Since drawing compact districts is not a cure-all, solving the gerrymandering problem also requires ways to measure how biased a given map is. In a 2006 ruling, the Supreme Court offered tantalizing hints about what kind of measure it might look kindly on: one that captures the notion of “partisan symmetry,” which requires that each party have an equal opportunity to convert its votes into seats.

The court’s interest in partisan symmetry, coming after its rejection of so many other possible gerrymandering principles, represents “the most promising development in this area in decades,” wrote two researchers—Nicholas Stephanopoulos, a law professor at the University of Chicago, and Eric McGhee, a research fellow at the Public Policy Institute of California—in a 2015 paper.

In that paper, they proposed a simple measure of partisan symmetry, called the “efficiency gap,” which tries to capture just what it is that gerrymandering does. At its core, gerrymandering is about wasting your opponent’s votes: packing them where they aren’t needed and spreading them where they can’t win. So the efficiency gap calculates the difference between each party’s wasted votes, as a percentage of the total vote—where a vote is considered wasted if it is in a losing district or if it exceeds the 50 percent threshold needed in a winning district.

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The two have proposed the efficiency gap as the centerpiece of a simple standard the Supreme Court could adopt for partisan gerrymandering cases. To be considered an unconstitutional gerrymander, they suggest, a district plan must first be shown to exceed some chosen efficiency gap threshold, to be determined by the court. Second, since efficiency gaps tend to fluctuate over the decade that a district map is in force, the plaintiffs must show that the efficiency gap is likely to favor the same party over the entire decade, even if voter preferences shift about somewhat.

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