Does Proportionality Increase Turnout? A Study of Adaptation to Oscillating Electoral Systems

Does proportionality increase turnout? This article studies a national electoral reform in early twentieth-century Argentina that was implemented asymmetrically across districts, creating a natural laboratory in which some districts oscillated between electoral systems across elections. Leveraging this unusual variation in a difference-in-differences design, this article shows that a shift from multi-member plurality rules towards a slightly more proportional system—which removed one-third of the contested seats from the grasp of the dominant party—led to a four percentage point increase in turnout. An investigation into causal mechanisms indicates that more proportional rules, by increasing the odds that smaller parties obtain seats, promoted strategic party entry and intensified electoral competition, both between and within parties. These findings complement existing quasi-experimental results that focus exclusively on elections in European democracies, confirm that parties are able to adapt immediately to changes in electoral rules, and refute the belief that proportionality does not affect turnout in Latin America.