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Electoral systems are the architecture within which party systems exist. In democratic polities, the electoral system shapes the number of political parties, their cohesiveness and the characteristics of representative democracy. It is an open question, however, whether electoral systems influence the ideological positioning of political parties. That the question remains unresolved is itself surprising. The expectation that electoral rules influence spatial positioning has existed since Downs first explicitly linked electoral systems to party positioning in an ideological space and argued that majoritarian systems induce centripetal electoral incentives, whereas proportional systems induce centrifugal electoral incentives.1 The theoretical literature largely echoes Downs, but it is also replete with studies demonstrating that both majoritarian and proportional systems may or may not encourage median voter behaviour. This might depend on whether the election is contested over one or more ideological dimensions, whether voters weigh 'valence' issues in their choice calculus, whether voters abstain if parties are too distant and a myriad of other considerations.2
This points to the importance of the empirical literature, which should provide insight as to what is, rather than what might be, given the modelling assumptions. But here, too, the relationship between electoral system and party ideological positioning is uncertain. There are few comparative studies that draw sufficient case variation in party-system spatial dispersion across nations and in electoral-system characteristics to provide definitive conclusions. Schofield's assessment that 'there is no empirical evidence for [party] convergence either in the U.S. polity or in those European polities whose electoral systems are based on proportional representation', combined with Ezrow's conclusion that there is 'no evidence that proportional systems promote extreme party positioning', illustrates that the empirical literature, taken as a whole, reveals no obvious relationship between electoral-system and party-system convergence or divergence.3 This is disappointing given the importance of electoral rules in theoretical studies of party positioning and the core importance of party positioning for the quality of democratic representation. In addition, the electoral system is one of the few foundations of electoral democracy that can be 'engineered' to achieve particular objectives.
I enter the discussion by comparing the extent of party-system compactness in thirty-one democracies as a function of electoral-system proportionality. Specifically, I seek to determine whether there are systematic patterns...