Gerrymandering: The Efficiency Gap

Abstract

As gerrymandering has become a powerful tool to create and maintain power in an increasingly polarized U.S. political landscape, the courts have wrestled with ways to definitively identify gerrymandering. We describe the efficiency gap, a quantitative measure of gerrymandering, together with a prescription for how to use it, developed by lawyer Nicholas Stephanopoulos and political scientist Eric McGhee [2015] to identify political gerrymanders to the courts.

We introduce definitions relevant to the efficiency gap; we offer examples to help understand it further. Because the efficiency gap was developed for use in litigation, we include some legal history so as to understand the motivations for the definitions. Finally, we look at some mathematical properties of the efficiency gap.