The bidirectional association between electoral rules and the effective number of parties: Evidence from the Chilean Congress

Abstract The relationship between electoral institutions and the party system is one of the main topics of political science. Here we delve deeper into a diachronic analysis using a database of almost 200 years of history. By collecting data on the effective number of parties (ENP) in Chile, an early multiparty system which has had numerous changes in electoral rules, we investigate the association between electoral reforms and ENP in both chambers of Congress. We classify 18 electoral reforms and explore 54 legislative elections held in Chile since 1833 until 2025. Major and technical changes in electoral rules account for noticeable changes in the ENP. The effect of electoral reforms is not always immediate, nor does their adoption correspond to a linear trajectory, and sometimes reforms reflect social and political changes that already occurred in the political system. The evidence suggests bidirectional association and nuanced dynamics between electoral rules and the ENP.