Quantifying Partisan Gerrymandering: An Evaluation of the Efficiency Gap Proposal.

• Author: Cover, Benjamin Plener

Introduction I. The Efficiency Gap's Power and Limits A. Gerrymandering as a Multinormative Structural Problem B. The Efficiency Gap as an Improved Measure of Partisan Symmetry C. Two Potential Concerns with the Efficiency Gap II. The Efficiency Gap's Conceptual Design A. McGhee's Efficiency Principle 1. Implications of the efficiency principle 2. A modified efficiency principle B. The Equal Voter Turnout Assumption 1. The turnout gap 2. The size of the turnout gap C. The Aggregation Method from District to Plan 1. The analytical technique: from district-level disparity to plan-level gap 2. The alternative approach: comparing wasted vote shares D. The Definition and Weight of Surplus Votes E. The Two-Party Assumption 1. Uncontested races 2. Voter suppression III. The Efficiency Gap's Relationship to Proportionality and Competitiveness A. The Relevance of Proportionality and Competitiveness B. The Simple Zero-Gap Plan 1. Measuring competitiveness 2. A district's wasted vote disparity is a discontinuous linear function of its margin of victory C. The Efficiency Gap and Proportionality D. The Efficiency Gap and Competitiveness 1. The competitiveness gap a. Definition and equality b. Unacknowledged measure convergence c. Unpacking versus decracking 2. Sensitivity and responsiveness a. Sensitivity as a function of responsiveness b. The robust, minimizing plan E. False Negatives and False Positives F. Definition and Weight of Surplus Votes IV. A New Wasted Vote Measure V. Doctrinal Implications and Conclusions A. Methodological Considerations B. Normative Considerations Introduction

We may be approaching a watershed moment in the U.S. Supreme Court's gerrymandering jurisprudence. In three cases over the last three decades, partisan gerrymandering has eluded the Court's grasp. (1) The Court has recognized that partisan gerrymandering poses a problem of constitutional significance (2) but has repeatedly fractured on whether and how to intervene. A minority of Justices have insisted that partisan gerrymandering presents a nonjusticiable political question susceptible to no judicially discernible and manageable standard, (3) while a majority of Justices have agreed that partisan gerrymandering is justiciable but have disagreed among themselves about the proper legal standard. (4) Justice Kennedy, the current swing vote, has rejected each proposal for assessing partisan gerrymandering claims while expressing hope that a suitable standard may one day materialize. (5) In the first thirty years after the Court held partisan gerrymandering justiciable, (6) dozens of plaintiffs raised claims of partisan gerrymandering, but not one was granted relief (7)--until recently.

In Whitford v. Gill, (8) plaintiffs challenged the 2012 Wisconsin State Assembly district map as a partisan gerrymander, relying in part on a newly proposed numeric measure and associated legal test called the "efficiency gap." (9) In 2014, political scientist Eric McGhee proposed the measure. (10) In 2015, McGhee and leading election law scholar Nicholas Stephanopoulos developed the measure into a legal test specifically designed to address concerns with prior proposals for assessing partisan gerrymandering. (11) In brief, the efficiency gap measure counts the relative number of votes "wasted" by each of two competing political parties; it thereby quantifies the relative efficiency with which each party is able to convert popular support (votes) into governmental power (seats). (12) The legal test classifies as an invalid partisan gerrymander any plan that produces a large, durable, and unjustified efficiency gap. (13)

Armed with this new measure and associated legal test, the Whitford plaintiffs not only survived the motions stage but also won at trial before a panel of three federal judges. (14) The majority opinion does not endorse wholesale the plaintiffs' proposal, but it extensively discusses the efficiency gap as strong evidence in support of its conclusion that the map was a partisan gerrymander. (15) Wisconsin appealed directly to the U.S. Supreme Court, (16) which stayed the panel's remedial order, ordered full briefing, and heard oral argument on October 3, 2017. (17) Whitford offers the Court the opportunity to decide whether the efficiency gap provides the legal test it has been waiting for. Were the Court to affirm the panel's finding of partisan gerrymandering--based on the efficiency gap analysis, other evidence, or some combination thereof--it would remake the law of electoral districting in advance of the 2020 redistricting cycle. (18)

As the Court considers Whitford, the efficiency gap measure and associated legal test warrant careful and comprehensive examination. Thus far, the reactions in popular media, (19) scholarship, (20) and litigation21 have been strong and conflicting. This Article contributes to this evaluative effort by offering a new analysis of the proposed 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.

This analysis bears on the questions before the Court in Whitford. Is partisan gerrymandering justiciable? If so, what is the governing legal standard? Under that standard, is the Wisconsin State Assembly plan a partisan gerrymander? And what role, if any, should the efficiency gap measure play in that standard? I would suggest the following answers to the first three questions: Yes, partisan gerrymandering is justiciable; the principle of partisan symmetry is an appropriate legal standard; and the Wisconsin plan is a partisan gerrymander. The fourth question is the subject of this Article.

The efficiency gap is one of multiple useful indicative measures of partisan asymmetry under circumstances like those in Whitford, where each party earns about half the votes and a large efficiency gap persists under plausible variations in voter behavior. However, 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. Note that this is precisely the approach suggested by leading academics in an amicus brief filed in Whitford. (22) Furthermore, the Court should acknowledge that partisan gerrymandering is not the only form of political gerrymandering that subverts democratic values and should signal its receptiveness to efforts to define and proscribe other forms of political gerrymandering. Just as excessive departures from partisan symmetry can trigger a partisan gerrymandering claim, perhaps excessive departures from competitiveness should trigger a bipartisan gerrymandering claim or excessive departures from seats-votes proportionality should trigger a minority protection claim.

Were the Court to embrace an approach of measurement refinement over time, this Article would prove relevant to the process through which "lower courts ... work out the precise contours of [partisan gerrymandering claim analysis] with time and experience." (23) Additionally, the efficiency gap measure represents a contribution to the election law and political science literatures independent from the role it may play in Whitford. Political scientists are exploring the relationship between partisan gerrymandering and other variables of interest, using the efficiency gap measure as the operational definition of partisan gerrymandering. (24) An evaluation of the efficiency gap measure is thus relevant not only to whether and how courts proscribe partisan gerrymandering but also to how political scientists study it. Finally, in developing the efficiency gap measure, McGhee has discovered significant, surprising relationships between seats-votes curves and properties of wasted vote measures, such as the fact that under traditional definitions parties waste an equal number of votes when a party translates a 1% increase in votes into a 2% increase in seats. (25) This Article identifies other relationships of interest between wasted vote measures, seats-votes proportionality, competitiveness, and voter turnout.

This Article proceeds in five Parts. Part I relates the necessary background in a way that frames the subsequent analysis, suggesting the utility and limits of the efficiency gap measure. Political gerrymandering is a multinormative structural problem the Court has struggled to regulate. The efficiency gap is designed to better measure partisan asymmetry using the ideal of equal wasted votes. But a legal standard for partisan gerrymandering must cohere with both an individual rights framework and a structural account of electoral democracy attentive to the multiple norms at stake. This suggests an inquiry into the efficiency gap's conceptual design and its relationship to competitiveness and seats-votes proportionality.

Part II explores the efficiency gap's conceptual design, examining five choices underlying the measure: the "efficiency principle" McGhee developed as a guide to the measure's design; the equal voter turnout assumption used to reduce the long-form equation to the simplified formula; the method of aggregating wasted votes to produce a single number; the definition and weight of surplus votes; and the two-party assumption.

Part III examines the efficiency gap's relationship to seats-votes proportionality and competitiveness. It shows that the efficiency gap can be understood as a competitiveness gap expressed in terms of turnout and margin of victory rather than wasted votes or undeserved seats. The efficiency gap measure may allow or even encourage mapmakers to draw plans that undermine electoral competitiveness and proportionality between votes...