[EM] Markus & Manipulability

[Markus Schulze]

Dear Mike,

you wrote (2 Feb 2002):
> Markus wrote (2 Feb 2002):
> > Due to Nurmi and Bartholdi, the more information you need
> > about the opinions of the voters resp. the more accurate this
> > information must be to be able to calculate a strategy, the less
> > vulnerable to strategies the used election method is. To my
> > opinion, this argument by Nurmi and Bartholdi is plausible.
>
> When academic authors write about methods with regard to strategy,
> they tend to speak of vulnerability to strategy. Strategy is
> taken to mean offensive strategy, by which someone is manipulating
> the method.
>
> It's (almost) common knowledge on this list that strategy is
> important as an undesirable _need_ for voters, rather than as
> a manipulation opportunity. However that's something that most
> academic authors, and those who worship them, remain ignorant of.
>
> When there's a problem with offensive strategy, the problem is
> the problem that that offensive strategy causes for others, the
> defensive strategy dilemma that it causes for them.
>
> I'm almost certain that it was Nurmi who, in one of his books or
> articles, rated the methods on their vulnerability to strategy.
> IRV rated best or near best in that regard, because offensive
> strategy is difficult in IRV. But what good does that do, when
> defensive strategy is necessary in IRV, regardless of whether or
> not anyone is using offensive strategy?
>
> Is it possible that the academics & their loyal are unaware that
> Plurality has a strategy problem not because someone can offensively
> manipulate it, but because Plurality forces voters to use a drastic
> defensive strategy, a kind of strategy needed to protect a sincere
> CW, or to enfore majority rule?
>
> With any non-probabilistic voting system, a majority can get their
> way about something that they all agree on. If there's an alternative
> that they all want to win, they can make it win. If there's an
> alternative that they all want to lose, they can make it lose.
> The former is usually easy, but the latter, with most methods,
> can require members of that majority to vote something else over
> their favorite.
>
> Strategy that's needed to protect the win of a sincere CW, or to
> enforce majority rule--I call that "defensive" strategy. It's the
> old "lesser-of-2-evils problem", and it's the reason why most of
> us want a better voting. And still the academics & their obedient
> copiers don't seem to understand that, and still consider
> "strategy problem" to mean vulnerability to manipulation.

When the strategists use a strategy as a reaction to the voting
behaviour of their opponents then they never know whether their
own strategy is "offensive" or "defensive" because they never
know whether the voting behaviour of the opponents is sincere or
insincere. Actually, for the strategy to work it is irrelevant
whether the voting behaviour of the opponents is sincere or
insincere. Therefore, it doesn't make much sense to differ
between "offensive strategies" and "defensive strategies" and
to say that the first kind of strategies should be as difficult
as possible and the second kind of strategies should be as simple
as possible.

Markus Schulze