Population Size, Urban Density, and Party ID: How Liberal is Massachusetts?

According to a Pew study, in 2014 Massachusetts was ranked the second most liberal US state, with 56% of adults identifying with or leaning towards the Democratic Party. Even though the majority (54%) of the Commonwealth’s voters in are unaffiliated with any party, registered Democrats still outnumber Republicans 3:1, and Hillary Clinton received twice as many votes as Donald Trump did in 2016.

What I’m trying to understand is whether and how this widely-recognized liberal identity varies across different cities and towns. There is good reason to think that geography and political affiliation are not independent from one another; in fact, there’s an extensive literature on the relationship between the two. Democrats cluster in cities, and they do it so much so that, as The New York Times put it, “Republicans don’t even try to win cities any more.” Books like The Politics of Resentment by Katherine Cramer document a deeply ingrained political, cultural, and economic divide between rural and urban areas.

It’s pretty clear that Massachusetts has some degree of an urban-rural political divide. Somerville — my hometown, the densest city in Massachusetts and the 16th densest city in the country — is a widely hailed as a liberal haven. Somerville is part of MA’s 7th congressional district, which recently held an election that WBUR identified as a “battle between progressives and Democrats.” MA-7 is so thoroughly blue that whoever won the primary was virtually guaranteed to also win the House seat. However, west of Somerville, Massachusetts does have a handful of towns that vote Republican, as the Boston Globe observed in late 2016. And perhaps the clearest signal of a geographic political divide is how the 2016 Democratic primary turned out. Bernie Sanders outvoted Hillary Clinton in rural counties across the country, and MA was no exception. As the Boston Business Journal reported, “the results [in Massachusetts] generally fell along urban versus rural lines.”

In order to illustrate the urban-rural divide in Massachusetts, I made an interactive map of statewide voter enrollment data, shown below:

Those who are familiar with maps of Massachusetts will notice that this one looks pretty similar to the state’s population density map.

Using the following two scatterplots, I show this association between the party ID of voters and the population size of those voters’ municipalities. This dataset contains all cities and towns in Massachusetts, omitting statistical outliers:

A few things jump out about these graphs. First, yes, the cities and towns tend to have a much higher percentage of Democrats than they do Republicans. Second, the data points in the Democrat plot are upward sloping. A higher percentage of Democrats is associated with a larger population size. To be clear, causation can’t be inferred from these data. Others have tried to link party ID to population size using much larger datasets, and it’s hard to determine which way the causation runs.

Although population size can be a useful way to categorize rural areas, exurbs, suburbs, and urban areas, it is not the only relevant metric. Population density, too, is an important tool used to distinguish urban areas from rural ones. As The Atlantic’s City Lab summarized in an article in 2012, “politics are inseparable from density.” Let’s take a look:

In the first graph, the percentage of Democrats in MA cities and towns is positively correlated with population density. In the second graph, the data are essentially flat and the correlation coefficient is near zero, indicating that there is almost no association between the percentage of Republicans and the population density.

The first scatterplot is also hereroskedastic, meaning that, when a trend line is fitted, the size of the error is correlated with population density, the x-variable. This simply means that there’s more variation at the left end of the distribution than there is on the right. Areas of medium-to-high population density follow the pattern better than areas of low density. As with the Democrat scatterplot, the Republican scatterplot suggests that low-density cities and towns have more variation in party ID than high-density ones.

In summary, the differences between these two graphs indicate that Massachusetts municipalities’ party enrollment data is highly related to how urban or rural each municipality is. And, in general, cities and towns in Massachusetts conform to expectations that denser areas are more liberal and less dense ones are more conservative.

Of course, I wouldn’t be me if I didn’t qualify these statements with their potential limitations.

City or town-level population density assumes uniformity of composition within each municipality, which isn’t always the case. My measure of density is the number of people per square mile in each municipality. Although widely-used, this is an imperfect measure of how people actually live, since the composition of cities and towns varies. For example, a municipality with a dense town center surrounded by low-density agricultural land would have a much lower overall density rating than an otherwise similar town without the surrounding open space. As there is no clear way to avoid this measurement issue, assessments of any individual city or town’s population density should be viewed as an approximation rather than an exact measure of how urban or rural the municipality is.

So, how liberal is Massachusetts? I think the simple answer is, pretty liberal, but it depends on where you live. My hope is that, when trying to understand the geography of political attitudes, we can more away from the “red state, blue state” narrative. State-level statistics matter enormously in studies of political representation in the Senate or Electoral College. However, when trying to understand the geography of political attitudes, maybe the more important question isn’t which state you live in, but rather, what does the building with your closest polling place look like?

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Data sources: Mass GIS, Secretary of the Commonwealth of Massachusetts Voter Registration Statistics, United States Census Bureau

Programs used: MS Excel, Google Fusion Tables, Tableau