Exclusion Zones of Instant Runoff Voting

• https://arxiv.org/abs/2502.16719
• Kiran Tomlinson, Johan Ugander, Jon Kleinberg

Voting, Instant runoff voting (Ranked choice voting)

there is an “exclusion zone” around the median voter such that if a candidate from the exclusion zone is on the ballot, then the winner must come from the exclusion zone. Thus, in one dimension, IRV cannot elect an extreme candidate as long as a sufficiently moderate candidate is running.

We prove that with voters uniformly distributed over any d-dimensional hyperrectangle (for d>1), IRV has no nontrivial exclusion zone. However, we also show that IRV exclusion zones are not solely a one-dimensional phenomenon.