Benjamin Cover has posted this draft on SSRN (forthcoming, Stanford Law Review).  Here is the abstract:

For three decades, the Court has failed to settle on a legal test for partisan gerrymandering, and such claims have uniformly failed – until now. Leveraging a new measure and associated test for partisan gerrymandering called the “efficiency gap,” plaintiffs challenging Wisconsin’s Assembly plan have prevailed before a three-judge federal panel. The measure defines partisan gerrymandering in terms of two parties’ relative efficiency at translating votes for their party into seats in government. The case is now before the Supreme 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’s conceptual premises and real-world performance. The measure may produce counterintuitive results because it favors one democratic norm – partisan fairness – over other norms like electoral competitiveness and the proportionality between votes earned and seats won. A mapmaker can achieve a low efficiency gap by drawing a bipartisan gerrymander that carves up the state into safe seats for incumbents and confers a legislative supermajority on a party that earns only a modest majority of ballots cast. Efforts to promote electoral competitiveness or a closer fit between votes earned and seats won may produce a high efficiency gap. And because the efficiency gap is a single measure based on observed election data, it is vulnerable to manipulation: By suppressing turnout of its competitor’s supporters, a party can artificially reduce a plan’s efficiency gap. The measure’s reliance on definitional choices and stylized assumptions about electoral circumstances limits its appeal as one based on observed election results rather than conjecture and hypotheticals. In light of these normative and methodological concerns, the article concludes that the efficiency gap may be a useful indicator of partisan gerrymandering when appropriately applied, but courts should not adopt it as the exclusive legal definition.