District Magnitude, Social Heterogeneity and Local Party System Fragmentation

Abstract

Scholars analysing party system structures have generally followed two approaches. The ‘institutional’ approach emphasizes the role of electoral laws, while the ‘sociological’ approach highlights the effect of socioeconomic heterogeneity. Recently, evidence based on cross-country data indicated that a high level of fragmentation requires both a permissive electoral system and substantial socio-economic heterogeneity. As electoral rules have their main impact at the level of the constituency, using country-level data may not be ideal, however. Moreover, previous results appear to be driven by the inclusion of single-member district states. Both these caveats are addressed in this article. Based on a sample of Belgian municipal elections over the period 1982–2000, I show that the model incorporating an interaction between heterogeneity and district magnitude significantly outperforms specifications that disregard the interaction effect (a) at the constituency level and (b) in (mediumsize) multi-member districts.