Recently, we tried to measure the strength of partisan bias in location choices. We examined every registered voter who moved from one location to another within the state of Florida from 2008 to 2010 (approximately 1.1 million people), as well as an additional random sample of 50,000 voters who moved from one state to another from 2005 to 2016. We compared the new-neighborhood choices of registered Democrats with those of demographically similar independents and registered Republicans who moved from the same zip code. (We might, for example, have compared the new neighborhoods of two white, 45-year-old men who both moved out of the 33609 zip code in Tampa, one a Republican and the other a Democrat.)
Our results confirm that Democrats systematically choose denser, more walkable, and more Democratic-voting neighborhoods when they move than do Republicans leaving the same place. However, the magnitudes of these partisan biases are quite small.
Next, we ran a large-scale simulation in which the rates of partisan sorting that we observed in our study were repeated over several election cycles. We found that Americans move frequently enough, and the partisan bias in moving is small enough, that if it were the only factor changing the geographic distribution of political preferences, then geographic polarization would actually go down substantially.
Moving patterns, it seems, are not the culprit; there is no intentional big sort. So what explains the existence and recent growth of geographic polarization? One part of the story is the partisan realignment that began in the civil-rights era, which has dramatically changed the composition of the parties’ electoral coalitions. The political scientist Lilliana Mason’s recent book, Uncivil Agreement, documents how partisan identities now align with social identities (race, religion, education level) far more closely than they did in the era when southern segregationists shared a party with northern labor. The shift of southern whites and, later, whites without a college degree to the Republican party has left Democrats as the party of educated white voters and racial minorities. Because both of these groups tend to live in cities, the realignment has increased geographic polarization without anyone having to physically move.
Read: Why Americans are so polarized: Education and evolution
Another part of the story is that places may change people more than people sort into places. Our study found that people who move tend to adopt the partisan leanings of their new locale. Examining voter registration data, we found that Republican voters who moved to more Democratic-leaning areas were more likely to change their partisan affiliation to the Democratic Party, compared with Republican voters who left the same place but moved to relatively more Republican-leaning areas, and vice versa for Democratic voters.