Three Practical Tests for Gerrymandering: Application to Maryland and Wisconsin

Partisan gerrymandering arises when many single-district gerrymanders are combined to obtain an overall advantage. The Supreme Court has held that partisan gerrymandering is recognizable by its asymmetry: for a given distribution of popular votes, if the parties switch places in popular vote, the numbers of seats would change in an unequal fashion. However, the asymmetry standard is only a broad statement of principle, and no analytical method for assessing asymmetry has yet been held to be manageable. Recently I proposed (68 Stanford Law Review 1263) three statistical tests to reliably assess asymmetry in state-level districting schemes: (a) a discrepancy in winning vote margins between the two parties' seats; (b) undue reliable wins for the party in charge of redistricting, as measured by the mean-median difference in vote share, or by an unusually even distribution of votes across districts; and (c) unrepresentative distortion in the number of seats won based on expectations from nationwide district characteristics. These tests use district-level election outcomes, do not require the drawing of maps, and are accessible via nearly any desktop computer. Each test probes a facet of partisan asymmetry. The first two tests analyze intent using well-established, century-old statistical tests. Once intents are established, the effects of gerrymandering can be analyzed using the third test, which is calculated rapidly by computer simulation. The three tests show that two current cases, the Wisconsin State Assembly (Whitford v. Nichol) and the Maryland congressional delegation (Shapiro v. McManus), meet criteria for a partisan gerrymander. I propose that an intents-and-effects standard based on one or more of these tests is robust enough to mitigate the need to demonstrate predominant partisan intent. The three statistical standards offered here add to the judge's toolkit for rapidly and rigorously identifying the consequences of partisan redistricting.