Dynamics in the fragmentation of political party systems

Fragmentation is an important dimension of political party systems. It concerns the proliferation of minor parties and the willingness of voters to vote for them. Past research relates party fragmentation to the voting system in a country; by Duverger's hypothesis, fragmentation should be highest in counties with proportional representation. The entropy measure is used to quantify fragmentation. The upper limit of fragmentation is analyzed in 24 countries, and the patterns of change in fragmentation are identified with mathematical difference equations, testing the idea that voters and parties evolve toward an equilibrium in fragmentation. The highest level of fragmentation is found comparable with a psychological limit on human information processing. In 17 of 22 countries, fragmentation moves toward an equilibrium, regardless of the type of voting system, and countries with higher equilibrium levels usually evolve more slowly. The results affirm the stability of democratic governments but raise questions about voters' strategies and suggest that some voters may try to control fragmentation when they vote.