Temporal spillovers in mixed-member systems: legislative incumbency and party-list voting in Taiwan

Abstract Does personal incumbency advantage generate lasting party-level gains across electoral tiers? We examine whether winning a single-member district (SMD) seat increases a party’s proportional representation (PR) vote share in the subsequent election under Taiwan’s mixed-member majoritarian system. Using constituency-level data from four legislative elections, we employ a regression discontinuity design that compares parties narrowly winning and losing SMD contests. Estimating this temporal spillover is complicated by “reverse contamination”: supporters of minor parties may strategically support major-party candidates in SMD races while continuing to vote for their preferred parties in the PR tier. Consequently, raw SMD–PR associations may understate the party-level returns to district incumbency. We address this problem by analyzing changes in constituency-level PR support demeaned by each party’s nationwide electoral shift, thereby isolating localized gains from broader changes in party popularity. The results show that narrowly securing an SMD seat increases the party’s PR vote share in the subsequent election. These effects are concentrated in southern Taiwan and in highly competitive constituencies. The findings demonstrate that personal incumbency can produce durable party-level benefits across electoral tiers