How Hard Is Bribery in Elections?

We study the complexity of influencing elections through bribery: How computationally complex is it for an external actor to determine whether by a certain amount of bribing voters a specified candidate can be made the election’s winner? We study this problem for election systems as varied as scoring protocols and Dodgson voting, and in a variety of settings regarding homogeneous-vs.-nonhomogeneous electorate bribability, bounded-size-vs.-arbitrary-sized candidate sets, weighted-vs.unweighted voters, and succinct-vs.-nonsuccinct input specification. We obtain both polynomial-time bribery algorithms and proofs of the intractability of bribery, and indeed our results show that the complexity of bribery is extremely sensitive to the setting. For example, we find settings in which bribery is NP-complete but manipulation (by voters) is in P, and we find settings in which bribing weighted voters is NP-complete but bribing voters with individual bribe thresholds is in P. For the broad class of elections (including plurality, Borda, k-approval, and veto) known as scoring protocols, we prove a dichotomy result for bribery of weighted voters: We find a simple-to-evaluate condition that classifies every case as either NP-complete or in P. ∗Supported in part by grants NSF-CCR-0311021, NSF-CCF-0426761, and NSF-IIS-0713061, two Friedrich Wilhelm Bessel Research Awards, and the Alexander von Humboldt Foundation’s TransCoop program. Also appears as URCS-TR-2006-895. An early version of this paper, titled “The Complexity of Bribery in Elections,” appeared in the proceedings of the 21st National Conference on Artificial Intelligence (AAAI-06) and was also presented at COMSOC-06 and NESCAI-07.