1. Introduction 2. Social heterogeneity and the number of parties: a theory 3. Describing social heterogeneity: measures and testable hypotheses 4. Social heterogeneity and party system fragmentation: empirical evidence across space and time 5. Israel: new parties for new groups? 6. Israel: testing hypotheses about sectarian party success 7. The United States: new parties for new groups? Testing hypotheses 8. Conclusion: party system fragmentation and beyond.
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