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The existence of a large incumbency advantage in the winner-takes-all plurality system of the United States is well documented. It is unclear whether incumbents in proportional systems should enjoy such a large advantage. Multi-seat constituencies make it difficult for individual incumbents to claim credit for the provision of local public goods and services. Moreover, multiple incumbents may dilute media attention thereby limiting name recognition advantage. We use a regression discontinuity design to estimate the causal effect of incumbency using election data from Ireland’s system of proportional representation with a single transferable vote (PR-STV). Incumbency causes an eighteen percentage point increase in the probability that a candidate in Ireland’s lower house of parliament wins a seat in the next election. Our results indicate that the protection of vulnerable incumbents from intra-party competition may be a source of incumbency advantage in multi-member district elections. [ABSTRACT FROM AUTHOR]