The first century B.C. was one of the most culturally rich in the history of the Roman Empire — the age of Cicero, Caesar and Virgil. But as much as historians know about the great figures of this period of Ancient Rome, they know very little about some basic facts, such as the population size of the late Roman Empire.

Now, a group of historians has used caches of buried coins to provide an answer to this question.

During the Republican period of Rome (about the fifth to the first centuries B.C), adult male citizens of Rome could be taxed and conscribed into the army and were also given the right to vote. To keep track of this section of the population (and their taxable assets), the Roman state conducted periodic censuses.

Unexplained increase

From the middle of the third to the end of the second centuries B.C., the adult male population was estimated to have risen from about 200,000 to 400,000 individuals. Those numbers, however, don’t jibe with censuses organized by the first emperor Augustus in the first centuries B.C. and A.D., which showed a population that had increased to about 4 million to 5 million males.

While the granting of citizenship to allies on the Italian peninsula accounts for some of the increase, there is still an estimated unexplained doubling or tripling in the Roman population before the first Augustan census in 28 B.C. Just what accounts for that increase is a matter of intense debate.

One camp explains the discrepancy by suggesting that the Empire began counting women and children in the census. While this would account for the relative increase, it would actually imply an overall decline in the population of Rome and there are no suggestions that the entire populace was counted in historical records.

On the other side of the debate are those who suggest that the population simply boomed. This would mean that the Roman Empire — and other premodern societies — achieved much higher economic output than previously supposed. It would mean that Roman history as it is now understood would have to be rewritten.

Coin clues

To help put an end to the debate, University of Connecticut theoretical biologist Peter Turchin and Stanford University ancient historian Walter Scheidel focused on the region's prevalence of coin hoards, those bundles of buried treasure that people hid to protect their savings during times of great violence and political strife. If the people who hid these bundles were killed or driven off, they wouldn't have been able to retrieve them, leaving them for archaeologists to find.

According to the researchers, mapping out the times when the coins were buried is a good indirect method for measuring the intensity of internal warfare and unrest, and therefore a key indicator of population demographics.

"Hoards are an excellent indicator of internal turmoil," Turchin said. "This is a general phenomenon, not just in Rome."

The model the two developed using the coin distribution and less controversial census data from earlier periods suggests that the population of Rome did in fact decline after 100 B.C., suggesting the census did likely begin to include women and children and that Ancient Rome wasn't substantially larger than historians had thought.

By these estimates the entire population of the Roman Empire — and not just its male population — was somewhere around 4 million to 5 million people by the end of the first century B.C.

"This may seem like an arcane dispute, but it isn't really because the difference is so large – 200 percent," Scheidel said. "This model is much more consistent with the low count. I'm not sure that by itself it has absolutely proven it, but it certainly provides additional evidence for the low-count hypothesis."

The findings are detailed in the Oct. 5 issue of the journal Proceedings of the National Academy of Sciences.