Beyond the Average: Revisiting How District Magnitude Shapes Party Systems

We argue that district magnitude shapes the number of electoral parties through two mechanisms: district permissiveness, which influences incentives for electoral coordination within districts, and variation in district magnitude across districts within a country. When district magnitude varies within countries, larger districts tend to support a greater number of parties. Small parties that succeed in these districts are subsequently incentivized to compete in smaller districts, thereby weakening overall electoral coordination. Using cross-sectional data from 54 democracies and longitudinal evidence from 30 European countries between 1945 and 2019, we find that within-country variation in district magnitude increases the number of electoral parties in permissive electoral systems and erodes electoral coordination over time. We further examine the contamination mechanism from larger to smaller districts using district-level data from legislative elections in Portugal and Spain, both of which employ simple Proportional Representation systems with varying district magnitudes.