Quantifying gerrymandering using the vote distribution

To assess the presence of gerrymandering, one can consider the shapes of districts or the distribution of votes. The efficiency gap, which does the latter, plays a central role in a 2016 federal court case on the constitutionality of Wisconsin’s state legislative district plan. Unfortunately, however, the efficiency gap reduces to proportional representation, an expectation that is not a constitutional right. We present a new measure of partisan asymmetry that does not rely on the shapes of districts, is simple to compute, is provably related to the “packing and cracking” integral to gerrymandering, and that avoids the constitutionality issue presented by the efficiency gap. In addition, we introduce a generalization of the efficiency gap that also avoids the equivalency to proportional representation. We apply the first function to US congressional and state legislative plans from recent decades to identify candidate gerrymanders.