PARTISAN GERRYMANDERING has long been reviled for thwarting the will of the voters. Yet while voters are acting disgusted, the US Supreme Court has only discussed acting — declaring they have the constitutional right to fix the problem, but doing nothing. But as better data and computer algorithms are now making gerrymandering increasingly effective, continuing to sidestep the issue could do permanent damage to American democracy. In Gill v. Whitford, the soon-to-be-decided challenge to Wisconsin’s 2011 state Assembly redistricting plan, the court could finally fix the problem for the whole country. Judging from the oral arguments, the key to the case is whether the court endorses the concept of “partisan symmetry,” a specific standard for treating political parties equally in allocating legislative seats based on voting.
In 2012, Wisconsin Republicans received about 48 percent of Assembly votes statewide but, because of how the districts were drawn, they won more than 60 percent of state Assembly seats. Here’s why this violates partisan symmetry: In 2014, under the same redistricting plan, when the Democrats received about 48 percent of the vote, they won only 36 percent of the seats. This was a gerrymander by and for Republicans. However, Democrats, when given the chance in other states, have been equally guilty of gerrymandering in their own favor.
Back in 1987, when we originally proposed the concept of partisan symmetry in the pages of the American Political Science Review, we hoped others might see its advantages too. As it turns out, almost all scholarly publications on the subject over the ensuing 30 years — analyzing numerous electoral systems and redistricting plans in the United States and countries around the world — also adopted our standard. Experts in legislative redistricting litigation, even those testifying on opposing sides of the same case, routinely appeal to partisan symmetry. But only if the Supreme Court also endorses the standard will partisan gerrymandering lose its power to bias democratic outcomes.
Partisan symmetry applies to an entire legislature, but we built it from an agreed-upon standard of fairness for voting in a single district: Namely, how many votes it takes to win shouldn’t depend on who you are or which party you represent. For example, the plurality voting rule mandates that whoever gets the most votes wins. Even young children understand plurality voting as an obvious and fair procedure. But precisely why is it fair? Plurality voting is fair because the parties are treated symmetrically. The winner is any candidate who gets the most votes.
Numerous other electoral rules satisfy the partisan symmetry standard in a single district, such as runoff elections, primary elections, etc. In fact, when choosing a restaurant with a group of friends, you probably use the unit-veto system, whereby any member of the group can veto an option. If so, then you too have chosen a rule to meet the same symmetry standard.
The partisan symmetry standard for a redistricting plan involving a legislature uses the same principle as for a single legislative district, requiring only that we treat the two parties symmetrically. For example, suppose the Democrats received 55 percent of the statewide vote and wind up with 70 percent of the seats. That would be fair only if the Republicans, too, would receive 70 percent of the seats when, in another election under the same redistricting plan, they happened to receive 55 percent of the votes. The rule would then be that any party receiving 55 percent of the votes would receive 70 percent of the seats, and so it is symmetric and thus fair.
Satisfying partisan symmetry still leaves redistricters with considerable discretion. For example, they could arrange for a party to receive the same proportion of seats as they receive in votes (as in proportional representation), or to receive a seat proportion equal to their vote proportion plus a bonus if they win a majority of votes; the bonus could even be as large as a winner-take-all rule. Proportional representation and winner-take-all are not usually chosen for US legislatures, but numerous other possibilities in between also meet the standard of partisan symmetry. For example, some states require that redistricting plans favor incumbents, which reduces the bonus for the majority party and the responsiveness of seats to votes. Other states require more competitive elections, which translates into a larger bonus and higher responsiveness. All these and others are allowed under partisan symmetry.
Determining whether an electoral rule for a single-member district meets the partisan symmetry standard is easy. Doing so for a redistricting plan is straightforward but, as in many other areas of the law, requires applying certain statistical methods. Fortunately, partisan symmetry methods have been refined, vetted, and validated in the 30 years since our original article; almost all methods in widespread use give similar answers for the same elections, and so the court need not choose among them. And software to ascertain the degree of deviation from partisan symmetry (known as “partisan bias”) is open source and freely available.
These methods can also easily indicate whether a proposed redistricting plan will be biased before any elections have been held.
Partisan gerrymandering illegitimately disrupts the connection between voters and officeholders, and its impact on American democracy is growing. This meddling in our electoral system is also entirely self-inflicted, with no foreign powers to blame. The court can and should fix this problem by simply requiring redistricters to follow the partisan symmetry standard.
Gary King is director of the Institute for Quantitative Social Science at Harvard University. Robert X. Browning is a professor in the Department of Political Science and Lamb School of Communication at Purdue University.