{"type":"ScholarlyArticle","title":"The Communication Complexity of Instant-Runoff Voting","authors":[{"name":"Élie de Panafieu","slug":"elie-de-panafieu","given":"Élie","family":"de Panafieu","affiliation":""},{"name":"François Durand","slug":"francois-durand","given":"François","family":"Durand","affiliation":""},{"name":"Jérôme Lang","slug":"jerome-lang","given":"Jérôme","family":"Lang","affiliation":""}],"abstract":"The communication complexity of a voting rule is the worst-case number of bits that n voters must transmit to a central authority under the most efficient elicitation protocol in an election with m candidates. We study the communication complexity of Instant-Runoff Voting (IRV). Conitzer and Sandholm [2005] established an upper bound of O(n (log m)^2), but did not provide a matching lower bound beyond Omega(n log m). We resolve this open problem by raising the lower bound to Omega(n (log m)^2) using the fooling set technique, thereby showing that the communication complexity of IRV is Theta(n (log m)^2). We further show that this complexity drops to Theta(n log m) under the single-peakedness restriction, and that both the IRV-Average variant and Single Transferable Vote (STV), the multiwinner extension of IRV, have the same asymptotic communication complexity as IRV. (Extended version with appendices: https://shs.hal.science/hal-05566718. Short video: https://youtu.be/gTXV3R2DS6o.)","doi":"10.24963/ijcai.2026/375","url":"https://ijcai.org/proceedings/2026/375","pdf":"https://ijcai.org/proceedings/2026/0375.pdf","markdown":"https://ijcai.org/proceedings/2026/375.md","bibtex":"https://ijcai.org/proceedings/2026/bibtex/375","volume":{"slug":"2026","title":"Proceedings of the Thirty-Fifth International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2026","url":"https://ijcai.org/proceedings/2026/","year":2026,"location":"Bremen, Germany","dates":"15-21 August 2026","editors":"Diego Calvanese","isbn":"978-1-956792-09-6","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2026"},"section":"Game Theory and Economic Paradigms","track":"Main Track","number":375,"pages":{"first":3376,"last":3384},"keywords":["Game Theory and Economic Paradigms: Computational social choice"],"videos":[],"references":[],"license_note":"Copyright International Joint Conferences on Artificial Intelligence."}