Nearly all systematic empirical work on the relationship between social diversity and the number of parties asserts the “interactive hypothesis”—Social heterogeneity leads to party fragmentation under permissive electoral rules, but not under single-member district, first-past-the-post (FPTP) rules. In this article, we argue that previous work has been hindered by a reliance on national-level measures of variables and a linear model of the relationship between diversity and party fragmentation. This article provides the first analysis to test the interactive hypothesis appropriately by using district-level measures of both ethnic diversity and the effective number of parties in legislative FPTP elections and considering a curvilinear relationship between the variables. We find that there is a strong relationship between social diversity and the number of parties even under FPTP electoral rules, thus suggesting that restrictive rules are not as powerful a constraint on electoral behavior and outcomes as is usually supposed.
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