Today we're witnessing the rise of a new urban phenomenon: the segregated affluent neighborhood. These aren't gated communities or condo high rises. They are isolated, rich neighborhoods that arise, seemingly spontaneously, from the shifting real estate in city centers. Armed with new data sets and computer-generated models, two urban planning researchers at UCLA have figured out where these enclaves come from and how they are changing our cities.

Income segregated neighborhoods

Urban planning professors Michael C. Lens and Paavo Monkkonen published the results of their research this month in the Journal of the American Planning Association after an intensive analysis of new data on the 95 biggest cities in the United States. Though it's widely known that income inequality has risen in US cities over the past 40 years, very little research has been done on how this affects neighborhoods. Most researchers assumed that income inequality led to segregated poor neighborhoods, or ghettos. But Lens and Monkkonen found it actually led to the opposite: enclaves of the ultra-rich.

Of course, cities have always had rich and poor neighborhoods. What's shrinking out of existence today are mixed neighborhoods that include people from different class and cultural backgrounds. And that's a problem, say economists who have done longitudinal studies of kids who grow up in income segregated cities. Kids who grow up in neighborhoods where everyone is poor tend to stay poor, while kids in mixed neighborhoods enjoy the kind of class mobility that the US prides itself on fostering. There are some obvious reasons for this. In many cities, wealthy neighborhoods have their own local school systems and community centers with abundant resources to prepare kids for college or skilled jobs. In poor neighborhoods, resources are stretched thin. There are few after-school enrichment programs and little support for kids who need help with learning. We can see the same discrepancies when it comes to hospitals, public parks, and other neighborhood amenities.

Running the numbers

Because cities are built environments controlled by local governments, urban planners often explore whether there might be some way to redesign cities to improve the lives of the people who live in them. If income segregation leads to poor economic outcomes for children growing up in cities, is there some way to encourage mixed neighborhoods? Before we can answer that question, we need to understand what's causing these rich enclaves in the first place.

That was exactly what Lens and Monkkonen set out to do in their research. They used a data set called the Wharton Residential Land Use Regulation Index (WRLURI for short), released in 2008, which includes information on city regulations from over 2,000 municipalities in the US. Cities get a WRLURI score based on 11 different factors that impact development, including whether there are density restrictions and open space requirements, as well as how many groups have to approve new building projects and whether community meetings are required for rezoning requests. WRLURI scores are used as a broad measure for a city's regulatory restrictiveness; the higher the score, the more restrictions apply to anyone trying to build things in the city.

Looking at the 95 largest cities in the WRLURI set, Lens and Monkkonen did something that no other city planning researchers had done before. They combined the WRLURI data with a statistical method for measuring income inequality developed by Stanford researcher Sean Reardon in 2004. Reardon's tool allows researchers to determine how income inequality maps to a geographical region, thus calling into relief which neighborhoods are poor and which are wealthy. As Lens and Monkkonen write, "It is thus possible to analyze whether more stringent regulation of urban development, as measured with the WRLURI, contributes to the segregation of higher- or lower-income households, or both."

Regulation and isolation

Controlling for variables like population, poverty rate, income inequality, and race (all taken from US Census data), the researchers measured how land uses from the WRLURI data affected income segregation in cities. In other words, they measured how city regulations affected the formation of wealthy and poor enclaves. What they discovered was fascinating. First, they determined that there was a strong correlation between heavily regulated city development and segregated rich neighborhoods. But not all regulations led to this state of affairs.

Three basic categories of regulation are associated with income segregation. As Lens and Monkkonen put it: