Worst-case Strategy-proofness

Abstract: We introduce a new non-manipulability axiom called worst-case strategy-proofness (WCSP). This axiom is weaker than strategy-proofness and stronger than non-obvious manipulability-worst (NOM-worst) by Troyan and Morrill (2020). WCSP focuses on non-manipulability in a worst-case scenario. We examine the implications of WCSP in a voting model. Although many voting rules, such as the plurality rule, the Borda rule, and the Dowdall rule, satisfy NOM-worst, they violate WCSP. We obtain a necessary and sufficient condition for the anti-plurality rule with fixed-order tie-breaking to satisfy WCSP in terms of the numbers of agents and alternatives.