Extreme partisan segregation goes down to a hyper-local level in the United States

Democrats and Republicans physically cluster together even in small geographic areas, such as cities and neighborhoods, according to new research published in Nature Human Behaviour. The study indicates that American voters live in politically isolated residential environments with very little local exposure to members of the opposing party.

“The geographic separation of Democrats and Republicans is something that is easily observable in a macro sense, in that we can look at county or state maps of the presidential vote and see clear red and blue areas, and this is phenomenon has been shown in research to be influence electoral outcomes, representation, and policy,” explained lead researcher Jacob R. Brown, a Ph.D. candidate at Harvard University.

“But the measurement of partisan segregation is usually quite blunt, relying on aggregate summaries, and we were interested in using individual-level data to measure exposure to Democratic and Republican neighbors with greater specificity than prior studies.”

For their study, the researchers used the residential addresses of every registered voter in the United States to calculate the partisan segregation of more than 180 million individuals.

“We measure the local residential partisan exposure of every voter in the country by identifying their 1,000 nearest registered neighbors, calculating the distance they live from each of these neighbors, and creating a weighted average of the proportion of their neighbors who are Democratic voters or Republican voters that gives greater weight to neighbors who live closer you,” Brown explained to PsyPost.

The researchers found a high degree of partisan segregation across the country. “Many voters live with very little exposure to neighbors of the opposite party,” Brown said. Democrat exposure to Republicans tended to be lower than Republican exposure to Democrats.

The most extreme political isolation was found among Democrats living in densely populated cities like New York, with 10 percent of them encountering a Republican only one out of 10 times in their neighborhood. A high level of political isolation was also observed among Republicans living in low-density rural areas.

But this political isolation was not just a result of the partisan-leanings of urban and rural populations. Democrats and Republicans tended to be segregated from one other within suburban areas, and even within neighborhoods.

“Even when Democrats and Republicans live in the same small geography, such as a city or neighborhood, we still see that they tend to live in different parts of these small geographies. This demonstrates a pervasive level of separation even conditional on living in the same larger area,” Brown said.

Partisan segregation also appeared to be distinct from racial and ethnic segregation. “Partisan segregation is correlated with racial segregation, but cannot be fully explained by it. For example, even just looking at same-race neighbors, we still see Democrats and Republicans clustering together,” Brown told PsyPost.

Previous research has found that political polarization among Americans has grown rapidly in the last 40 years, and the researchers fear that political isolation could be a driving force behind ideological extremism. But the causes and consequences of the observed segregation are still unclear.

“Future questions include how living in homogeneous neighborhoods influences voters, in their political participation and their own partisan attitudes,” Brown said. “The level of micro-segregation shown in the paper also motivates inquiries into what forces drive political segregation. These are ongoing research efforts by myself and colleagues.”

The study, “The measurement of partisan sorting for 180 million voters“, was authored by Jacob R. Brown and Ryan D. Enos.


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