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There is an assumption in much of the electoral engineering literature that domestic episodes of electoral system choice occur in a vacuum, isolated from international influences. Yet this assumption remains largely untested, despite the comparative focus of much of that literature. This article focuses on part of this gap by considering two electoral mechanisms that seek to limit party system fragmentation under proportional representation - low district magnitudes and high electoral thresholds - and shows that the mechanisms have spread across many European countries during the post-1945 period. Analyses reveal that national legislators are more likely to adopt one of these electoral mechanisms when a large number of peer countries have made similar choices within the last two or three years. This effect is robust to various model specifications and to the inclusion of multiple controls. The article also offers some qualitative evidence from case studies and parliamentary debates.