Assessing the Causes of District Homogeneity in U.S. House Elections

Abstract

The argument that U.S. House districts have become more demographically homogenous over time—either through intentional gerrymandering, geographic self-sorting, or some combination of the two— is a central component of most explanations of the growth of partisan polarization in Congress. In recent research, we developed a highly sophisticated method of creating simulated, randomized districts in all states with more than one member of the House (see Powell, Clark, and Dube 2015, 2016). In this paper we build on that prior research to examine the extent to which geographic self-sorting may or may not account for trends in district homogeneity. To do so, we compare the homogeneity of actual districts as drawn to repeated iterations of randomized, simulated districts within each state. We extend this analysis to examine changes in the margins of victory in these districts. Thus, this paper presents a thorough examination of the hypothesis that geographic self-sorting (or intentional gerrymandering) is creating safer House seats and a more polarized legislative environment in Congress.