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Coin hoards may reveal population histories

Oct. 5, 2009
Courtesy University of Connecticut
and World Science staff

Bur­ied stashes of coins can help re­veal the popula­t­ion his­to­ry of a giv­en time pe­ri­od, a new study sug­gests.

The research focused on the first cen­tu­ry BC in Italy, a cul­tur­ally a bril­liant age, un­equaled by any oth­er pe­ri­od in Ro­man his­to­ry. It was a time of Cic­e­ro, Cae­sar, Vir­gil, Hor­ace and oth­er ma­jor lit­er­ary fig­ures. 

Bund­les of bu­ried Ro­man coins in­di­cate the in­ten­sity of the re­gion's vio­lence and po­li­ti­cal strife, re­search­ers say. (© Ju­pi­ter Im­ages)
Yet some bas­ic fact­s—like the ap­prox­i­mate popula­t­ion size of the late Ro­man Re­pub­lic—re­main un­der in­tense de­bate. De­pend­ing on who his­to­ri­ans be­lieve was counted in the early Im­pe­ri­al cen­sus­es, the It­alian popula­t­ion ei­ther de­clined or more than dou­bled in that cen­tu­ry. 

If the high­er count is right, much of Ro­man his­to­ry would have to be re-written and it would have huge im­plica­t­ions on the pop­u­lar view of the eco­nom­ic po­ten­tial and so­cial struc­ture of an­cient Rome, ac­cord­ing to his­to­ri­an Wal­ter Schei­del of Stan­ford Un­ivers­ity in Cal­i­for­nia and the­o­ret­i­cal bi­ol­o­gist Pe­ter Turchin of the Un­ivers­ity of Con­nect­i­cut.

The two researchers tried to re­solve the de­bate by fo­cus­ing 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. The pair worked on the the­o­ry that more stashes means a drop­ping popula­t­ion, due to the great­er fre­quen­cy of vi­o­lence.

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

Turchin and Schei­del de­vel­oped a sim­ple math­e­mat­i­cal mod­el that used coin hoards to proj­ect popula­t­ion dy­nam­ics be­fore and af­ter 100 BC. The mod­el pre­dicts de­clin­ing popula­t­ion af­ter 100 BC and sug­gests the vig­or­ous popula­t­ion growth sce­nar­i­o of the “high count” is highly im­plau­si­ble, Schei­del said.

Turch­in’s and Schei­del’s mod­el was de­vel­oped us­ing cen­sus da­ta of the pe­ri­od be­fore 100 BC when Ro­man popula­t­ion his­to­ry is rel­a­tively un­con­tro­ver­sial. The mod­el’s tra­jec­to­ry suc­cess­fully cap­tured ma­jor de­mo­graph­ic trends dur­ing that pe­ri­od, the re­search­ers said. 

They then tested the mod­el us­ing hoard da­ta af­ter 100 BC and found the tra­jec­to­ry mir­rored the num­bers pos­tu­lat­ed by the low-count the­o­ry. “Judg­ing by the num­ber of hoards found dur­ing the first cen­tu­ry BC, this pe­ri­od was as ca­lam­i­tous as the war with Han­ni­bal,” a gen­er­al from North Af­ri­ca who wreaked hav­oc in It­aly, Turchin said. “Ac­tu­ally, it was even worse, be­cause there was not just one, but two large clumps of hoards.” 

Turchin and Schei­del are ad­vo­cat­ing great­er col­la­bora­t­ion be­tween schol­ars of the hu­man­i­ties and sci­en­tists. “The re­sults in this ar­ti­cle in­di­cate that a for­mal ap­proach com­bin­ing mod­eling with da­ta anal­y­sis can com­pen­sate for the scarcity of re­li­a­ble sta­tis­tics from pre-modern so­ci­eties,” said Turchin, who has coined a term for such col­la­bora­t­ions “Cliody­nam­ics” and has de­vot­ed a web­site to the new sci­ence: http://cliody­nam­ics.info. “I’m very much in fa­vor of such col­la­bora­t­ions,” Schei­del said.

The findings are published on­line this week in the re­search jour­nal Pro­ceed­ings of the Na­tion­al Acad­emy of Sci­ences.

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Buried stashes of coins can reveal, through their distribution, the population history of a given time period, new research contends. The first century BC in Italy was culturally a brilliant age, unequaled by any other period in Roman history. It was a time of Cicero, Caesar, Virgil, Horace and other major literary figures. Yet some basic facts—like the approximate population size of the late Roman Republic—remain under intense debate. Depending on who historians believe was counted in the early Imperial censuses, the Italian population either declined or more than doubled in that century. If the higher count is right, much of Roman history would have to be re-written and it would have huge implications on the popular view of the economic potential and social structure of ancient Rome, according to historian Walter Scheidel of Stanford University in California and theoretical biologist Peter Turchin of the University of Connecticut. In a study, the two tried to resolve the debate by focusing 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. The pair worked on the theory that more stashes means a dropping population, due to the greater frequency of violence. “Hoards are an excellent indicator of internal turmoil,” said Turchin. “This is a general phenomenon, not just in Rome.” Turchin and Scheidel developed a simple mathematical model that used coin hoards to project population dynamics before and after 100 BC. The model predicts declining population after 100 BC and suggests the vigorous population growth scenario of the “high count” is highly implausible, Scheidel said. Turchin’s and Scheidel’s model was developed using census data of the period before 100 BC when Roman population history is relatively uncontroversial. The model’s trajectory successfully captured major demographic trends during that period including the short-lived population increase before the Second Punic War, demographic population growth in the second century BC. They then tested the model using coin hoard data after 100 BC and found the trajectory mirrored the numbers postulated by adherents of the low-count theory. “Judging by the number of hoards found during the first century BC, this period was as calamitous as the war with Hannibal,” a general from North Africa who wreaked havoc in Italy, Turchin said. “Actually, it was even worse, because there was not just one, but two large clumps of hoards. It is very difficult to imagine how a population could grow during a period of such violence and the model provides a precise quantitative statement of this.” Turchin and Scheidel are advocating greater collaboration between scholars of the humanities and scientists. “The results in this article indicate that a formal approach combining modeling with data analysis can compensate for the scarcity of reliable statistics from pre-modern societies,” said Turchin, who has coined a term for such collaborations “Cliodynamics” and has devoted a website to the new science: http://cliodynamics.info. “I’m very much in favor of such collaborations,” Scheidel said.