Modeling the effect of mandatory district compactness on partisan gerrymanders

Geographic compactness standards have been offered as neutral and effective standards constraining redistricting. In this paper, we test this allegation. Redistricting is treated as a combinatoric optimization problem that is constrained by compactness rules. Computer models are used to analyze the results of applying compactness standards when political groups are geographically concentrated. Several population models are used to generate populations of voters, and arbitrary plans are created with combinatoric optimization algorithms. We find that compactness standards can be used to limit gerrymandering, but only if such standards require severe compactness. Compactness standards are not politically neutral—a geographically concentrated minority party will be affected by compactness standards much differently than a party supported by a geographically diffuse population. The particular effects of compactness standards depend on the institutional mechanism that creates districts.