This report addresses two questions: first, whether the spatial distribution of the American electorate has become more geographically clustered over the last 40 years with respect to party voting and socioeconomic attributes; and second, whether this clustering process has contributed to rising polarization in the U.S. House of Representatives. We find support for both hypotheses and estimate that long-term geographical clustering of voters is responsible for roughly 30 percent of the increase in polarization in the House between the 93rd and 112th Congresses. An important ancillary finding is that the within-district percentage of adults who are married dwarfs other socioeconomic variables — including those measuring race, education, income, and urbanicity — as a predictor of local partisanship, as measured by both the party affiliation of the House representative and the presidential vote share.

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