[EM] I need an example of Condorcet method being subjected to the spoiler effect if any

[Kristofer Munsterhjelm]

robert bristow-johnson wrote:
> 
> so, we have a CW...  add a candidate, if that candidate does not become 
> the winner, nor cause a cycle, then the Condorcet Winner we had before 
> continues to be the CW with the added candidate.  (boy, i guess we're 
> rephrasing the same thing multiple times!)

Yup.

>> River is independent of Pareto-dominated alternatives - a candidate is 
>> Pareto-dominated if everybody who ranks both him and some other 
>> (specific) candidate, rank the other candidate above him (e.g. X is 
>> Pareto-dominated by Y if all voters who rank both X and Y say Y>X, and 
>> there's at least one such voter).
> 
> i wouldn't mind if someone explains this.  i don't know what 
> "Pareto-dominated" is about.  can someone expound?

A is Pareto-dominated by B if all voters who express any difference in 
preference between A and B, prefer B to A. Note that a voter may simply 
leave both unranked - that wouldn't count towards either's Pareto 
dominance. If all voters equal-rank B and A, that doesn't count towards 
any Pareto-dominance, either.

A is Pareto-dominated (period) if there's some other candidate by which 
it is Pareto-dominated.

The strong Pareto criterion states that Pareto-dominated candidates 
shouldn't win. This makes sense, because say X won and was 
Pareto-dominated. Then people could (rightly) complain that everybody 
who expressed some preference between X and some other candidate Y, 
preferred Y, and therefore Y should have won.

Independence from Pareto-dominated alternatives then simply means that 
Pareto-dominated candidates can't be spoilers either - they can't even 
change who wins.

>> I imagine these resistances would mostly come into play in smaller 
>> elections. Still, they're nice to have, and their existence 
>> immediately tells parties not to try exploiting certain weaknesses 
>> (because it won't work).
> 
> yes, that's the whole point.  this is why i am not yet afraid of someone 
> strategically voting to push a Condorcet election into a cycle.  it 
> would be an unsafe way to accomplish a political goal.  how could anyone 
> predict what would happen?

A party might still try, thinking they could pull it off; but complying 
with a strategic criterion stops that dead, because there's no way it's 
going to work. It doesn't even have to entertain the thought of trying. 
With advanced methods, the barrier imposed by these criteria might be so 
steep that the only remaining strategies are those where a sizable 
fraction of the electorate has to cooperate, and then it's practically 
strategy-proof in large public elections (barring "disorganized 
strategy" like the "a sizable fraction goes on a Burial spree because 
each individual voter thinks they have nothing to lose" of Warren's DH3).