Urban/Rural Populations

More evidence that modern American politics are driven more by the urban/rural divide than any other single factor. https://t.co/ohPeY3Onw5

— Dr. John Barentine (@JohnBarentine) October 24, 2016

I saw this tweet this morning as part of a thread, and it didn't quite seem right to me.  The urban/rural factor is certainly one component, but it seems like race and income should also be major factors.  Unfortunately I didn't fully dig through the census site to find a data set that contains all those variables, so this is just an analysis of the census county based urbanization statistics and the 2012 presidential election outcomes on a county basis.  To be completely fair, this is only an analysis of the subset of those two data sets that have identical spelling/spacing for the county names, so 3069 counties from the 3222 available in the census sample.  A quick check seems to indicate that Alaska and Puerto Rice aren't matched between the two.

In any case, plotting the vote fraction for Barack Obama as a function of urban population fraction doesn't show a overly strong trend:

There is a lot of scatter in this relation, but there is an uptick for the most urban counties.

Smoothing the data by taking medians/robust sigma values in 0.02 wide bins shows the uptick clearer.

The purple "fit" is simply a constant with a break to linear placed to match the binned data.

This suggests that for the vast majority of counties, there's little change, but once a certain threshold of urbanization is reached, the median county becomes more likely to vote Democratic as it urbanizes.  A fuller description would try to disentangle this effect from racial, economic, educational, and simple population based components.

One interesting thing to note is that only about a third of the country lives in a county below that break point.  This plot is just the CDF of the population data plotted as a function of the urbanization metric.