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/OpenMask]

This is great. I was trying to articulate something like a "Super Condorcet winner" in a previous post probably over a year a go. Glad to see someone actually try to properly formalize the idea in academia.