Having developed new measures of social heterogeneity, it is time to begin testing my theory about how a democracy's social heterogeneity shapes the fragmentation of its party system. One of the core arguments I advanced in Chapter 2 was that a group of individuals newly added to a democracy's citizenry is likely to successfully form its own sectarian political party, increasing the fragmentation of the party system, when the characteristics of the group and systemic features of the polity take certain favorable forms. That is, I argued that several factors besides the electoral system indirectly condition the effect of social heterogeneity on party system fragmentation. I also argued that the polity's prior social heterogeneity directly conditions this relationship. The question now is whether the empirical evidence supports these arguments.

In this chapter, I provide some answers to this question by conducting a quantitative empirical analysis that assesses the evidence across both space and time. My dependent variable in this analysis is party system fragmentation. I use linear regression to analyze the relationship between this dependent variable, the independent variable of social heterogeneity, and some of the hypothesized conditioning variables. I say “some” because not all of my hypotheses could be tested in this manner due to the difficulty of developing cross-national measures of the variables. Later chapters will turn the empirical lens on the key mechanism of interest, sectarian party success, and subject the remaining hypotheses to empirical scrutiny using case studies of Israel and the United States. The case studies will also provide additional tests of the hypotheses tested here, a triangulation designed to increase the study’s internal validity.