Council Size and District Representation: How Institutional Rules Shape Fragmentation in Local Councils

This article examines how institutional rules shape fragmentation in local councils by analyzing the effects of council size and district representation. While extensive research has focused on national-level party systems and electoral rules, less attention has been paid to how institutional variation affects political representation at the local level. Using panel data from 308 Portuguese municipalities across six local elections (2001-2021), this research investigates two main research questions: What factors influence fragmentation of local councils in Portuguese municipalities? How does institutional variation affect fragmentation? Several theoretical arguments suggest that larger councils and district representation should increase fragmentation. For council size, I propose mechanical effects (lower thresholds for representation), proportionality effects (better translation of votes to seats), supply-side effects (more candidates and parties), demand-side effects (reduced strategic voting), and political competition effects (more competitive opportunities). For district representation, I emphasize geographic representation effects (capture of distinct local interests), candidate access (lower barriers to entry), electoral competition (attraction of diverse candidates), and representation effects (voice for neighborhood interests). The empirical analysis employs multiple measures of fragmentation as dependent variables – the Effective Number of Parties (ENP) index, Shannon's Entropy Index, and Simpson's Diversity Index – and uses beta regression and fractional response regression models. The analysis controls for various socioeconomic and demographic factors, including population size, age distribution, education, income, voter turnout, and media exposure. Special attention is paid to the treatment of independent lists, which present unique challenges for measuring fragmentation in local councils. The findings contribute to our understanding of how institutional rules affect local political representation and have important implications for the design of local electoral systems. By focusing on Portuguese municipalities, this study provides insights into the dynamics of local democracy in a relatively understudied context while addressing broader theoretical questions about institutional effects on political representation.