How are people, infrastructure and economic activity organized and interrelated? In infinite ways depending on infinite dynamic factors. The question borders on absurdly ambitious. But a paper published last week, “Scaling and universality in urban economic diversification,” examines these connections with notable results. In an effort to measure and characterize the economic diversity of cities and its role in productivity, innovation and economic development, the authors unexpectedly discovered what they call “a systemic behavior common to all cities.”

To understand the commonality at work, first think of the city as an ecosystem. Think of types of businesses as “species.” Ecosystems in the natural world often share common patterns in distributions of species. “That got us thinking,” says Hyejin Youn, an Oxford fellow and the lead researcher, “maybe [the same consistency] happens in the city too. Only instead of the food web, it’s people and money and businesses that require one another.” (Youn is quick to clarify that this is merely a metaphor and that the human economy is obviously more complicated than simply a consumer-resource network.)

Just as ecosystems vary greatly — from deserts to tide pools — so too do cities. Cities have different abundances of specific sectors. San Jose’s tech is to Detroit’s manufacturing. “We usually think of cities as unique. Detroit is very different from Silicon Valley,” Youn says. “Abundances of certain industries are their signatures.” But, it turns out, what governs the distribution of those abundances stays the same across the board.

The monster data set the team analyzed for this study was the National Establishment Time-Series, which was created by Walls and Associates, an economic development agency based in Oakland. The set contains more than 32 million establishments nationwide and includes data on employment, sales, business performance, jobs gained and lost, businesses born and failed, and changes in major markets.

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The team also found that as cities grow, the total number of establishments is linearly proportional to its population size (more people, more businesses). Seems logical maybe, but they got specific: On average, they found, “a new work place is created each time the city size increases by 22 people.”

When the researchers tried to measure diversification — the rates at which different types of businesses enter the mix — things got complicated. The scheme that the researchers used for classification of the data may seem obvious, but was perhaps the trickiest aspect of the study design. It comes down to the muddy question of how to measure or even define “diversity.”