The Growth of Political Bubbles

For many, the 2016 election revealed the large degree of geographic and cultural separation within America. While coastal areas and big cities moved more Democratic than they already were, more rural areas and those in the so-called “heartland” of the U.S. turned politically redder than before. Political bubbles have become much clearer in the aftermath of this past election, as it seems Americans increasingly interact with and live among others that share their same worldview and political inclinations.

I wanted to test this ideas of growing political bubbles—which has the effect of more people living in more solidly Democratic and Republican areas and fewer living in mixed or “purple” regions. I turned to county-level vote totals in the 2008, 2012, and 2016 elections calculated margins of victory in favor of either the Democratic or Republican presidential candidate. The margins help describe the political environment—the type of bubble—in which voters live, ranging from solidly blue to solidly red. Then, based on total votes cast in a county and the margin in the same county, I could see how many voters fell into each type of political environment (and how many lived in bubbles). I plot this in the below graph.

Just as an example to follow along what’s going on, 13.73 percent of 2012 voters lived in counties where the presidential vote margin was between 10+ points in favor of the Democratic candidate, Barack Obama.


The most purple types of bubbles—ones in which the margin between the Democrat and Republican is small and thus have many people of opposite parties living in proximity of each other—are located in the center of these graphs. Where the Democratic margin of victory was between 0 and 10 points, 15.9 percent of 2008 voters, 15.1 percent of 2012 voters, and 12.1 percent of 2016 voters resided. For counties that saw between a 0-10 point close Republican victory, 10.1 percent of 2008 voters, 11.2 percent of 2012 voters, and 9.8 percent of 2016 voters lived there. Thus, fewer voters are living in the most politically diverse counties in the country, with the biggest decrease occurring in 2016—an election with Clinton and Trump as candidates saw fewer counties with mixed vote preferences.

The next most politically diverse counties (Democrat +10 margin) and (Republican +10 margin) also have fewer voters living in them, with 2016 introducing the largest decrease. So where are all these voters now living? In much more one-sidedly blue or red counties. Relative to 2012, the Dem +20, Dem +30, Dem +50, Dem +60, and Dem +70 bubbles—areas where the vote is much more similar among everyone and befitting of a “political bubble” label—all see increases in number of voters in the 2016 election. On the other end of the spectrum, the same growth of bubbles occurs for every group from Rep +20 through Rep +80.

As an example, 1.7 percent of 2008 voters lived in counties where McCain won by 50-60 points (+50), 2.5 of 2012 voters lived in counties where Romney won by 50-60 points, and 4.9 of 2016 voters lived in counties where Trump won by 50-60 points. In general, the distribution of voters is becoming flatter across degree of political bubbles—more and more voters now live in counties with an increasingly one-sided blue or red vote. It’s not a seismic change, but it’s gradually trending toward greater bubbles and 2016 accelerated that path.

To better visualize what’s going on (and reduce the number of bubble buckets), below is the same graph but simplified to 20-point margin increments.


The same story of growing political bubbles materializes. Eight years ago, 26.1 percent of all voters lived in counties where the margin between Democratic and Republican vote shares was less than 10 percent; in 2016, that number has dropped to 21.9. Fewer voters are now living in counties where Democrats won by 10-30 points and 30-50 points, but more voters now live in counties that are decidedly blue (by more than 50 points). While 7.5 percent of voters lived in counties that saw a 30-50 point Republican victory in 2008, 13.1 percent of voters now live in this territory. The portion of electorate living in the most Republican counties—50 points or more red—has increased by more than five percentage points as well.

Finally, to measure just how much bubbles on either side are growing (not in terms of a Democratic or Republican bent), the below graph shows where voters live based on absolute differences in major party support (smaller vote margins—such as “0-10 % Diff.”—indicate more diverse political areas and larger vote margins—such as “50+ % Diff.”—indicate areas with voters who are more politically alike).


Slight pluralities of voters still live in more politically mixed counties, but it’s changing. 26.1 percent of 2008 voters and 26.4 percent of 2012 voters lived in counties where the margin between the Democratic and Republican candidates was less than 10 points—only 21.91 of voters did so in 2016. The percentage of voters living in counties with a 10-20 point margin (for example, where the Democrat won 55 percent and Republican won 40 percent) also dropped from 26.5 in 2008 to 20.2 in 2016. The share of voters living in 30-40 point margin counties grew slowly, but the biggest change occurs at the extreme end: the percentage of voters living in counties where the Democrat or Republican candidates won by more than 50 points grew from 10 in 2008 to 16.8 to 2016. In other words, more voters are now living in counties that contain very internally similar vote preferences—either one-sidedly Democrat or Republican. Political bubbles were already starting to grow a bit, but the 2016 election accelerated this process, contributing a clear increase in county-level geographic sorting.