Quantifying Partisan Gerrymandering: An Evaluation of the Efficiency Gap Proposal

Electoral districting presents a risk of partisan gerrymandering: the manipulation of electoral boundaries to favor one political party over another. For three decades, the U.S. Supreme Court has failed to settle on a legal test for partisan gerrymandering, and such claims have uniformly failed. Until recently. Plaintiffs prevailed before a three-judge federal panel in Wisconsin by leveraging a new measure called the efficiency gap, which quantifies partisan gerrymandering in terms of two parties\u27 relative efficiency at translating votes for their party into seats in government. The case is now before the Court, which may embrace the efficiency gap approach and thereby remake the law of electoral districting. Through a synthesis of mathematical and legal analysis, this Article examines the efficiency gap measure, focusing particularly on its underlying methodological choices and electoral assumptions as well as its relationship to competitiveness, seats-votes proportionality, and voter turnout. The efficiency gap is a useful indicative measure of partisan gerrymandering under the circumstances of cases like the one currently before the Court, in which each party earns about half the votes and a large efficiency gap persists under plausible variations in voter behavior. Relying in part on the efficiency gap measure, the Court should rule in favor of the plaintiffs. However, a mapmaker can achieve a below-threshold efficiency gap with a skewed bipartisan gerrymander that carves a state up into uncompetitive districts denying minority parties sufficient representation. For example, a party that earns only 59% of the vote can secure a filibuster- and veto-proof 75% supermajority of the legislature with a below-threshold efficiency gap. For this and other reasons, the Court should not adopt the efficiency gap as the exclusive definitional measure of partisan gerrymandering, such that a plan would be invalid if and only if it exhibited a large, durable, and unjustified efficiency gap. Instead, the Court should permit some flexibility for scholars, litigants, and courts to refine measurement approaches over time and under varying circumstances. One approach worth future exploration is a variation on the efficiency gap that defines a surplus vote in terms of the full margin of victory and compares wasted vote shares instead of totals. Finally, the Court should be aware that any measure, like the efficiency gap, that compares votes to seats entails the perverse risk that partisan voter suppression may operate to reduce the apparent severity of partisan gerrymanders