The effect of incumbency in a mixed-member electoral system

In this study, I demonstrate that incumbency advantage can arise for different reasons across electoral systems by examining a mixed-member system. My findings from a regression discontinuity design indicate that an incumbency advantage exists in both district and proportional representation (PR) tiers. Additionally, I provide evidence that distinct mechanisms underpin these advantages: parties strategically assign favorable ballot positions to incumbents from both tiers when competing for district seats, but this party-driven advantage is more pronounced for PR incumbents. By contrast, voter support mechanisms mainly favor district incumbents. Finally, I demonstrate that this incumbency advantage is short-lived for both tiers, which implies that an entry-level political office does not necessarily serve as a springboard for career politicians in the South Korean mixed-member system.