It is 42 BC and you are a citizen of ancient Rome. Julius Caesar has been dead for two years, and civil war is raging between his assassins and the army of his adopted son, Octavius. Life is tumultuous and unstable, so you decide to follow the lead of many of your neighbors and hide your savings. Each week you collect your hard-earned wages and bury the coins in a secluded area on the outskirts of town, intending to retrieve them later. Of course, you keep the location of your money secret. But what if you never make it back there? What if, like so many others, you are called to the battlefield and killed? To the benefit of modern-day scholars, your money will remain buried and untouched for many centuries after your death.
Nearly two millennia later, clusters of ancient Roman coins, called hoards, are still being uncovered in remote areas all over Italy. As these coin hoards have been unearthed over the years, scholars have used them to glean valuable information about the society from which they came.
Recently, two professors, one a historian and the other a scientist joined together in an unlikely scholarly collaboration to investigate whether the coins could help them paint a more accurate picture of the population count of ancient Rome. Their findings were published in a study that could re-write the history books.
Several years ago, Walter Scheidel, a historian at Stanford University, met University of Connecticut scientist Peter Turchin at a history conference. While there, they discovered a mutual interest in ancient Roman culture. They kept in touch, and through the course of a series of conversations realized that by working together, they could offer a new perspective on an issue that has been a topic of debate among historians for decades.
Rome at the beginning of the first millennium is a subject of intense fascination for many scholars. Marked by periods of both internal and external warfare, the territory expanded from a republic to an empire in 27 BC and continued to extend its boundaries throughout Europe. Much of what we know about Rome is extracted from historiographical sources – ancient texts and documents, but these records have varying degrees of reliability.
While browsing some history books, Prof. Turchin, who specializes in data models, became troubled by remarkably high population estimates of 20 million citizens of ancient Rome.
If the high figure is indeed accurate, then our understanding of Rome is dramatically altered: in that case, the Roman state would never have experienced a shortage of manpower, and the Roman economy would have been able to feed many more mouths than in later periods.
“I was reading up on the first century BC, which I know was a disintegrative phase, when the population typically declines or remains stagnant, so when I read about the high count, I thought, ‘it can’t be true’,” says Turchin.
Turchin shared his concern with Scheidel who agreed that it was unlikely that Roman Italy could have sustained such a large population during times of strife and political upheaval.
Scheidel was particularly compelled to explore the population issue because if the population of ancient Italy had been as large as assumed by the high count, it would have been unsurpassed until the nineteenth century and the Roman economy would have been much more productive than is commonly accepted. This might completely change established theories of how the European economy has developed over millennia.
The two scholars determined that an inter-disciplinary research project might shed more light on this important facet of Roman history. Scheidel’s historical expertise combined with Turchin’s quantitative skills allowed them to build a new statistical model that could more accurately project the trajectory of population growth in ancient Rome. Since the ancient coin hoard information was available and coin hoards from later historical periods had already been shown to reflect periods of instability, it was the ideal data for them to use as the foundation for their investigation.
Ancient Romans were interested in their population size too, and actually compiled census statistics, but Professor Scheidel explains that opaque information about census methodology is precisely why the population count has been debated among historians. A census counting only adult males was conducted every five years during much of the republican era (most regularly in the third and second centuries BC.) The census polls continued into the early Imperial era (in the late first century BC and early first century AD) but documentation about who was counted in these later polls is lacking. Without documentation historians are left with two different ways of interpreting the data; one resulting in a significantly higher population count than the other. Scholars dispute these two possible counts at the beginning of the imperial era. The conservative count assumes that the census included all citizens, and places the population at around 6 million. The high count assumes that the census only counted adult males, following the pattern of the republican era. This count would raise the population in Italy to between 15 and 20 million people, because for every man they would estimate two additional family members.
Documents about the census do not clarify if women and children were counted. Nor is it known how fully people who lived further from the central city were included. Even the purpose of the census remains unclear.
“It can serve military purposes, if you count all men of military age. It can be used for taxation, if you count all citizens, or for political purposes, if you count everyone who could vote, which were adult men. We don’t know,” says Professor Scheidel.
Scheidel and Turchin decided that mathematics might hold the key to settling the imperial era debate, and this is where the coins fit into the equation.
The number of coin hoards has an inverse relationship with population growth. Citizens tend to stash their money away in times of warfare and instability. If the coin hoards remained buried, it is
likely that their owners were killed or disabled before they could retrieve them. A high number of these coin hoards suggests strife and population decline. This idea is one that has been tested and confirmed when applied to other places and periods in history.
Starting with the basic exponential model of population growth, Scheidel and Turchin applied a parameter meant to depict the effect of coin hoards.
“We took data from the period before 100 BC when everyone agrees that what was counted in the census then was only adult males. There is no argument about that,” says Turchin. “We tried different parameters until we found the right combination that gave us the smallest difference from the actual data.”
The census data from the republican era revealed the relationship between coin hoards and population. Using this information, they then predicted the trajectory for the population of the imperial era, when the Roman approach to the census was unclear.
The results were remarkably in line with the low count theory, suggesting that women and children were indeed counted in the census. This method represents a shift from the way the census was conducted during the republican era. The low population count also confirms the prevalence of warfare in the early Roman Empire.
Scheidel and Truchin agree that they wouldn’t have been able to conduct this sort of groundbreaking research within their respective disciplines. This study has just affirmed each of the professor’s strong feelings about the importance of combining specialized skills. The project also highlighted the fact that there’s much to be gained by fostering more interdisciplinary opportunities.
“This is a rare opportunity in both history and science,” says Scheidel. “It doesn’t happen very often. Scientists are busy with studies in the lab, and humanists don’t usually seek out the scientists.”
Interdisciplinary collaborations are common in the sciences, but much less so in the humanities. Professor Scheidel notes that the traditionally organized structure of history research does not typically lend itself to interdisciplinary studies, but he explains, there are many natural opportunities for cross-disciplinary investigation.
“History is incredibly multi-faceted. A historian must understand everything from climate, to demography, from economics, to sociology.” Scheidel continues. “It is impossible for anyone to become an expert in all of these fields, yet all aspects are needed to create a comprehensive historical picture.”
Professor Turchin acknowledges parallel opportunities in his scientific work, “I have always been interested in demography, but my statistical experience is limited.” Turchin says. “To tease out meaningful information and not just assumptions, you need formal statistics.”
He adds, “Interdisciplinary research is really the way to go. Even though everyone says that, it’s really important. Most administrators really don’t understand it. Everyone is still sitting in his or her separate department with little communication. It really takes individuals like Walter and me getting together on our own.”
Because the project so clearly illustrates the benefits of collaboration, Scheidel is hopeful that it will encourage other scholars to seek out inter-disciplinary opportunities.