The latest evidence of the political influence of density.

The below graph from Conor Sen, an armchair demographer in Atlanta, has been making the rounds this morning in my Twitter feed. It neatly reflects a political phenomenon we've written about before: Yes, cities generally tend to lean more Democratic, and rural states more Republican, but the fine-grained relationship between politics and population density is actually quite remarkable.

This scatter plot looks at congressional districts, which are ranked according to the Cook Parisan Voting Index. That measure essentially reflects how Republican or Democratic a district leans (by percentage of voters, not ideological purity) relative to the national average. A Democratic district shown above as +10, for example, gave the Democratic candidate in the last two presidential elections on average 10 points more of the local vote (say, 63 percent compared to 53 percent) than the nation-wide vote.

Sen compared that data with Census data on population density by congressional district. Here is how a smarter person than me interpreted the resulting picture:


But you may also be struck by the shape of that trend line (Sen is quick to note, by the way, that he's not a statistician). It roughly suggests a political tipping point somewhere around a population density of about 800-1,000 people per square mile. That's actually a number that we've seen before. Here is a different take on the same question, looking at presidential votes by county in the 2012 election, via Dave Troy:

Troy concluded that, "at about 800 people per square mile, people switch from voting primarily Republican to voting primarily Democratic." Richard Florida looked in more depth at that finding last November with a broader conversation on what this trend really says about our differing political preferences and needs in crowded cities and leafy exurbs.

Feel free to weigh in below on why you think your politics may be tied to the proximity of your neighbors (or maybe they're not?). Sen is a bit more zen about what all of this portends for the state of the country in an email: "The big realization I had a couple years ago was if the last era (call it 1982-2007) was driven by debt, the next one (2008-??) appears to be driven by demographics, and I'm trying to get ahead of the curve."

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