Robust group manipulation with indifferences

Abstract

We define two belief-free notions of coalitional non-manipulability that rule out coalitions in which some agents are indifferent. Strong robust group strategy-proofness typically yields negative results as it often rules out desirable rules. Semi-strong robust group strategy-proofness permits desirable rules in some environments. The differences between these properties highlight a crucial link between standard truthtelling properties: all members of a successful deviating coalition believe the outcome changes, and indifferent members believe they are essential to implement the change.