Who Is My Neighbor? The Spatial Efficiency of Partisanship

September 28, 2020

Statistics and Public PolicyRelative to its overall statewide support, the Republican Party has been over-represented in congressional delegations and state legislatures over the last decade in a number of US states. A challenge is to determine the extent to which this can be explained by intentional gerrymandering as opposed to an underlying inefficient distribution of Democrats in cities. We explain the “spatial inefficiency” of support for Democrats, and demonstrate that it varies substantially both across states and also across legislative chambers within states. We introduce a simple method for measuring this inefficiency by assessing the partisanship of the nearest neighbors of each voter in each US state. Our measure of spatial efficiency helps explain cross-state patterns in legislative representation, and allows us to verify that political geography contributes substantially to inequalities in political representation. At the same time, however, we also show that even after controlling for spatial efficiency, partisan control of the redistricting process has had a substantial impact on the parties’ seat shares. Supplementary materials for this article are available online.