Partisan spatial sorting in the United States: A theoretical and empirical overview

We develop a variance-like index of heterogeneity in partisanship and use it to measure spatial sorting. We prove that our index is the only one (up to a linear transformation) that satisfies seven theoretical properties, all of which are intuitively desirable. Based on this index we document the long-run evolution of geographic sorting along partisan lines in the American electorate. We provide evidence that spatial cleavages have increased dramatically since the mid-twentieth century. At no point since the Civil War have partisans been as clustered within the boundaries of individual states as today. Nonetheless, even when geographic sorting is measured at the precinct level, differences across communities tend to be significantly smaller than differences within. In this sense, the American electorate continues to be more diverse within than across areas.