On the Efficiency of An Election Game of Two or More Parties: How Bad Can It Be?

Abstract: An election campaign among two or more parties can be viewed as a game of two or more players, each of which has its own candidates as the pure strategies. People, as voters, comprise supporters for each party, and a candidate brings utility for the supporters of each party. Each party nominates exactly one of its candidates to compete against the other party's. A candidate is assumed to win the election with greater or equal odds if it brings more utility for all the people. The payoff of each player is the expected utility that its supporters get. The game is egoistic if every candidate benefits its party's supporters more than any candidate from a competing party does. In this paper, we first prove that it is NP-complete to determine whether an election game in a succinct representation, which is called the general form, has a pure-strategy Nash equilibrium even if it is egoistic. Next, we propose a fixed-parameter tractable algorithm to compute a pure-strategy Nash equilibrium of an egoistic election game and show that a naive constant time algorithm leads to a (1+e)-approximate pure-strategy Nash equilibrium when the winning probability is computed by a softmax function. Finally, perhaps surprisingly, we show that the price of anarchy for egoistic election games is upper bounded by the number of parties. Our results suggest that an election becomes unpredictable in terms of stability and efficiency when more than two parties are involved, and, to some extent, also provides supporting arguments for why the two-party system is prevalent in democratic countries.