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Political polarization manifests itself geographically in large part because partisan preferences are strongly correlated with population density. Most casual observers of American politics are aware that in recent election cycles, Republicans have outperformed Democrats in rural areas and vice versa in urban areas, but the density-partisanship connection is evident not only at this macro level but also at very micro scales. Work by the political scientist Jonathan Rodden has shown that within metropolitan areas, and even within places few would consider metropolitan—Muncie, Indiana, for instance—Democrats tend to live in denser areas than Republicans. This is a recent development: For most of the first half of the 20th century, the correlation between voting and density was zero.

What explains the rapid growth of geographic polarization in the United States? One popular theory, most famously articulated by the journalist Bill Bishop in his 2008 book, The Big Sort, is that Americans choose to live in neighborhoods where most residents share beliefs similar to their own. As Bishop writes, “Americans [have been] busy creating social resonators, and the hum that filled the air was the reverberated and amplified sound of their own voices and beliefs.” Residential mobility, as Bishop sees it, leads to echo-chamber neighborhoods where people can avoid interacting with anyone who disagrees with them on political issues.

In a variant of this theory, political sorting is an unintended consequence of sorting based on lifestyle attributes that, for whatever reason, correlate with political beliefs. Political tastes are simply brought along for the ride as conservatives who pine for three-car garages move to the suburbs and liberals who yearn for mass transit move downtown.

These theories are intuitively appealing, but they have two serious shortcomings. First, most people are highly constrained in where they can choose to live—by their job, their family, and the housing market. No matter how strongly a liberal new arrival to the Bay Area might like to live in the comfortable embrace of dense and left-wing San Francisco, unless she can afford the city’s sky-high rents, she’ll have no choice but to settle for the region’s sprawling and (relatively) conservative periphery.

Second, Americans move a lot. The Census Bureau’s Current Population Survey typically finds that around 12 percent of respondents moved within the past year. This high turnover rate means that unless partisans are much more likely to move to a co-partisan neighborhood than an opposite-party one, all that shuffling around would tend to homogenize and smooth out their distribution, like sugar cubes stirred into a cup of coffee.

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