[EM] Re: Alex: Nash Equilibrium
nkklrp at hotmail.com
Tue Nov 23 18:30:41 PST 2004
It's been a while since I made that post, but as I recall I felt that your
definition was too vague because it contained the phrase "no set of voters"
rather than the more restrictive phrase "No set of voters with identical
Some definitions use more restrictive language than others. But that isn't
what "vague" means.
So a you could more accurately have said, "The voting equilibrium that
you've defined is different from the one that I have defined".
In general, a definition is vague if it could mean more than one thing, or
if it's in doubt whether or not some particular thing fits that definition.
In particular, from that meaning, the definition of a criterioin is vague if
it can't be definitely established whether some particular method passes
that criterion. And, also from that meaning, the definition of an
equilibrium is vague if it can't be established whether or not a particular
outcome is an equilibrium of the kind defined.
Now, you forgot to tell us what two different meanings my definition could
have. Or what outcome can't be determined to be, or not be, a voting Nash
equilibrium, as I defined that term.
Or are we a being a little vague about what we mean by "vague"? :-)
Using the phrase "no set of voters" rather than "no set of voters with
identical preferences" is too vague and can lead to situations with no Nash
Here you repeat your claim about vagueness, and I refer you to my reply to
As for situations with no Nash equilibrium, it's easier for a situation to
be an equilibrium by your definition, because you restrict what voters are
referred to. That means only that your definition, and your equilibrium, is
Could there be times when your more-easily-found equilibrium suits a problem
or project better? Sure. And there could just as easily be times when it
would suit as well. That's the advantage of there being more than one
equilibrilium, you know.
We have 2 equilibria that are extensions of Nash equilibrium now, and we
have two names for them. Hey, that works out just right! :-)
This may come as a surprise to you, but game theorists actually use even
more than two kinds of equilibrium. In fact, they have a whole taxonomy of
them. If you can just forgive and tolerate there being more than one :-)
his would violate Nash's Nobel-winning discovery that (with suitable
definitions) there are always equilibria.
"Violate a discover"y. It isnt entirely clear what it means to "violate a
discovery". I'm not claiming that there isn't a Nash equilibrium, as Nash
defined it, for individuals chaning their strategy alone, under the
conditions under which Nash said that there's always such an equilibrium.
If trhere isn't always a voting Nash equilibrium, as I've defined it, that
doesn't violate Nash's discover, which is about the equilibrium that he
defined, for idivuals chaning their strategy alone.
Nash spoke of situations that no one individual can improve (for himself) by
changing his strategy alone.
I speak of situations that no set of voters can improve for themselves by
changing their vote if no one else does.
That sounds like a reasonalbe, natural, and obvious extension of Nash
equilibrium to voting.
It's an obvious generalization of Nash equilibrium as Nash defined it.
Yes it's more general than your equilibrium definition.
A more complete, but wordier statement of my definition would be:
A voting Nash equilibrium is an outcome such that no set of voters, by
changing their vote if no one else does, can change that outcome to make an
outcome that all of them prefer to it.
Suppose that we use Approval Voting. If we don't define the factions
carefully, and we have a Condorcet Cycle, then it will always be the case
that a majority of the voters prefer some candidate to the current winner,
and acting in concert that majority could elect somebody that it prefers to
the current winner simply by approving that preferred candidate and no other
candidate. (They might have to vote insincerely to do it, but the point is
that they could do it.)
As I said, there will be outcomes that are Nash equilibria as you define it
for voting, and not as I define it for voting. That's ok.
And there may be situations where there's an equilibrium of the kind that
you derfine, but there isn't a voting Nash equilibrium. That's ok too.
I'll give my definition of Nash equilibrium in elections, and a rationale
for why I prefer that definition to various other possible definitions.
You seem to believe that there can be only one Nash-derived voting
equilbrium, and that this town isn't big enough for your Nash-derived voting
equilibrilum and mine. Actually, as I said, game theorists use even more
than two kinds of equilibrium. I aplogize if telling you that makes you
angry about or bothered by the use of more than one Nash-dereived voting
by game theorists. :-)
Also, notice that at no point did I make reference to whether or not the
voting method uses ranked ballots.
But could you have been expected to? Why should a definition of a voting
equilibrium specify what kind of balloting is used?
Even in non-ranked methods (e.g. Approval Voting) your strategic incentives
depend only on your ordinal preferences, not your cardinal preferences.
Wrong. Cardinal ratings have a large part in Approval strategy, even though
whether you'd prefer A winning to B winning only depends on your ordinal
prefernce between them.
By the way, I've always heard "preference" used in the ordinal sense. You
prefer Smith to Jones. For cardinality, we speak of ratings or utilities,
Maybe you can find a reference somewhere in which someone spoke of cardinal
preferences, but that isn't how we've been using "preference" on EM.
If you prefer A to B, changing your strategy to elect A rather than B is
always in your best interests, regardless of whether you think the
difference between the candidates is minor or huge.
But you often don't know whether denying an Approval vote to B will let C
win instead of B, when you like B better than C. In situations like that,
your cardinal utility ratings of A, B, & C affect your Approval strategy.
But, yes, with only 2 candidates, your statement is true.
Above is my definition, and if it matches yours then I guess we agree on
No your definition doesn't match mine, but I wasn't aware that that meant
that we disagree on something. It isn't entirely clear what we disagree on,
because it isn't clear what you're saying.
If you're saying that there can be only one correct definition of a
Nash-derived voting equilibrium, then I disagree with you. You're wrong.
A little history:
Steve & Alex have called the definition of voting Nash equilibrium "Mike's
definition". Maybe I should let you believe that, and take credit for being
the first to define it. But I'm going to be honest, and admit that Blake
first defiined it on EM.
One person, who shall remain anonymous, because his correspondence was
offline, wants to call voting Nash equilibrium "Ossipoff equililbrium". As
much as I'd like to agree with that name for it, I'm going to be honest and
admit that Blake first defined it on EM.
I had earlier defined a different Nash-equilibrium-derivative for voting,
one that somewhat resembles yours. Then later I found that the one that
Blake defined worked better for what I was using it for. Of course it's also
much more general.
Because of its generality, voting Nash equilibrium is the equilibrium that
most deserves to be called the obvious extension of Nash equilibrium to
But, Alex, do you really have nothing better to do than to jealously try to
establish that your kind of Nash-derived voting equilibrium should be the
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