單一選區下三政黨的合作與競爭之賽局分析

Three-Party Game of Cooperation and Competition in Single Member District Election

觀察臺灣近年選舉過程中，政黨間常協商共同合作的可能性，經雙方協調後各政黨可選擇合作與否。本文建立三政黨賽局模型，分析單一席次選舉中，相對小黨在何種條件下願意與實力較強的大黨合作競選，並討論各黨決策的合理性。根據均衡結果得出，只有當滿足一定的合作利益條件時，才會產生政黨合作情形，合作對象亦不侷限於實力最強的政黨。另外在資訊不對稱情況下，存在一組分離均衡，其中強者s型態與弱者w型態大黨的政黨實力差距愈大時，小黨愈容易與w型態的大黨合作競選，此外，若小黨無競選成本時，將不會有政黨合作之現象。

Observing the process of the recent elections in Taiwan, we found that different political parties often negotiated with each other for the possibility of cooperation. Each party would decide whether to cooperate or not. This paper develops a three-party game theoretical model of single-seat elections to analyze the conditions under which that minor political party would agree to cooperate with major political parties, and discusses the rationality of decision from each political party. According to the equilibrium results, cooperation between political parties only occurs when certain conditions are met. The cooperative partner is not restricted to the most powerful party. In addition, there is a separating equilibrium in the case of asymmetric information. This leads to the conclusion that the larger difference between the strong type and the weak type of the major political party is, the more common minor political party would cooperate with the weak type of the major political party. Moreover, no cooperation takes place when the minor political party has zero cost in political elections.