[EM] ironclad pro-Condorcet argument?

Warren Schudy wschudy at WPI.EDU
Mon Aug 23 14:37:38 PDT 2004

On Mon, 23 Aug 2004, James Green-Armytage wrote:

> Has anyone clearly advanced this pro-Condorcet argument? I think that it
> is devastating to methods which are not Condorcet efficient. If someone
> else has made this same argument already, please let me know, so that I
> can cite it in the paper I’m trying to write on weighted pairwise.
> 	If there is a Condorcet winner with regard to the sincere preference
> rankings of voters, and the voting method is plurality, then the Vote is
> only at equilibrium when the Condorcet winner is selected.
> changing their vote.

Your argument implicitly relies on the assumption that half the population 
can coordinate a change their votes, which is very difficult, especially 
with secret balloting. That is why the stability of the 2-party duopoly in 
US presidential elections does not contradict your theorem. (I suspect 
that McCain pairwises beats Bush and Kerry, but he is not elected 
under plurality because the current scenario is stable against any small 
number of voters changing their votes.)

I don't think this is a good argument for Condorcet since its consequences 
are at variance with common knowledge, because of the assumption 
mentioned above.

Unfortunately, I'm not sure what a better definition of a voting
equilibrium would be. However, I will point out that others have published 
about voting equilibria in some detail. Here's a reference for an article 
that develops an interesting definition of a voting equilibrium:

Myerson, Roger B. and Robert J. Weber. 1993. "A Theory of Voting
Equilibria." American Political Science Review. 87:102-114.

There's also a more readable webpage (by one of the authors of the above 
paper) that cites that paper: 


| Warren Schudy                           |
| WPI Class of 2005                       |
| Physics and computer science major      |
| AIM: WJSchudy  email: wschudy at wpi.edu   |
| http://users.wpi.edu/~wschudy/          |

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