Approval-Based Multi-Winner Rules and Strategic Voting

Martin Lackner, Piotr Skowron

Abstract: We investigate the possibility of strategic voting in approval-based multiwinner rules. In particular, we define three axiomatic properties that guarantee resilience to certain forms of strategic voting: independence of irrelevant alternatives (IIA), monotonicity, and SD-strategyproofness. In this paper, we systematically analyze multiwinner rules based on these axioms and provide a fine-grained picture of their resilience to strategic voting. Both our axiomatic and experimental analysis show that approval-based multiwinner rules are generally very susceptible to strategic voting---with one exception: multiwinner approval voting.