Monday, October 12, 2009

Answering ancient history questions with an equation   posted by Razib @ 10/12/2009 04:03:00 PM

Peter Turchin is at it again, Coin hoards speak of population declines in Ancient Rome (ungated version):
In times of violence, people tend to hide their valuables, which are later recovered unless the owners had been killed or driven away. Thus, the temporal distribution of unrecovered coin hoards is an excellent proxy for the intensity of internal warfare. We use this relationship to resolve a long-standing controversy in Roman history. Depending on who was counted in the early Imperial censuses (adult males or the entire citizenry including women and minors), the Roman citizen population of Italy either declined, or more than doubled, during the first century BCE. This period was characterized by a series of civil wars, and historical evidence indicates that high levels of sociopolitical instability are associated with demographic contractions. We fitted a simple model quantifying the effect of instability (proxied by hoard frequency) on population dynamics to the data before 100 BCE. The model predicts declining population after 100 BCE. This suggests that the vigorous growth scenario is highly implausible.

The figure to the left shows the reasoning. A simple model which related population size (dependent) to coin hordes (independent) was fitted before 100 BCE. The correlation between coin hordes to population size and political stability are well attested for many polities. In any case, using the model and projecting outward with the coin hordes known for the early imperial period a theory which suggests that multiplicative increases in census size during the Julio-Claudian age were a function of a shift in the accounting method (instead of simply males, including the whole household) was supported. The high count was already implausible on other grounds (e.g., if true, that means that Italy never attained the early Roman imperial population size until the mid-19th century), but that the model fits so well with the lower projections previously offered by other scholars is very suggestive. Contra extreme subjectivists some models of the past are probably right, and some are probably wrong.

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