Laine, JeanLang, JeromeOzkal-Sanver, IpekSanver, M. Remzi2026-04-042026-04-0420251434-47421434-4750https://doi.org/10.1007/s10058-025-00382-4https://hdl.handle.net/11411/10390We investigate the Pareto-efficiency of ordinal multiwinner voting rules, that is, voting rules based on ordinal preference profiles over candidates. Defining Pareto-optimality of a committee requires relating the voters' rankings over individual candidates to their preferences over committees. We consider two well-known extension principles that extend rankings over candidates to preferences over committees: the responsive extension and the lexicographic extension. As the responsive extension outputs partial orders, we consider two Pareto-optimality notions: a committee is possibly (respectively, necessary) Pareto-optimal if it is Pareto-optimal for some (respectively, every) completion of these partial orders. As the lexicographic extension principle outputs a total order, it leads to only one Pareto-optimality notion. We then define several notions of Pareto-efficiency of multiwinner rules, depending on whether some (respectively, all) committees in the output are Pareto-optimal for one of the latter notions. We review what we believe to be a complete list of ordinal multiwinner rules that have been studied in the literature, and identify which Pareto-efficiency notions they satisfy. Our finding is that, somewhat surprisingly, these rules show a huge diversity: some satisfy the strongest notion, some do not even satisfy the weakest one, with many other rules at various intermediate levels.eninfo:eu-repo/semantics/openAccessExtension PrincipleMulti-Winner RulesCommitteesD71Article2-s2.0-10500555081310.1007/s10058-025-00382-410.1007/s10058-025-00382-4Q3Q4WOS:001491356200001