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Authors:

• Martin OSBORNE
  University of Toronto

• Rabee Tourky
  University of Melbourne

We study party formation in a general model of collective decision-making, modeling parties as agglomerations of policy positions championed by decision-makers. We show that if there are economies of party size and the policy chosen is not beaten by another party in pairwise voting, then players agglomerate into exactly two parties. This result does not depend on the magnitude of the economies of party size or sensitively on the nature of the individuals' preferences. Our analysis encompasses a wide range of models, including decision-making in committees with costly participation and representative democracy in which the legislature is elected by citizens.