[EM] FPTP is not strategy proof!!!

[Markus Schulze]

Dear Stephane,

Russ Paielli wrote (6 Aug 2003):
> Plurality is also "extremely difficult to manipulate" in this way,
> isn't it? What kind of "offensive strategy" could you possibly use
> in plurality? None that I can think of.
>
> Suppose I want candidate A to win a plurality election. I can't think
> of any way I could possibly help A win except by voting for him. Can
> you? If I vote for A, I'm voting sincerely -- not strategically.
>
> Yes, anyone who does not prefer one of the two major parties may need to
> vote *defensively*, but that's not "manipulating" the system. That's
> just making a rational decision based on the deficiency of the voting
> system -- the very same thing that would also be needed in IRV.

I wrote (7 Aug 2003):
> Suppose FPP is being used. Suppose your sincere first choice is the
> Libertarian candidate. Suppose that you know that the Libertarian
> candidate doesn't have a chance and that you therefore vote for
> the Republican candidate since you prefer the Republican candidate
> to the Democratic candidate. Then, of course, you vote insincerely
> in that sense in that the term "insincere" is used in the literature
> since your casted vote doesn't reflect your sincere opinion.

Russ Paielli wrote (7 Aug 2003):
> In the first case, I really preferred the Libertarian, but I *gave up*
> on him and effectively voted *against* the Democrat (by voting for the
> Republican). How you can possibly construe giving up on your first
> choice as "manipulating" the system is beyond me. "Manipulating" the
> system would be somehow voting insincerely to give the Libertarian
> a *better* chance of winning -- not giving up on him! But you can't
> possibly do that in plurality. The one and only possible way of
> helping the Libertarian (or any other candidate) win is to vote
> for him. But then you are voting sincerely, hence you are not
> "manipulating" anything. Plurality is therefore beyond manipulation.

In short: A voter who uses "compromising" is still voting "sincerely
-- not strategically." This voter is "just making a rational decision
based on the deficiency of the voting system." "Plurality is therefore
beyond manipulation."

Markus Schulze