One of the main reasons Republicans have a big advantage on the congressional map in the years ahead has nothing to do with politicians gerrymandering districts; it has to do with Americans gerrymandering themselves.

Basically, the argument (which I have espoused) is that Republicans have an advantage when it comes to map-making because their voters are much more diffuse and spread out, while Democratic voters tend to be more concentrated in urban areas. So while Republicans have a bunch of districts that they generally win with 55 or 60 percent, Democrats have many more districts that are 70 percent-plus Democratic performing.

Thus, the half of Americans who tend to favor Democrats are concentrated in fewer districts, leaving Republicans with many more districts -- and states -- that lean their way.

One point of contention on this theory, though, is the "why." Why are Americans sorting themselves in this way? Are people who just happen to live in more rural areas simply drawn to the Republican Party and city-dwellers to Democrats, or are Republicans actually physically moving away from the cities so they can live near people who agree with them politically (and vice versa for Democrats)?

Well, according to new data from the Pew Research Center, it's pretty clear that politics do inform people's decisions when they move -- to a significant extent.

The data show that 77 percent of people who are "consistently liberal" favor urban communities, while 75 percent of "consistent conservatives" favor a more spacious lifestyle. And there's a pretty smooth progression as you move along the political continuum between the two.

Indeed, just 4 percent of consistent conservatives say that if they could live anywhere, it would be in the city, while 76 percent prefer small towns and rural areas. Conversely, just 31 percent of consistent liberals say they prefer small-town or rural life, while 67 percent favor the city and suburbs.

And again, the in-between looks exactly how you'd expect:

Now, just because liberals like cities and Republicans like small towns doesn't mean that people are actually moving accordingly. That's not quite what these data show -- at least not by themselves.

What they do show, though, is that it's pretty clear people at least desire the kind of setting that tends to fit their politics. And, thus, when they move, they will definitely be drawn to such areas -- if its feasible to move to them.

But how conscious are they of this? Are people actually attempting to move to new places with the express purpose of being around people with whom they agree politically?

The answer: Yes. Many people are.

Pew also asked people whether the political leanings of their neighbors was important to them, and 50 percent of consistent conservatives and 35 percent of consistent liberals said that they preferred politically similar neighbors.

Now, that might not seem overwhelming. After all, the other half of consistent conservatives and 65 percent of consistent liberals aren't saying that their neighbors' politics matter.

But the data above suggest that, even among those who don't outwardly admit their desire for politically similar neighbors, their subconscious is telling them to move to neighborhoods that just happen to include such people.

So why does this all benefit the GOP?

As the second graph shows, the GOP's aversion to the city is much more pronounced than Democrats' aversion to small towns and rural areas. While 31 percent of consistent liberals and 44 percent of "mostly liberals" are happy as a peach in politically red small towns and rural areas, just 4 percent of consistent conservatives and 14 percent of "mostly conservatives" feel most at home in the blue city.

That suggests Republicans are much more likely to flee urban areas -- leaving ultra-concentrated Democrats districts -- than Democrats are to flee small towns and leave similarly concentrated GOP districts out there.

All of which reinforces -- and could even expand -- the GOP's "natural gerrymandering" advantage.