Thus far, this book has focused on the interactive effect on political outcomes of electoral rules and the level of democratic and party system development. In this chapter, we shift our focus to how electoral rules interact with a different category of political context, social diversity, to affect the party system.

A long line of research has argued that the number of parties in a country is the product of an interactive process between the number of social cleavages and the electoral system, whereby social heterogeneity has little effect on the number of parties under “restrictive” rules such as first-past-the-post (Duverger 1954; Ordeshook and Shvetsova 1994; Amorim Neto and Cox 1997; Clark and Golder 2006; Singer and Stephenson 2009). Many scholars posit that as the number of distinct groups in society increases, so will the number of parties, but that this relationship will exist only under “permissive” electoral rules (such as PR with high district magnitude and little or no legal threshold of representation). The incentives of the electoral system under restrictive rules drive voters, groups, and elites to withdraw their support from candidates who are unlikely to be competitive, and instead line up only behind those truly “in the running” – thereby leading to a small number of parties (or candidates), irrespective of the number of distinct groups in society.