A new study from the Complexity Science Hub (CSH) has uncovered a strikingly consistent pattern in the economic development of U.S. cities, one that has held firm over 170 years of history. The researchers, Simone Daniotti, Matte Hartog, and Frank Neffke, drew upon an extraordinary dataset of 650 million U.S. census records, 6 million patents, and a wealth of other historical sources spanning nearly two centuries. Their findings reveal that while cities evolve and diversify over time—shifting from craftsmanship and manufacturing to services and engineering—they nonetheless maintain a surprisingly constant level of what the study terms “coherence.” This concept describes how well a city’s economic activities fit together, and the researchers found it to be a stable feature of urban transformation.

This trend is not confined to older, established cities. The same regularity was observed on the West Coast, which developed later and initially grew in relative isolation from the rest of the country. In 1850, cities such as San Francisco and Los Angeles were emerging during the Gold Rush, far behind their eastern counterparts in terms of occupational diversity. At that time, fewer than half of the export-oriented occupations present across the United States were represented on the West Coast. Yet within fifty years, that share rose to nearly 90 per cent, reflecting a dramatic and rapid diversification. Despite the pace and scale of these changes, the coherence of West Coast cities remained stable and comparable to that of the older cities on the East Coast.

The researchers argue that this persistence indicates a fundamental constraint on how cities can transform. Cities may abandon industries of the past and develop entirely new ones, but they do so in a way that preserves their overall coherence. This finding sheds light on the experiences of places such as Pittsburgh and Boston, which endured long periods of decline before re-emerging with renewed vitality. In both cases, the transition away from heavy manufacturing toward high-tech industries and services required a gradual reshaping of the local economy. The lesson is that cities cannot simply leap from one economic identity to another; they must follow pathways that maintain a balance in their industrial structure.

The study also highlights how coherence interacts with city size. Larger cities consistently exhibit lower levels of coherence, with researchers identifying a steady decline of around 4% for every doubling of population size. This pattern has remained essentially unchanged despite profound shifts in technology and society, from the advent of railroads and telephones to the emergence of computers and artificial intelligence. The U.S. population has grown from 23 million in 1850 to 332 million in 2022, spreading steadily westward; yet, the relationship between size and coherence has remained remarkably constant. The implication is that larger populations allow for greater diversity, but they also naturally reduce the coherence of economic activity.

These insights carry significant implications for urban policy. While many cities aspire to expand into emerging technologies and diversify their economies, the study suggests that structural limits must temper such ambitions. A city’s existing base of skills, infrastructure, and institutions is costly to maintain, and excessive diversification can undermine its coherence. Policymakers must therefore strike a balance: smaller cities, in particular, must remain compact in their economic capabilities, while larger cities have more room to spread out across multiple industries. Ultimately, the degree of diversity a city can sustain is closely tied to its size, and realistic ambitions must be benchmarked against those of peer cities.

Underlying this research is a new understanding of coherence as the glue that holds a city’s economy together. It reflects the similarity of occupations, industries, or technologies within the same urban system. It is shaped by three factors: the variety of activities present, the distribution of those activities across the workforce, and the degree of difference between them. A highly coherent city concentrates on a narrower set of closely related industries, such as Detroit during its heyday in the automotive industry. In contrast, a less cohesive city spans many disparate sectors, as in New York. By revealing how coherence remains constant even amid significant transformation, the study opens the door to rethinking how cities grow, diversify, and adapt over the long term.

More information: Simone Daniotti et al, The coherence of US cities, Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.2501504122

Journal information: Proceedings of the National Academy of Sciences Provided by Complexity Science Hub