The effects of electoral rules on party systems have been well known since Duverger first proposed his famous law. Often considered ‘second order’ in terms of issues and voting behaviour, many European Parliament elections are held under different electoral rules to national elections. This article examines the consequences of these differences and hypothesizes that where a more permissive electoral system is used for European Parliament elections, the size of the party system at European Parliament elections will grow towards what we would expect from the European Parliament electoral rules in isolation, and that this will lead to a subsequent growth in the size of the national party system. Using multi-level mixed-effect growth curve modelling support is found for both these hypotheses.