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82 U. Chi. L. Rev. 831 (2015)
Partisan Gerrymandering and the Efficiency Gap

handle is hein.journals/uclr82 and id is 843 raw text is: Partisan Gerrymandering and the Efficiency Gap Nicholas O. Stephanopoulost & Eric M. McGheett The usual legal story about partisan gerrymandering is relentlessly pessimis- tic. The courts did not even recognize the cause of action until the 1980s; they have never struck down a district plan on this basis; and four sitting justices want to vacate the field altogether. The Supreme Court's most recent gerrymandering deci- sion, however, is the most encouraging development in this area in a generation. Several justices expressed interest in the concept of partisan symmetry-the idea that a plan should treat the major parties symmetrically in terms of the conversion of votes to seats-and suggested that it could be shaped into a legal test. In this Article, we take the justices at their word. First, we introduce a new measure of partisan symmetry: the efficiency gap. It represents the difference be- tween the parties' respective wasted votes in an election, divided by the total num- ber of votes cast. It captures, in a single tidy number, all of the packing and crack- ing decisions that go into a district plan. It also is superior to the metric of gerrymandering, partisan bias, that litigants and scholars have used until now. Partisan bias can be calculated only by shifting votes to simulate a hypothetical tied election. The efficiency gap eliminates the need for such counterfactual analysis. Second, we compute the efficiency gap for congressional and state house plans between 1972 and 2012. Over this period as a whole, the typical plan was fairly balanced and neither party enjoyed a systematic advantage. But in recent years-and peaking in the 2012 election-plans have exhibited steadily larger and more pro-Republican gaps. In fact, the plans in effect today are the most extreme gerrymanders in modern history. And what is more, several are likely to remain extreme for the remainder of the decade, as indicated by our sensitivity testing. Finally, we explain how the efficiency gap could be converted into doctrine. We propose setting thresholds above which plans would be presumptively unconsti- tutional: two seats for congressional plans and 8 percent for state house plans, but only if the plans probably will stay unbalanced for the remainder of the cycle. Plans with gaps above these thresholds would be unlawful unless states could show that the gaps either resulted from the consistent application of legitimate pol- icies or were inevitable due to the states'political geography. This approach would t Assistant Professor of Law, The University of Chicago Law School. ff Research Fellow, Public Policy Institute of California. This Article builds on our earlier legal and political science work on redistricting. It is part of a larger project aimed at grasping the consequences-and improving the law- of this important and intricate activity. For helpful comments, we are grateful to Bruce Cain, Jowei Chen, Chris Elmendorf, Andrew Gelman, Michael Gilbert, Ruth Greenwood, Bernie Grofman, Rick Hasen, Benjamin Highton, Simon Jackman, Vlad Kogan, Justin Levitt, and Rick Pildes. We are pleased as well to acknowledge the support of the Robert Helman Law and Public Policy Fund at The University of Chicago Law School. 831

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