[EM] Suppose, for a moment, there were never any cycles...

Kristofer Munsterhjelm km_elmet at t-online.de
Sun Jan 22 09:28:11 PST 2023

On 22.01.2023 10:08, robert bristow-johnson wrote:
> On 01/21/2023 5:18 PM EST Kristofer Munsterhjelm <km_elmet at t-online.de> wrote:
>> If there is a CW, then IIA always holds, yes. Suppose X is the CW and Y
>> is someone else. Then running the election without Y doesn't change
>> anything: X still beats everybody else pairwise and wins.
>> Thus if somehow you could prevent both sincere and tactical Condorcet
>> cycles, the method would be strategy-proof. (Strictly speaking, this
>> doesn't invalidate Arrow's theorem because a method that magically
>> forbids Condorcet cycles would fail universal domain.)
> Well I don't really expect to prevent cycles, but just by doing
> nothing, I can plan on them being extremely rare, 0.2% of the time.  So
> it's a faux-suppression of cycles, a pseudo-effect.  Either way, I can
> say that the reason that IRV is good at avoiding the spoiler effect is
> solely because 99.4% of the time IRV elects the Condorcet winner.  It's
> as effective at suppressing the spoiler effect as it is effective at
> electing the Condorcet winner.  FairVote has to credit any success they
> claim for IRV to Condorcet.  Because when IRV fails to elect the CW, it
> doesn't do shit about preventing the spoiler effect and all the
> pathologies cascading from that failure to prevent the election from
> being spoiled.

Like Kevin, I would say it's worse. The way Plurality goes, after a few 
spoiled elections, people figure out that they can't throw away their 
vote by voting for a third party.

For IRV, I imagine that the typical progression would be that IRV allows 
minor parties to grow (since they don't affect the outcome), then a 
viable three-way contest happens, and then IRV more or less becomes a 
random draw. Then the voters either figure out that they have to 
compromise after all (which leads back to two-party rule), or they 
repeal IRV.

But somehow, the step from small third party to three-way contest 
doesn't seem to happen all that often in the US. I don't know why, but 
whatever it is, it masks IRV's flaws.

In any case, if people go back to two-party voting to protect themselves 
from IRV's failures, then those 0.6% have an outsize influence -- just 
like with Plurality.


More information about the Election-Methods mailing list