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The Supreme Court is deciding a gerrymandering case. Here’s the social science that the justices need to know.

Evidence shows that partisan gerrymandering is undermining democratic representation. But it can be diagnosed.

- May 31, 2019

Over the past year, federal trial courts have overturned several instances of what they deemed to be partisan gerrymandering, including in state legislative maps in Wisconsin and Michigan, and congressional maps in North Carolina, Maryland, Ohio, and Michigan. Moreover, the Pennsylvania Supreme Court overturned a partisan gerrymander in that state’s congressional map.

Now, the U.S. Supreme Court is set to rule on partisan gerrymandering in the coming weeks. One of the key questions facing the court is whether judges can reliably detect the most egregious gerrymanders and avoid a flood of new litigation. It is increasingly clear that social science provides straightforward metrics to detect the most extreme gerrymanders.

Moreover, social-science evidence suggests that reining in gerrymandering would improve democratic representation in the United States.

The partisan gerrymanders under scrutiny are really extreme

In recent years, political parties have increasingly relied on gerrymandering to lock in large majorities of the seats in congressional delegations and state legislatures. In gerrymandered states, one party can win a large majority of the seats with a minority of the votes, while the other party would need to win an overwhelming majority of the votes even to reach parity in the legislature. Thus, the relationship between the number of votes and number of seats is distorted in a way that favors the gerrymandering party.

There are several ways to measure partisan advantage in a districting plan, including the efficiency gap, partisan symmetry, declination and the mean-median difference. These metrics all capture distortions in the vote-seat relationship. In a recent work, Nicholas Stephanopoulos and I averaged these measures to estimate the partisan bias in each state’s districting process — that is, whether these distortions advantaged one party. The graph below draws on data from all congressional elections from 1972 to 2018 in states with more than six congressional seats.

Distribution of partisan bias in U.S. House district maps.
Distribution of partisan bias in U.S. House district maps.

The graph shows just why partisan gerrymandering has become so controversial: the states that have produced litigation really do have historically high levels of partisan bias in their congressional maps. Michigan, Ohio, Pennsylvania and North Carolina have some of the largest pro-Republican biases in the past 40-plus years, and Maryland has a large pro-Democratic bias. For instance, Ohio’s congressional map had a higher level of pro-Republican bias in 2012 than 99.5 percent of previous congressional maps.

Blame the gerrymandering party, not just electoral geography

What these states have in common is that one party controlled their redistricting process in 2011-12. While geographic sorting and other factors certainly affect the districting process, a lot of academic research shows that political control of the redistricting process enables parties to gerrymander districts and gain a large partisan advantage in subsequent elections. Moreover, the effect of gerrymandering on legislative elections often swamps the effect of geography.

Partisan gerrymandering lowers political competition and increases political inequality

Partisan gerrymandering has a number of other downstream effects on the health of political parties. Stephanopoulos and I found that candidates are less likely to contest districts when their party is victimized by gerrymandering. Candidates who do choose to run are more likely to have weak résumés. Donors are less willing to contribute money. And ordinary voters are less apt to support the targeted party.

Gerrymandering also affects policymaking. The growing partisan polarization in Congress and in state legislatures means that legislators nearly always vote the party line. So voters who face a partisan bias because of gerrymandering do not have their views fairly represented in the legislature. This means that they have little voice on important issues facing our country — ensuring real inequality in whose voices count in writing laws and making policy.

There’s not much voters can do about this

Though a large majority of Americans oppose gerrymandering, voters can’t do much about it. Legislators are unlikely to voluntarily relinquish their party’s advantage in elections. In states that allow ballot initiatives, advocates can propose independent redistricting commissions and other reforms directly to voters.

But the Supreme Court may soon strike down the ability of voters to enact independent commissions for congressional elections via ballot initiatives as a violation of the Constitution’s elections clause. And even if the Supreme Court doesn’t do this, only about half the states allow ballot initiatives. So voters in half of the country can’t abolish partisan gerrymandering without judicial intervention. This is why reformers have had to bring legal challenges.

What social science can tell the Supreme Court

One key question before the Supreme Court is whether there are standards to detect an extreme partisan gerrymander. During oral arguments this year, Justice Samuel A. Alito Jr. asked, “How do you determine whether [a particular plan] is constitutional?”

In fact, the five federal courts that have struck down gerrymanders that they deemed partisan have shown that social science provides manageable standards to evaluate both undue partisan intent by mapmakers and partisan bias in the effects of a districting plan. Thus, courts have the tools they need to detect extreme gerrymanders.

First, courts can detect partisan intent in a districting plan using computer algorithms that ignore partisan considerations when drawing districts and instead draw thousands of alternative district maps on the basis of traditional goals, such as equalizing population and maximizing geographic compactness. These simulated maps can be compared to the map being challenged to assess whether legislators’ partisan goals led them to deviate from traditional goals. They can also be used to show that there’s no nonpartisan “justification” for a plan’s observed partisan bias.

Second, courts can detect the partisan effect of a districting plan by comparing the level of partisan bias in a plan to plans over the past several decades. This shows whether a particular plan is a historical outlier. I conducted this sort of analysis in several recent court cases. As the earlier graph showed, the five states whose maps have been challenged in federal court all have historically high levels of partisan bias.

Overall, there is a growing body of evidence that partisan gerrymandering is undermining democratic representation in the United States. Moreover, there are ways to detect the most egregious gerrymanders. As the Supreme Court mulls the issue, these facts should be at the forefront of its thinking.

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Chris Warshaw is an assistant professor of political science at George Washington University.