Keeping the party together

Under what conditions do political parties split? This paper presents a model of intra-party politics to explain party unity in parliamentary systems. The theory derived from an incomplete information game predicts that parties split with positive probability, which rises with the cost of dissent following a failed attempt to split and falls with the cost of forming a new party. Party unity also is predicted to be high when the leadership faction’s weight within the party is large. The model’s results have implications for the relationship between party unity and the majority status of the government party, party system polarization as well as intra-party polarization.