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Does gerrymandering cause political polarization?

Andrew Prokop is a senior politics correspondent at Vox, covering the White House, elections, and political scandals and investigations. He’s worked at Vox since the site’s launch in 2014, and before that, he worked as a research assistant at the New Yorker’s Washington, DC, bureau.

Gerrymandering is often blamed for our political system’s current polarization. The argument is that incumbents tend to be more extreme if their districts are gerrymandered, because when they’re sure to win the general election, they’ll focus on appealing to the more extreme members of their base in order to prevent a primary challenge.

But political science research indicates that polarization has much broader causes.

The main evidence for this is from studies that seek to measure the ideological position of each member of Congress, as Keith Poole, Howard Rosenthal, and Nolan McCarty do here. When this data is compared with electoral district information, we can see whether representatives in gerrymandered districts are far more extreme than others. And they’re not.

First of all, the Senate isn’t gerrymandered at all, and it is quite polarized, too. ”The United States Senate follows a very similar trajectory with polarization in the House,” McCarty wrote. That alone is a very strong indication that gerrymandering isn’t the main cause of political polarization.

Second, let’s look at the House districts, and how they relate to polarization. This chart by McCarty shows how heavily George W. Bush won each congressional district on the X axis, and codes the ideology of each member of Congress on the Y axis. If gerrymandering caused polarization, we should see both Democrats and Republicans in swing districts looking more like each other than their brethren who have been gerrymandered into safe districts. But we don’t:

Gerrymander Nolan McCarty

Representatives do become somewhat more ideologically extreme in more lopsided districts, and gerrymandering could certainly help explain this. But there’s a clear gap between Democrats and Republicans in all districts — and gerrymandering can’t explain that basic polarization.

”Democrats and Republicans are just polarized, no matter whether their district is red, blue, or purple,” political scientist John Sides wrote. “It’s hard to imagine that creating more competitive districts will mitigate polarization. Members in purple districts are pretty polarized, too.”