Scholars have long argued social diversity, and electoral institutions interactively shape party systems: diversity has little effect on the effective number of parties (ENP) in single member plurality (SMP) systems but increases ENP in proportional ones. We argue instead that where diversity is salient enough to generate demand for parties, it also hinders strategic coordination, preventing SMP rules from reducing the number of parties and producing a correlation between diversity and ENP. In contrast, non-salient forms of diversity have little impact regardless of institutional rules. We test this intuition using data from South Africa’s municipal mixed-member system and explore its highly salient racial cleavage and less salient ethnic one. We find racial diversity correlates with ENP in SMP systems while ethnic diversity correlates with ENP in neither SMP nor proportional representation systems. Our study contributes to mounting evidence questioning the interactive hypothesis and points to the importance of the salience of social divisions in shaping party systems.
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