In this new paper, I weave recent research in political science, economics, psychology and more into an account of political polarization and the rise of populist nationalism as a surprising and overlooked side-effect of urbanization.

I claim that we’ve failed to fully grasp that urbanization is a relentless, glacial social force that transforms entire societies and, in the process, generates cultural and political polarization by segregating populations along the lines of the traits that make individuals more or less responsive to the incentives that draw people to the city. I explore three such traits — ethnicity, ideology-correlated aspects of personality, and level of educational achievement — and their intricate web of relationships. The upshot is that, over the course of millions of moves over many decades, high density areas have become economically thriving multicultural havens while whiter, lower density places are facing stagnation and decline as their populations have become increasingly uniform in terms of socially conservative personality, aversion to diversity, and lower levels of education. This self-segregation of the population, I argue, created the polarized economic and cultural conditions that led to populist backlash.

Because the story of urbanization just is the story of a strengthening relationship between density, human capital and economic productivity, it’s also the story of relative small town and rural decline. The same process that has filtered better-educated, more temperamentally liberal whites out of lower density places has left those places with less vibrant economies, but also with more place-bound, ethnocentric populations.

It’s no shock that leavers leave and stayers stay. What’s surprising is that, if you’re white (and if you’re not, you’re almost certainly urban), the personality traits that make you more or less inclined to leave or stay — that make you more or less magnetized to the rising attractive force of the city — also predict how socially conservative or liberal you’ll tend to be, and which political party you’ll tend to support.

So the pull of urbanization has segregated us geographically on those traits, and it has done it so thoroughly that Democratic vote share now rises, and Republican vote share drops, in a remarkably linear fashion as population density rises. The relationship between density and party affiliation is, with few exceptions, similar everywhere — in “red states” and “blue states,” in sprawling metro regions and bucolic small towns — and majorities tend to flip at the density typical of a big city’s outer suburbs. I call this partisan polarization on population density the “density divide.”

When populations segregate geographically on traits relevant to ideology and party affiliation, political polarization is sure to follow. The increasing concentration of the economy in big cities, which is both a cause and effect of urbanization, amplifies this polarization. Rising prosperity reliably produces a liberalizing, tolerant, positive-sum mood. Material insecurity, in contrast, tends to elicit a grim, zero-sum, us-or-them mindset. Because the sunshine of prosperity has become increasingly focused on urban populations, lower density white populations — which, thanks to the sorting logic of urbanization, are already more conservative and ethnocentric — have been left with objectively darkening prospects and a mounting sense of anxiety that is, at once, economic and ethno-cultural.

This combination of conditions created a political opportunity Donald Trump managed to exploit. Because urbanization is reshaping societies everywhere around the world, it has created similar conditions, and similarly illiberal strongman leaders, in other countries as well. If we’re going to be able to do anything about the acrimony of polarization and the peril of ethno-nationalist populism, we’re going to have to get the story straight. This cross-disciplinary account of the social and psychological forces behind the density divide is my preliminary attempt to put us, finally, on the right track.

You can download the full paper here.

An audio version, conveniently broken into six podcast-sized parts, is available here.