Bucklin Voting is Broadly Resistant to Control

Abstract: Electoral control models ways of changing the outcome of an election via such actions as adding/deleting/partitioning either candidates or voters. These actions modify an election's participation structure and aim at either making a favorite candidate win ("constructive control") or prevent a despised candidate from winning ("destructive control"), which yields a total of 22 standard control scenarios. To protect elections from such control attempts, computational complexity has been used to show that electoral control, though not impossible, is computationally prohibitive. Among natural voting systems with a polynomial-time winner problem, the two systems with the highest number of proven resistances to control types (namely 19 out of 22) are "sincere-strategy preference-based approval voting" (SP-AV, a modification of a system proposed by Brams and Sanver) and fallback voting. Both are hybrid systems; e.g., fallback voting combines approval with Bucklin voting. In this paper, we study the control complexity of Bucklin voting itself and show that it behaves equally well in terms of control resistance for the 20 cases investigated so far. As Bucklin voting is a special case of fallback voting, all resistances shown for Bucklin voting in this paper strengthen the corresponding resistance for fallback voting.