Coins buried by anxious Italians in the first century B.C. can be used to track the ups and downs of the Roman population during periods of civil war and violence.

In times of instability in the ancient world, people stashed their cash and if they got killed or displaced, they didn't come back for their Geld. Thus, large numbers of coin hoards are a good quantitative indicator of population decline, two researchers argue in in the *Proceedings of the National Academy of Science *Monday.

And it turns out that during the periods we know to have been violent — the Second Punic War, the Social War and various civil wars — hoarding behavior soars, providing a statistical peek into the life of your average Italian 2,000 years ago.

"During that period, we have a good literary record. Caesar died because he was killed by assassins," said Peter Turchin, a population ecologist at the University of Connecticut. "We don't know what happened to common people. [Coin hoards] tell us what happened to common people."

The new work could help settle a long-standing historical debate about the Roman population. Census figures from the end of the second century B.C. show a population of adult males of around 400,000. Then, the record goes blank, and census figures from around a hundred years later show a population of 4 to 5 million. Some of the population explosion is explained by the extension of Roman citizenship to various groups, but far from all of it. From this evidence, a group of historians known as the "high-count" hypothesizers have argued there was excellent population growth during that period.

Another group of historians had an alternate explanation for the appearance of population growth. They figured that the later census takers had started to count women and children, rather than just adult males, as part of the official population stats.

Without more information about the ancient world, it was difficult to settle the argument one way or another. Turchin, though, knew from previous work that warfare and instability don't tend to deliver robust population growth in societies.

"Population growth and instability are negatively correlated," Turchin said.

He teamed up with Walter Scheidel, a Stanford classicist and historian, to create a model of population growth and decline based on coin-hoard data gathered over the years. Their model's predictions match well with the early census data. The population projections they derive make the "high-count" hypothesis "highly implausible," they argue.

That has important implications for Roman history, but not too much history will actually be rewritten. It was the high-counters who were hoping to revise the Italian population up from what earlier historians had assumed.

Image: A hoard of Roman coins from the Bristol Museum.

flickr/Synwell Liberation Front.

See Also:

WiSci 2.0: Alexis Madrigal's Twitter, Google Reader feed, and green tech history research site; Wired Science on Twitter and Facebook.**