A striking example of this new approach comes from the University of São Paulo in Brazil, where two economists have reconstructed the monetary circulation of the Roman Republic using the remains of approximately four million coins unearthed in excavations conducted over the past two centuries. Focusing on the period from 155 BCE to 2 CE, Eduardo Amaral Haddad of the School of Economics, Business, and Accounting and Inácio Fernandes Araújo of the Luiz de Queiroz College of Agriculture combined techniques from regional economics, spatial analysis, and geographic information systems with large international archaeological databases. Their study, published in the journal Humanities and Social Sciences Communications, part of the Nature group, arrives at a provocative conclusion: the consolidation of Roman rule depended less on military conquest than on the economic integration of the territories that conquest brought under Roman control.

The logic of the method rests on a simple but powerful observation. Every coin preserved by archaeology carries three pieces of information: where it was minted, when it was produced, and where it was found roughly two thousand years later. Taken individually, each record says very little. But when millions of records are analyzed together, patterns emerge that no single artifact could reveal. The paths taken by money, the intensity of economic exchange between regions, the degree of integration across the Mediterranean, and even the institutional evolution of one of antiquity’s largest economies all leave traces in the aggregate distribution of coinage. In effect, the researchers treated coin hoards not as collections of curiosities but as data points describing the economic relationships that structured the Roman Republic.

The project had unlikely origins. In 2014, Haddad was on sabbatical at Princeton University, working on mainstream economics questions, when he began attending a weekly seminar in the Department of Classical Studies out of personal interest. At one meeting, he watched a presentation that used shipwreck remains and pottery shards to reconstruct trade networks in the ancient Mediterranean. The idea stayed with him. Shortly afterward, while exploring the university library, he found a catalog of Roman coins containing exactly the information he needed: minting dates, production locations, and excavation sites. He photocopied the catalog, reasoning that the same tools economists use to study flows of people, goods, and income between modern cities could be turned on the Roman Mediterranean. What began as a hobby eventually became a long-term research program, deepened by a distance-learning graduate course on the ancient Mediterranean at the University of Leicester in the United Kingdom.

Carrying out the analysis required solving a problem that had frustrated earlier attempts at large-scale reconstruction: fragmentation. Information on Roman coins was long scattered across museums, libraries, private collections, and researchers’ archives, making systematic study nearly impossible. That changed as institutions such as the American Numismatic Society began coordinating international projects to digitize and standardize these collections under common recording protocols. The main source for the study was Coin Hoards of the Roman Republic Online, a database dedicated to hoards from the Republican period. The researchers also drew on ORBIS, a Stanford University platform that simulates travel along the roads, rivers, and sea routes of the Roman world and estimates the time and cost of journeys between hundreds of locations, as well as the Pleiades gazetteer and the Roman Road Network database, which provided georeferenced information on cities, roads, and ports across the ancient Mediterranean.

After careful curation, the team assembled a dataset of roughly four million coins organized into 24,646 hoards, corresponding to 5,167 distinct pairs of minting and discovery sites. Rather than analyzing individual coins, whose numbers are distorted by differences in preservation, loss, and reuse over the centuries, the researchers worked at the level of these archaeological records. Their first question was deceptively simple: was the spatial distribution of the coins random, or did it follow a pattern? Statistical tests drawn from regional economics and economic geography gave an unambiguous answer. The coin finds clustered in ways that were far from chance, concentrating along the main trade routes of the Roman world. Cross-referencing the coin distributions with the road network revealed something even more striking: a clear spread of coinage radiating outward from the city of Rome itself, following the infrastructure the Republic had built.

Tracing the money was only the first step. To understand why some regions saw intense monetary circulation while others remained peripheral, the researchers needed to model the Roman economy itself. They organized information from the historical and archaeological literature into a framework inspired by social accounting matrices, a tool normally used to analyze contemporary economies. The model describes the relationships among the principal economic actors of the time, including the government, households, landowners, merchants, slaves, and the army, and represents the flows of goods and payments linking them. It also distinguishes between types of production and consumption, from food and raw materials to manufactured goods and luxury items, the latter capable of traveling far greater distances. The model also captured a gradual but profound transformation: the progressive monetization of the Roman economy, as payments for supplying the army, maintaining slaves, and funding public activities shifted from payment in kind to payment in coin, a shift visible in the archaeological record itself.

It was at this point that one of the study’s most consequential findings emerged. A widely held interpretation assigns the Roman army the dominant role in spreading currency through conquered territories, on the intuitive logic that advancing legions carried soldiers who received wages and suppliers who traded goods. The results only partially confirm this. Military structures were decisive in the initial phase of expansion, introducing monetary circulation into newly conquered lands. But their influence waned as territories were permanently incorporated. Currency took root only once those regions developed economic, religious, administrative, and civic structures capable of generating a lasting demand for money. The army, in other words, acted as a catalyst that opened regions to monetization, but it was the economy that made the currency permanent. This helps explain why interpretations focused exclusively on military action miss a fundamental dimension of Roman expansion: conquest was only the beginning, and genuine integration required markets, cities, institutions, religious centers, and enduring networks of exchange.

The study also documents the evolution of Roman economic geography over time. In the earliest periods analyzed, coins tended to remain relatively close to where they were minted. As the Republic expanded, coins began appearing at ever greater distances from their points of origin, and in the statistical models this appears as a progressive weakening of the effect of distance on circulation. Regions once separated by geographic barriers were being stitched together through a common network of transportation, markets, and institutions. The researchers describe this as perhaps the most significant finding of the study, because it suggests that coin circulation can serve as an indirect indicator of economic integration across the ancient world. The spatial analysis further revealed a set of nested functional regions: at the center, the city of Rome, dominated by public administration; around it, a highly integrated economic core in the Italian Peninsula, where circulation reflected market activity; beyond that, an intermediate belt where administrative, economic, and military expenditures coexisted; and, at the frontier, zones of recent expansion where conquest-related spending predominated until pacification allowed civil and commercial activity to take hold. Together, these findings recast the spread of a common currency as the connective tissue of an empire in the making.

Subject of Research: Mapping coin circulation and economic networks in the Roman Republic using digital archaeology and spatial analysis

Article Title: Traces of nearly four million coins reveal how Rome achieved economic integration

Article References: Traces of nearly four million coins reveal how Rome achieved economic integration. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Roman Republic, coin hoards, economic integration, digital archaeology, spatial analysis, numismatics, monetization, regional economics, ancient Mediterranean, trade networks, ORBIS, social accounting matrices

The finding arrives as the global trading system becomes increasingly dependent on overlapping networks of regional agreements. By 2023, more than 356 regional trade agreements were in force, while the World Trade Organization had received more than 585 notifications related to such arrangements. These agreements include free-trade areas, customs unions and other preferential frameworks that reduce tariffs or establish common rules among participating economies. Their effects are not uniform. The terms of each agreement, the sectors it covers, the products exchanged and the economic characteristics of its members can all shape the outcome. Some agreements may rapidly increase trade in manufactured goods, while others may affect agriculture, services or transportation only after years of adjustment.

The central statistical challenge is timing. Regional trade agreements rarely begin affecting commerce on a single, clean date. Negotiations may influence firms before a treaty formally enters into force, while tariff reductions and regulatory changes can be phased in over many years. Countries may also join different agreements at different moments, and some agreements eventually expire or are replaced. This staggered pattern makes it difficult to compare countries fairly. A simple before-and-after calculation can confuse the effect of an agreement with unrelated changes in economic growth, exchange rates, wars, financial crises or shifts in global demand. The economists therefore applied recent advances in difference-in-differences estimation, a family of methods used to compare changes over time between units exposed to a policy and units that are not.

Traditional difference-in-differences models often rely on two-way fixed effects. In a typical application, country-pair observations are adjusted for persistent differences between pairs and for shocks affecting all countries in a given year. The approach can be useful, but it becomes problematic when treatment effects vary across countries or over time. Under staggered adoption, a country pair treated early may inadvertently serve as a comparison group for a pair treated later, even though the earlier agreement is already influencing trade. If the effects grow or shrink over time, the resulting estimate can blend together distinct phases of the policy response and, in some circumstances, give misleading weights to different treatment cohorts.

Abreha and Robertson’s design explicitly accommodates these complications. Their study uses bilateral trade data covering more than 200 countries from 1962 through 2020, drawn from the Centre d’Études Prospectives et d’Informations Internationales, or CEPII. The long time span allows the researchers to observe trade relationships well before agreements are introduced and for many years afterward. The wide country coverage also extends the analysis beyond the narrower samples common in earlier work. Rather than treating an agreement’s influence as identical immediately after implementation and a decade later, the model permits dynamic effects that can vary with time since adoption. That distinction is crucial because companies may need years to redirect supply chains, establish distribution networks, meet new standards or develop relationships with customers abroad.

The resulting pattern is one of gradual expansion rather than an instant trade surge. The estimated effect grows during the years following implementation and reaches approximately 25 to 30 percent after ten years. This estimate is larger than the effect obtained from standard two-way fixed-effects models, indicating that conventional specifications may understate the longer-term consequences of trade agreements when they fail to represent staggered adoption and changing treatment effects. The result does not mean that every agreement produces the same increase, nor that the agreement alone explains every change in commerce. Instead, it represents an average estimated response across a broad and historically diverse set of bilateral relationships, under the assumptions built into the researchers’ econometric framework.

One especially important feature of the analysis is its treatment of anticipation. Trade policy can influence behavior before the legal implementation date. Firms may begin investing, signing contracts or shifting sourcing plans after an agreement is announced but before tariffs or other provisions formally change. Importers and exporters may also adjust inventories in expectation of future rules. If researchers mark the policy’s beginning only at the enforcement date, some of the genuine response may already have occurred and could be incorrectly assigned to the pre-treatment period. Abreha and Robertson find that accounting for anticipation changes the estimated timing, magnitude and statistical significance of the dynamic trade response. The result reinforces the idea that economic policies have calendars more complicated than a single legal start date.

The paper also expands the scope of the evidence beyond manufacturing, incorporates agreements that can expire and uses a lagged dependent-variable specification as an alternative way to address the persistence of trade relationships. Bilateral commerce is highly durable: countries that trade extensively in one period are likely to continue doing so in the next because firms have already built logistics networks, established suppliers and learned how to navigate one another’s markets. A lagged dependent variable captures part of that persistence, although it also introduces additional identifying assumptions that must be considered when interpreting the estimates. By examining non-absorbing treatments—agreements that do not remain permanently active—the study reflects the fact that international policy arrangements can end, change status or be superseded.

The award committee selected the article from eligible papers published in the journal during 2025, including papers rated in the top quarter or better during the review process. The committee was chaired by N. Gregory Mankiw of Harvard University and included members of the Atlantic Economic Journal’s editorial board and sponsors of the International Atlantic Economic Society Endowment Fund. The recognition highlights not only the headline estimate but also the methodological problem behind it: trade agreements are complex interventions whose effects may vary by country, sector and time. The authors acknowledge important boundaries to their conclusions. Their study does not examine cases in which agreements create entirely new trading relationships, and it does not separately account for individual provisions such as intellectual-property protections or regulatory rules. Those details could explain why some agreements generate stronger effects than others. Even so, the analysis offers a more technically realistic picture of how trade policy reshapes the global economy—slowly, unevenly and often beginning before the paperwork says it has started.