Measuring Pivotalness: How Coalition Probability Shapes Small Party Fortunes

• In Event: Executive Politics

Abstract

One of the biggest questions in political science is how small interest groups can achieve their policy goals and get representation against the “tyranny of the majority.” In this paper we argue that within multiparty (parliamentary and presidential) systems pivotal parties can get more political positions, and more policy benefits beyond their sizes. We define pivotalness by creating a new probabilistic theory of coalition formation. We argue that the probability that a given coalition forms is never zero, and the probability can vary based on the cost-benefit calculations of the parties. We weigh coalition probability between different parties after an election by their sizes and the compatibility of their preferred policy demands. We make a distinction between programmatic and distributive demands and in case of the programmatic demands the compatibility of the issue preferences. Then we calculate new coalition probability-weighted versions of established power measures like the Shapley-value and the Banzhalf-index. We test our theory with postwar (or democratic period) data from multiparty industrial democracies in Western Europe, Asia, and Latin America. We calculate the pivotalness factor of each party that achieved legislative representation in these systems. We find that parties that have higher weighted power indices enter into coalitions more often and when they do they get ministerial positions in proportion to their pivotalness factor.

Authors

• Zsuzsanna Blanka Magyar, Institut Barcelona d'Estudis Internacionals

• Marika Danielle Csapo, Institut Barcelona d'Estudis Internacionals