This is the fourth in an ongoing series of posts about income segregation, urban planning, and economic opportunity. In the first, we examined three different ways of looking at income segregation: the proportion of people living in low-income neighborhoods, high-income neighborhoods, or both “extremes.” In the second, we looked at another kind of income segregation, measured along the entire income spectrum, and distinguished between segregation and inequality. In the third, we examined how income segregation has changed, both since 1970 and since the Great Recession.

Over the last two weeks, we’ve written about how income segregation is really many different kinds of sorting; how to measure several of the most important kinds; how and why to distinguish between segregation per se and inequality; and how income segregation has changed over the last 40 years.

But we’re not just interested in analyzing and diagnosing income segregation for its own sake. We’re interested in how it intersects with outcomes we care about—notably economic opportunity for people with low incomes, and the strength of common civic culture—as well as policy levers that cities and other governments can use to improve those outcomes.

That makes a recent study led by Reid Ewing of the University of Utah particularly valuable. Ewing et al’s paper is one of the first to rigorously analyze the relationship between economic mobility, income segregation, urban sprawl, inequality, and other potential correlates of economic mobility.

The study builds off of an innovative “compactness index” developed by Ewing and Shima Hamidi to compare levels of sprawl between metropolitan areas. Previously, researchers like Raj Chetty and his team at the Equality of Opportunity Project had used commute times as a proxy for sprawl, but that’s obviously related to a number of other factors beyond the spread-out-ness of the urban environment.

The big takeaway from Reid’s study is that in metropolitan areas that are more compact—that is, less sprawl-y—children born into the lowest fifth of the income distribution are much, much more likely to reach the top fifth as adults. On average, if City A is twice as compact as City B, low-income children in City A will be 41 percent more likely to reach the top fifth of the income distribution than low-income children from City B. (In the real world, “twice as compact” is roughly the difference between sprawl-y metropolitan Nashville and less sprawl-y metropolitan St. Louis.)

St. Louis. Credit: Ron Reiring, Flickr
St. Louis. Credit: Ron Reiring, Flickr


Reid et al also find that lower levels of income segregation—specifically, segregation of the poor—is associated with greater mobility, confirming the findings of other researchers.

But when it comes to the interaction of sprawl with income segregation, things are a little more ambiguous. The study actually finds a negative relationship between compactness and segregation—that is, more compact metropolitan areas tend to be somewhat more segregated by income.

That raises a few questions. First, if compactness is associated with more income segregation, and more income segregation is associated with lower mobility, then how can compactness itself be associated with higher mobility? The answer to that, according to Reid et al, is that other effects—perhaps most notably access to jobs—overwhelm the income segregation effect and cause the net effect to be positive.

Second, why would more compact metro areas tend to be more segregated? There are a number of possibilities here. One is that there is some inherent connection between relatively dense built environment and segregation—but it’s not totally clear what the mechanism there would be. On the other hand, there are a number of possible third factors that might lead to the appearance of such a relationship. One is that in the US, more compact urban areas tend to be older urban areas—and older urban areas are ones where larger proportions of neighborhoods were around to be affected by the more blatant policies that promoted racial segregation, including redlining, restrictive covenants, and widespread racist violence. These historically racially segregated neighborhoods are, today, very disproportionately likely to be areas of concentrated poverty. In that way, the mechanism wouldn’t be that compact development leads to segregation, but that older cities are both more compact and more segregated.

Another possibility is the “modifiable aerial unit problem.” Reid et al measure segregation by Census tracts, which are drawn to have very roughly equal numbers of residents. That means that they’re much smaller in denser cities—and, perhaps, more sensitive to block-by-block sorting than in more sprawling regions, where tracts can include many different blocks that aren’t really in the same neighborhood.

Census Tracts at the same scale in Brooklyn and suburban New Jersey. Credit: Social Explorer

Screen Shot 2016-05-13 at 5.56.30 PM
Census Tracts at the same scale in Brooklyn and suburban New Jersey. Credit: Social Explorer


Or perhaps there’s something distinct that Reid’s compactness index is picking up that other definitions of sprawl don’t include. After all, previous research has in fact found a connection between looser zoning regulations—that is, ones that allow more compact multi-family development—and less income and racial segregation, presumably because a wider variety of housing types is more likely to include a wider variety of housing prices.

But in any case, clearly more research is needed. In the meanwhile, Reid’s study provides more evidence both that more compact urban areas provide more economic opportunity to the low-income, and that income segregation is a key lever of opportunity as well.