincerely lower to make him lose). IRV isn't
vulnerable to burying while most other methods are. Therefore, I consider
Monroe's criticism feasible.

Blake Cretney's terminology:
http://www.condorcet.org/emr/defn.shtml

******

You wrote (17 Aug 2003):
> I would like to know if anyone has a way to blow Monroe's argument
> out of the water, beyond my own relatively lukewarm counter.

I would point to the fact that IRV is vulnerable to "pushing over"
(i.e. ranking a weak candidate higher than one's preferred candidate)
while those methods that meet monotonicity aren't.

******

You wrote (17 Aug 2003):
> P.S. By the way, here is another example similar to the one I used in
> this posting, where C is a less wretched loser, but where manipulation
> is easier:
>
> Sincere preferences
> 30: A>B
> 25: B>A
> 23: C>A
> 22: C>B

When IRV is being used, then I suggest that the 25 BAC voters should use
bullet voting so that the 30 ABC voters would have to give their first
preferences to candidate B to keep candidate C from winning.

******

You wrote (18 Aug 2003):
> Certainly you can't punish individual voters for voting insincerely. What
> you could do is punish anyone, let's say a party official or employee, who
> openly reaches out to the supporters of that party and instructs them to
> vote insincerely. For something like this to have an impact in a public
> election, it would be necessary to reach out to *a lot* of people, so it
> wouldn't be too hard to find out about it.

That reminds me of Australia. Here, it wasn't allowed to _ask_ the voters
to use bullet voting. But it was allowed to _educate_ the voters about the
possible consequences of truncated ballots.

Similarily, even when the parties in the USA weren't allowed to _instruct_
their supporters to vote in a certain manner, it wouldn't be possible to
forbid these parties to _educate_ their supporters about the consequences
of voting in a certain manner.

Markus Schulze