In Continuing Defense of the Efficiency Gap: How a Recent Decision in North Carolina Further Supports the Argument that the Efficiency Gap is a Viable Measure to Determine Partisan Gerrymandering

By: Elvin Perez


On January 9, 2018, a federal district court in North Carolina ruled that a 2016 congressional district map drawn by that state’s legislature was an unconstitutional partisan gerrymander under the First and Fourteenth Amendments.[1]  The ruling in this case, Common Cause v. Rucho, marked the first time that a federal court has struck down congressional districts as partisan gerrymanders, since prior courts have struck down only state legislative maps based on partisan considerations.[2]  In its decision, the court dismissed the concerns of the defendants and used the efficiency gap as one of the primary statistical measures for its Fourteenth Amendment analysis, which measures “wasted votes” cast in favor of the respective parties and divides them by the total number of votes cast in an election.[3]  This measure, which determines levels of partisan bias, is currently before the Supreme Court in Gill v. Whitford[4], where they will decide whether to accept its premise, or—like previous measures—relegate it to the dustbin of history.  If the decision made by the district court in Rucho is any indication, it is that there is support for the efficiency gap as a viable measure to determine partisan gerrymanders.

In Rucho, the court examined the analysis of Dr. Simon Jackman, a professor of political science at the University of Sydney and an expert on statistical methods in elections, who applied the efficiency gap on the disputed redistricting plan.[5]   His results showed that the redistricting map drawn by the North Carolina Legislature gave the Republican party an advantage of 19.4 percent, which is more than the 8 percent advantage that Dr. Jackman calculated to be the limit before a redistricting map becomes unfairly biased.[6]  These results were disputed by the defendants, and it is here where the court gave its defense of the efficiency gap from some of its most common criticisms: (1) that the efficiency gap, as a mathematical formula, does not take into account a number of  unique considerations that effect the outcome of elections, such as the quality of candidates, money raised, and the impact of wave elections, and (2) that the Constitution does not entitle supporters of a particular party to representation in a state’s congressional delegation in proportion to their statewide vote share.[7]  With its attack of these two criticisms, the district court advanced the argument that the efficiency gap is a viable measure to use in determining partisan gerrymanders.

Often one of the most common criticisms of the efficiency gap is that it’s mathematical formula does not factor-in other considerations in the outcome of elections, and as a result, the redrawing of district lines should be left to politicians and others.  Chris Winkelman, General Counsel for the National Congressional Republican Committee, has expressed some of these criticisms: “The history of redistricting in the United States, if it teaches us anything, teaches us that redistricting is a political act, was intended to be a political act, and has always been a political act.”[8]  The defendants in Rucho tried to advance these same arguments, but the court rejected them, citing to the methodology used to create the threshold for determining what a partisan gerrymander is under the efficiency gap.  The methodology cited involves a comparison of the currently disputed plan with previous congressional elections spanning 44 years, eliminating the argument that election-specific factors explain the large advantage that certain political parties have in elections and instead focusing  on the result found through quantitative analysis.[9]  This defense supports the viability of the efficiency gap as a measure to determine partisan gerrymander because it accounts for different variables that judicially-created measures cannot, leading to more equitable maps. Another common criticism that the defendants tried to use was that the efficiency gap promotes proportional representation, which is not found in our Constitution.  In reality, the efficiency gap does not contain assumptions of proportionality. [10]  Instead, as the court correctly points out, the efficiency gap is premised on making sure that the process of mapping votes into seats is the same for both sides.[11]  As such, this clarification dispels a misconception of the efficiency gap that prevents other courts from adopting it, and advances the argument that the measure is viable in determining whether a redistricting plan is an illegal partisan gerrymander.

While the court in Rucho did mention drawbacks to the efficiency gap, they clearly believed that the benefits outweighed the costs by disposing of two popular critiques of the measure.  In doing so, the ruling in Rucho advances the argument that the efficiency gap is a viable measure to determine illegal partisan gerrymanders, and lends greater weight towards its acceptance by the Supreme Court.


[1] See Common Cause v. Rucho, No. 1:16-CV-1026, 2018 WL 341658, at *1 (M.D.N.C. Jan. 9, 2018).

[2] See David A. Graham, North Carolina’s Landmark Ruling Against Partisan Gerrymanders, The Atlantic (Jan. 9, 2018),; See Rucho, 2018 WL 341658, at *1.

[3] See Nicholas Stephanopoulos & Eric McGhee, Partisan Gerrymandering and the Efficiency Gap. 82 U. of Chi. L.R. 831, 851 (2015) (explaining “wasted votes” as votes cast for both a losing candidate and votes cast in excess of what a winning candidate needs to prevail); Rucho, 208 WL 341658, at * 49.

[4]  Gill v. Whitford, 137 S.Ct. 2289 (2017).

[5] See Rucho, 2018 WL 341658, at *1, 49.

[6] See id. at *50.

[7] See id. at *52-53.

[8] Chris Winkelman Et Al., Symposium: Mind the gap? The efficiency gap, its failures and the “problem” of geography and choice in redistricting, SCOTUSblog (Aug. 8, 2017, 2:56pm),

[9] See Rucho, 2018 WL 341658, at *53.

[10] See Stephanopoulos, supra note 3, at 887.

[11] See Rucho, 2018 WL 341658, at *52.