Robert Gebeloff and David Leonhardt have a recent article in The Upshot (NYT’s excellent online site for political analysis) titled “The Growing Blue-State Diaspora.” In it, they argue that increased migration from blue states to red states has been making red states bluer.

The analysis is based on actual migration data, but is based (as far as I can tell) on a mere assumption that migrants from blue states tend to be bluer than native voters in the red states they move to.

Is that assumption correct? Maybe not. For example, I was raised in Texas and lived there most of my life. But I’m one of those high-education liberals, people who are often uncomfortable living in Texas (other than in Austin). And so I left Texas and came to Washington DC, a place where I feel much more culturally and politically in sync. Could it be that this happens a lot — that liberal people are especially likely to leave red regions and settle in blue regions? And, likewise, are conservative people especially likely to leave blue regions and settle in red regions?

Something like this is behind the analyses of Bill Bishop and Robert Cushing in “The Big Sort.” The idea here is that mobile Americans have increasingly sorted themselves into like-minded communities.

Roughly, there are three possibilities:

(1) Diaspora: Migrants from blue states are more likely to be Democrats than natives of the red states they move to.

(2) Assimilation: Regardless of how they start off, migrants from blue states end up about as likely to be Republicans as those in the red states they move to.

(3) Big Sort: Migrants from blue states are actually more likely to be Republicans than others — after all, they chose to leave the blue states for redder pastures.

To check this, I turned to the U.S. General Social Survey (years 2000 to 2012). The GSS contains both the region in which the person currently lives (Region) as well as the region in which the person was raised (Reg16). I focused on two broad regions: (1) the Northeast (including New England and Middle Atlantic, roughly from Maine down through Pennsylvania and New Jersey) and (2) the South (including East South Central and West South Central, roughly from Tennessee to Texas).

These data support the Big Sort predictions. People who leave the Northeast are more likely to be Republicans than people raised in the Northeast who stay. People who leave the South are more likely to be Democrats than people raised in the South who stay. Similarly, people who migrate from other U.S. regions to the Northeast are more likely to be Democrats than natives of the Northeast. And people who migrate from other U.S. regions to the South are more likely to be Republicans than natives of the South.

In short, and somewhat weirdly, migrants from blue states make red states redder (and blue states bluer), and migrants from red states make blue states bluer (and red states redder). Just like I made Texas a little redder when I left and made DC a little bluer when I arrived. Migration patterns exaggerate rather than mute regional political differences as people seek out like-minded neighbors.