Strategic oversupply: minor parties and contamination effects in Japan’s mixed-member majoritarian elections

Abstract This paper investigates the contamination effect in Japan’s mixed-member majoritarian (MMM) electoral system by examining whether the presence of a party’s candidate in a single-member district (SMD) increases its vote share in the proportional representation (PR) tier. While mixed systems are designed to combine the benefits of majoritarian and proportional rules, scholars have debated whether these systems operate independently across tiers or whether contamination occurs. Using panel data from Diet elections in Japan between 2013 and 2025, I analyse how the changing candidacy status for Japanese parties impacted their PR vote share. The results from propensity score-weighted two-way fixed effects models indicate a significant contamination effect for some parties: minor parties tend to gain higher PR vote shares in districts where they field SMD candidates, creating incentives to nominate non-viable candidates. Also, candidates with prior political or professional experience were more effective in boosting their party’s PR vote share. This study highlights how the interplay between tiers in MMM systems affects party strategies and disproportionality, and underscores the strategic dilemma faced by parties under Japan’s increasingly fragmented party system.