Traces of nearly 4 million ancient Roman coins have offered new insights into how the Roman Republic achieved economic unity across its vast territories. Researchers from the Regional and Urban Economics Lab at the University of São Paulo (NEREUS-USP) used methods from regional economics, spatial analysis, and geographic information systems, combined with large international archaeological databases, to reconstruct the movement of money during the Roman Republic, from 155 BCE to 2 CE. Their findings, published in the journal Humanities and Social Sciences Communications, suggest that the expansion of Roman influence was not solely driven by military force, but also by the economic integration of newly conquered regions. The study was conducted by Eduardo Amaral Haddad, a professor at the School of Economics, Business and Accounting (FEA-USP), and Inácio Fernandes Araújo, now a professor at the Luiz de Queiroz College of Agriculture (ESALQ-USP). They analyzed data from about 4 million coins found in archaeological excavations over two centuries, grouped into 24,646 hoards—collections of coins found together—linked to 5,167 pairs of minting and discovery sites. This large dataset allowed the researchers to trace how coins moved across the Roman world. The project began in 2014 when Haddad, during a sabbatical at Princeton University, became interested in using archaeological data to study ancient trade networks. He discovered a catalog of Roman coins that contained the necessary information for his study and began using tools from regional and urban economics to analyze these networks. For many years, information on Roman coins was scattered across museums, libraries, private collections, and researchers’ archives, making it difficult to study large-scale patterns. However, recent efforts by institutions like the American Numismatic Society to digitize and standardize these collections have made it easier to study the movement of coins across the ancient world. The main source for the study was the Coin Hoards of the Roman Republic Online (CHRR), a database focused on hoards of Roman Republic coins. The researchers also used digital archaeology platforms like ORBIS, developed by Stanford University, which simulates travel across ancient Roman roads, rivers, and sea routes, estimating travel times and costs. Other tools, such as the Pleiades gazetteer and the Roman Road Network database, provided georeferenced information on cities, roads, and ports in the ancient Mediterranean. Using spatial analysis techniques common in economics and geography, the researchers found that the distribution of coins was not random. Instead, they formed statistically significant clusters along the main trade routes of the Roman world. The study also sought to explain why some regions had more intense monetary circulation than others. To do this, the researchers created a model inspired by social accounting matrices, a tool used in modern economic analysis, to represent the relationships between key economic actors of the time: the government, households, landowners, merchants, slaves, and the military. The study suggests that while the military was crucial in the early stages of expansion, its influence diminished as newly conquered territories became more integrated into the Roman economy. The army helped open these regions, but economic factors ensured the long-term use of Roman currency. Over time, coins were found farther from where they were minted, indicating increasing economic integration. The researchers also identified distinct economic zones within the Roman Republic, from the administrative center of Rome to the Italian Peninsula, which had a highly integrated economy, and to the more recently conquered regions, where military and administrative spending was more common. As these areas became more stable, economic and commercial activities grew in importance.