In The Righteous Mind, Jonathan Haidt cites Dave Wasserman,
Our counties and towns are becoming increasingly segregated into “lifestyle enclaves,” in which ways of voting, eating, working, and worshipping are increasingly aligned. If you find yourself in a Whole Foods store, there’s an 89 percent chance that the county surrounding you voted for Barack Obama. If you want to find Republicans, go to a county that contains a Cracker Barrel restaurant (62 percent of these counties went for McCain).1
It turns out that there’s a decisive break between Democratic and Republican support at a population density of 800 persons/per square mile. Why might that be? Colin Gordon suggests,
There is, of course, a lot going on here, including a long history of regional and metropolitan patterns in partisan alignment. But a hopeful reading of the map would go something like this: People who live close to one another are more likely to know someone of a different color, a different income group, or a different sexual orientation. They therefore rely upon and appreciate the provision of public goods and public services (transit, parks, garbage collection), even as they consume fewer public dollars than their less-densely populated counterparts.
On Quora, Kelly Martin, answering a question about why Manhattan overwhelmingly votes Democratic, explains,
For the same reason that virtually every densely populated urban area in the United States overwhelmingly votes for Democrats: because Republican principles of governance would leave highly developed urban areas with insufficient tax revenue to maintain the infrastructure services without which those densely populated areas could not continue to support human habitation at those densities. (For those who don’t believe this, consider this: the third rail of Chicago city politics is snow removal, and an administration that fails at snow removal will not survive the next election.)
Population density is one of the best predictors of partisan voting outcomes: the higher the population density of a voting precinct, the more likely that precinct will vote Democratic. Indeed, the only large cities in the US that do not reliably elect Democrats are cities (such as Indianapolis, Indiana and Jacksonville, Florida) that have incorporated large tracts of lower-density suburban territory into their municipal boundaries. Manhattan is the most densely populated tract of land in the United States, and thus on that basis alone one can predict—correctly—that Manhattanites will reliably elect Democrats.
It’s a compelling argument, but it could be mistaken in one of two ways. First, greater density generally implies economies of scale, so government outlays per capita for infrastructure might be lower than in rural areas. For instance, Internet access is more costly in rural areas than urban areas.
Perhaps Martin’s unstated premise is that the output of critical infrastructure specific to dense urban areas is above the minimum long run average cost, and suffers from diseconomies of scale. Given Tiebout competition, cities might offer value in many other ways that attract and retain residents, despite an inability to minimize the long run average cost of infrastructure.
Second, even if urban-specific infrastructure would benefit from economies of scale, it’s possible that there are systematic ideological biases that obscure this knowledge from urban Democratic voters.
Might it be that cities attract and breed people who have high openness to new experiences? Linda Krueger on Quora supposes,
Those in urban areas are simply exposed to more people, more diversity, more cultures, more ideas. They know the world is made up of differing religions, beliefs, and lifesyles because they see it outside their door every day and coexisting peacefully as everyone moves through their day.
Rural areas are isolated from much of the world because the world and all its diversity is not right outside their doors. Instead they get to create a view of others and of opposing beliefs that will not be challenged to any great degree simple because their environment is more homogeneous. In that scenario fear is built on the unknown. They can assume immigrants are taking jobs even though none have taken their jobs. They can assume terrorist are just waiting to destroy them because they fear and are leery of outsiders. They can assume homosexuality is a curse to a moral society because a foundamental religious foundation gives them a sense of community, a strength of righteousness, and a guide to follow that comes from a high authority they can rely on and refer to.
Without any definitive answers, I’d guess that a difference in tolerance and openness to new experiences is what’s primarily driving the effect, and the other political beliefs follow via irrational clustering.
1 Based on research by David Wasserman of The Cook Political Report, reported by Stolberg 2011, as cited in Haidt, Jonathan (2012-03-13). The Righteous Mind: Why Good People Are Divided by Politics and Religion (Kindle Locations 6731-6732). Knopf Doubleday Publishing Group. Kindle Edition. Haidt, Jonathan (2012-03-13). The Righteous Mind: Why Good People Are Divided by Politics and Religion (Kindle Locations 5210-5213). Knopf Doubleday Publishing Group. Kindle Edition.
CC BY-ND 2.0-licensed photo “night density” by urban feel