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Taken to an Extreme, Gerrymandering Could Lead to a One-Sided Congress

For the last year, the U.S.’s two major political parties have battled to draw district lines to their own advantage in the decennial redistricting process. A new study by Yale SOM’s Kai Hao Yang and Alexander Zentefis finds that partisan gerrymandering, pushed to the limit, could exclude the views of half the country from the legislative process.

In 2021, as Democrats struggled to pass a package of voting reforms, Senator Chuck Schumer told The Nation that “partisan gerrymandering strips the American people of their right to have a truly representative government.” Ellen Sciales, of the Sunrise Movement, suggested that gerrymandering must be halted “before we can no longer save our democracy.”

Gerrymandering is the process of drawing the borders of voting districts to either dilute or amplify the power of voters from one political party. It has come under particularly intense scrutiny since 2019, when the Supreme Court ruled gerrymandering for party advantage cannot be challenged in federal court, and in the wake of the highly partisan redistricting process that followed the 2020 census.

A new working paper coauthored by Yale SOM’s Kai Hao Yang and Alexander Zentefis suggests that these concerns aren’t overblown. If pushed to the limit, gerrymandering could radically reshape the makeup of Congress, with major implications for the legislation that could be passed.

Their results are rooted in a mathematical model that defines the possible ways to divide a group of voters along a spectrum of policy positions—more or less left-leaning, more or less right-leaning, centrist—into election districts.

“We find that, at the extreme, gerrymandering can lead half the population to lose representation of their views in Congress.”

Without gerrymandering, a truly representative government, as Senator Schumer suggested, would mean that the composition of the officials elected from these districts would mirror the political positions of the population. For instance, if the country were 60% Democratic, Congress ought to be 60% Democratic as well.

Yang and Zentefis instead find that in the most extreme case, gerrymandering can create maps in which candidates from only one ideological wing are elected, to the exclusion of the other side. That is, by strategically redistributing voters into districts, one can build a map in which every district elects candidates to the right of the median voter or a map in which every district elects candidates to the left of the median voter—or any map between these two poles.

“The set of possibilities that can be created is quite broad,” Zentefis says. “We find that, at the extreme, gerrymandering can lead half the population to lose representation of their views in Congress.”

What does such a map mean for legislation? If the position of the median voter is thought of as the number 50 and the two ideological extremes are 1 and 100, mapmakers can design a map to enable the passage of a piece of legislation that sits as distant from the median as 25 on one side or 75 on the other. And if lawmakers are given a choice between two pieces of legislation—the median preference and anything else—mapmakers can create a map in which the median always loses, even when it is put up against a measure that is much more extreme.

“It’s well known that when there’s a referendum between two policy positions the median is always going to win,” Yang says. But gerrymandering means that a representative system in which elected officials make policy can produce the opposite result. “If you take the median and take any other policy, then there exists a map under which the Congress produced from that map will prefer the alternative policy.”

Short of banning partisan gerrymandering or handling redistricting over to a bipartisan commission, are there policy solutions that would mitigate the power of gerrymandering? The authors say that altering the voting procedure in legislative bodies can help. They note, for instance, that raising the number of votes needed to pass legislation from a simple majority to a supermajority could dampen the effect of gerrymandering. Of course, this is currently the functional practice in the U.S. Congress, due to the Senate filibuster, which has been blamed for creating excessive inaction. But it also makes it virtually impossible to pass legislation that is favored only by a narrow majority of legislators.

“Raising the threshold of what fraction of people need to agree to pass legislation has the benefit of not letting policies pass that are way beyond the median,” Zentefis says. “But it has obvious downsides too, like entrenching the status quo.”

The world of politics is messy, of course, and Yang and Zentefis, recognizing the limits of a simple model, complicate the math to better reflect the real world. They restrict maps to 435 districts, for instance, and they make each district contiguous and contained within a single state. With these (and other) extensions, the results hold.

“Qualitatively, what we find remains,” Yang says.

Zentefis says.

Department: Research