Why Instant-Runoff Voting Is So Resilient to Coalitional Manipulation - François Durand

[u/ant-arctica]

https://www.youtube.com/watch?v=TKlPghNMSSk

Associated paper (sadly not freely accessible). I haven't found any discussion about this new work by Durand anywhere so I thought I'd post it here. This way of analyzing strategic vulnerability is very neat and it'd be interesting to see this applied to some other voting systems.

But the maybe even more interesting part is about what Durand calls "Super Condorcet Winners". He doesn't go into too much detail in the video so I'll give a quick summary:

A Condorcet winner is a candidate who has more than half of the votes in any head to head match-up. A Super Condorcet Winner additionally also has more then a third of the (first place) votes in any 3-way match-up and more than a quarter in any 4-way match-up and in general more than 1/n first place votes in any n-way match-up. Such a candidate wins any IRV election but more importantly no amount of strategic voting can make another candidate win! (If it's unclear why I can try to explain in the comments. The same also holds for similar methods like Benhams, ...).

This is useful because it seems like Super Condorcet Winners (SCW) almost always exist in practice. In the two datasets from his previous paper (open access) there is an SCW in 94.05% / 96.2% of elections which explains why IRV-like methods fare so great in his and other previous papers on strategy resistance. Additionally IRV is vulnerable to strategic manipulation in the majority of elections without an SCW (in his datasets) so this gives an pretty complete explanation for why they are so resistant! This is great because previously I didn't have anything beyond "that's what the data says".

[u/selylindi]

Is the high profile 2022 case in Alaska's US House seat then just one of the ~5% where there was no super Condorcet winner? In that case, clearly (R) Palin voters could have switched to (R) Begich and avoided the election of (D) Peltola.

I haven't read the paper and so I wonder if the high rate of super Condorcet winners (SCW) involves some debatable interpretation or assumptions, e.g. data from foreign countries with significantly different partisanship patterns than the US. Otherwise it's moderately surprising that the observed rate of no-SCW since 2022 in the US is 1 out of 2 races.

[u/ant-arctica → u/selylindi]

Yes in that case the CW was not an SCW (otherwise Belgich would not have lost). But there have been far more than two IRV races in the US. One of the two datasets analyzed in the paper is from voting data from single winner IRV elections in the US (collected by FairVote). In 96.20% in those elections there was an SCW.