How do electoral systems shape incumbency advantage? Some recent studies have argued single-member districts (SMD) make incumbents more favorable than multi-member districts (MMD), but no study simultaneously examines why and how much they affect incumbency advantage. To compare with the incumbency effects under both systems, this paper utilizes the case of Japanese prefectural elections which combines SMD with MMD. Our results using a regression discontinuity design show that incumbency (dis)advantage depends on district magnitude (M). Although incumbents are more likely to victory under SMD as well as M=2 districts, there are only negligible incumbency advantages under districts of more than M=3. That is, incumbency advantage decreases as district magnitude increases. We also suggest that in Japanese local elections, the incumbency effect is promoted by deterring challengers who were barely defeated in the previous election to rerunning.
