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A new way to measure a city’s diversity — and what it means that Boston ranks surprisingly high

By Robert David Sullivan
June 12, 2011

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The biggest story to come out of the 2010 Census is that America is getting more and more diverse. We talk a lot about how diversity is changing the United States in a larger sense, but when it comes to measuring how it affects our daily lives, that’s harder to pin down.

Perhaps the most useful way to think about what “diversity” means in practice is to look at how it increases our variety of experience — specifically, how it boosts the chances you’ll encounter a lot of different people where you live. A simple head count doesn’t come close to measuring this phenomenon.

Here’s one way to do it: “diversity density.”

This is a new measure (OK, I invented it) that tries to gauge the real experience of diversity in a city by looking at two things at once. First, is there a wide range of ethnic and racial groups in your city — as opposed to a binary division between white and black, or native and immigrant? And second, is your city’s density high enough so that you really encounter people from different ethnic backgrounds on sidewalks and other shared space, as opposed to simply driving past their neighborhoods on your way to the mall?

The diversity density index measures both at once. And if you use data from the most recent census, you see something surprising: Boston is the third-most diverse city in America, outside of New York and San Francisco.

Diversity density counts the number of people per square mile who do not claim membership in either of the county’s two largest racial/ethnic groups. The result gives you a rough approximation of the likelihood of running into people of a variety of different ethnic backgrounds during a brisk walk across town.

Suffolk County, most of whose residents are in Boston, has 12,338 people per square mile, making it the seventh-most crowded county in the United States. Take away its two largest ethnic groups (non-Hispanic whites and Hispanics), and there remain 3,957 people in other categories per square mile — the sixth highest concentration of all US counties and county equivalents. (Of the five leaders, four are boroughs in New York City.)

This may be surprising to those who remember the Boston of 50 years ago, an overwhelmingly white city with a reputation for racist attitudes that only got worse with the introduction of school busing. Since then, the Hub has lost a sizable number of white residents and has attracted a large immigrant population that is unusually varied in terms of race, language, and educational levels.

In Boston today, you can see the diversity density effect — and how the city has changed — by taking the Orange Line. A few decades ago, its ridership was almost entirely white above Downtown Crossing and overwhelmingly black on its southern route. Today, it’s a rainbow express from one end to the other.

Aside from the changing feel of the subway, what does diversity density mean for a city?

In general, a high diversity density seems to be correlated with a stable or growing economy, as found in New York, Boston, and Washington. Other highly ranked areas include California’s Orange County and New Jersey’s Middlesex County, home of Rutgers University.

Conversely, the struggling cities of Baltimore and St. Louis, as well as Detroit’s Wayne County, rank lower on diversity density than their sizes would suggest. They’re not quite as dense as Boston, and are far less broadly diverse: At least 90 percent of the populations in all three places are either black or non-Hispanic white. All of these places have been marked by racially polarized politics followed by “white flight” to the suburbs. And in contrast to Boston, San Francisco, and New York, they all lost population from 2000 to 2010, and the number of white residents dropped by more than 10 percent in each.

Both diversity and density are components of modern city life, but there has long been debate over whether they should be goals in themselves. While diversity has been a goal of city planners hoping to reduce racial tensions, both diversity and density have only recently come to be seen as potential engines for the growth and health of cities.

Studies have shown that the face-to-face networking that can happen in crowded cities is associated with a growing and innovative economy. As for racial diversity, sociologist Richard Florida popularized the idea of tolerance as one of the “three Ts” driving 21st-century economic development (“talent” and “technology” being the other two), though critics have argued that it is rising educational levels, not diversity, that drive the economies of “creative cities” such as Boston and Minneapolis (which remains overwhelmingly white). University of Michigan professor Scott Page, author of “The Difference: How the Power of Diversity Creates Better Groups, Firms, Schools, and Societies,” stresses that “cognitive diversity” is what drives a city’s economy, but adds that “by being in a [diverse] city, you can see differences that keep your mind fertile” and open to new ideas.

The idea of a link between diversity and creativity is strong enough that leaders in Portland, Ore., reacted with dismay to census data showing that its downtown has lost minority residents over the past decade, with Mayor Sam Adams telling The Oregonian, “The exodus from the central city causes me great concern; it is alarming.”

Japonica Brown-Saracino, a professor of sociology at Boston University and author of “A Neighborhood That Never Changes: Gentrification, Social Preservation, and the Search for Authenticity,” cautions that some cities that now score highly on the diversity density index could simply be on their way to becoming like Portland, as new “creative class” residents drive up housing prices and push less affluent groups out. But it’s not a foregone conclusion: She also notes the growing presence of “social preservationists,” who are conscious of, and try to head off, the disruptive effects of gentrification. (One example of social preservationists: the protesters of supermarket chain Whole Foods’ plan to take over a space in Boston’s Jamaica Plain once occupied by a Latino grocery store.)

To be sure, neighborhoods that look diverse statistically don’t necessarily have much face-to-face interaction between people of different ethnic groups — and Page, for his part, notes a paradox that can arise when a city is both dense and diverse: “People find it easier to find other people like themselves, and you can lose the benefits that diversity can bring.” If ethnic groups (and other groups, such as gays and lesbians) form independent social networks and patronize only businesses that cater to them, “you can have a diverse city that’s really just a collection of silos.”

There’s not much that public policy can do to avoid the silo syndrome, Page says, adding that “people at the micro level have to want to interact with people from different backgrounds.” Fortunately, Boston has a big advantage in its high level of education attainment, Page says: “If you interact with diverse people in college, that tendency carries over to your adult life.”

That tendency may be especially strong in the Hub, where the huge student population can make anyone — no matter what age or educational level — feel as if he or she is living on a college campus, one in which waves of newcomers from around the globe are as natural as the changing of the seasons. An openness to new ideas has allowed Boston to reinvent itself several times, and our high degree of diversity density may give us a big advantage in doing so again.

Robert David Sullivan is a freelance writer living in Malden.