The Complexity of Online Manipulation of Sequential Elections

Most work on manipulation assumes that all preferences are known to the manipulators. However, in many settings elections are open and sequential, and manipulators may know the already cast votes but may not know the future votes. We introduce a framework, in which manipulators can see the past votes but not the future ones, to model online coalitional manipulation of sequential elections, and we show that in this setting manipulation can be extremely complex even for election systems with simple winner problems. Yet we also show that for some of the most important election systems such manipulation is simple in certain settings. This suggests that when using sequential voting, one should pay great attention to the details of the setting in choosing one's voting rule. Among the highlights of our classifications are: We show that, depending on the size of the manipulative coalition, the online manipulation problem can be complete for each level of the polynomial hierarchy or even for PSPACE. We obtain the most dramatic contrast to date between the nonunique-winner and unique-winner models: Online weighted manipulation for plurality is in P in the nonunique-winner model, yet is coNP-hard (constructive case) and NP-hard (destructive case) in the unique-winner model. And we obtain what to the best of our knowledge are the first P^NP[1]-completeness and P^NP-completeness results in the field of computational social choice, in particular proving such completeness for, respectively, the complexity of 3-candidate and 4-candidate (and unlimited-candidate) online weighted coalition manipulation of veto elections.Comment: 24 page

Tacit Lobbying Agreements: An Experimental Study

We experimentally study the common wisdom that money buys political influence. In the game, one lobbyist has the opportunity to influence redistributive tax policies in her favor by transferring money to two competing candidates. The success of the lobbying investment depends on whether or not the candidates are willing to respond and able to collude on low-tax policies that do not harm their relative chances in the elections. In the experiment, we find that lobbying is never successful when the lobbyist and candidates interact just once. By contrast, it yields substantially lower redistribution in about 40% of societies with finitely-repeated encounters. However, lobbying investments are not always profitable, and profit-sharing between the lobbyist and candidates depends on prominent equity norms. Our experimental results shed new light on the complex process of buying political influence in everyday politics and help explain why only relatively few corporate firms do actually lobby.lobbying, redistribution, elections, bargaining, collusion

Tacit Lobbying Agreements: An Experimental Study

We experimentally study the common wisdom that money buys political influence. In the game, one lobbyist has the opportunity to influence redistributive tax policies in her favor by transferring money to two competing candidates. The success of the lobbying investment depends on whether or not the candidates are willing to respond and able to collude on low-tax policies that do not harm their relative chances in the elections. In the experiment, we find that lobbying is never successful when the lobbyist and candidates interact just once. By contrast, it yields substantially lower redistribution in about 40% of societies with finitely-repeated encounters. However, lobbying investments are not always profitable, and profit-sharing between the lobbyist and candidates depends on prominent equity norms. Our experimental results shed new light on the complex process of buying political influence in everyday politics and help explain why only relatively few corporate firms do actually lobby.lobbying, redistribution, elections, bargaining, collusion