Abstract:
Plurality with runoff is a widely used voting rule, consisting of two rounds, the second round being a duel between two finalists selected at the first round using plurality scores. The talk will also consist of two rounds.

Round I: Plurality with runoff, no!
Using examples from French presidential elections (especially the one that took place this year), I will comment on the numerous drawbacks of the rule, and its few advantages, namely the existence of a runoff and of a time gap between the two rounds.

Round II: Approval with runoff
We claim that it is possible to retain the idea of a runoff and avoid the catastrophic drawbacks of its coupling with plurality, by having voters cast approval-type ballots at the first round. With approval-type ballots, there are various ways to select the finalists. We leverage known approval-based committee rules and study the obtained runoff rules from an axiomatic point of view. Then we analyse the outcome of these rules on single-peaked profiles, and on real data.

Joint work with Jérôme Lang, Jean-François Laslier and Remzi Sanver.