The last installment in this series (first one here) adopts a more critical stance towards the article by McConnell et al., Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity. As I said in the second post, the only quantitative part of this study was coming up with the lead pollution curve, while all the comparisons between the lead curve and historical events in Rome and elsewhere in the Western Mediterranean were qualitative (with silver content of the Roman denarius the only exception). Here’s the relevant figure:
Figure 3 of McConnell et al. 2018
The problem with an “eyeball” comparison between a quantitative curve and a set of qualitative events is the danger of confirmation bias. Or put simply, cherry-picking of data. It’s way too easy to find an explanation for any uptick and down-tick in the curve when one has a large list of events to choose from. McConnell et al. appear to be guilty of this.
One of the major messages of the article is that wars in the Western Mediterranean, and especially those affecting the Iberian Peninsula, have a depressing effect on silver production (which lead emissions proxy). But the authors also use the opposite effect to explain one of the upticks:
Longer-term declines possibly were linked to disincentives to investment in war-torn regions. For example, lead emissions dropped notably at the outbreak of the first Punic War (264–241 BCE) but rose in the later years as Carthage increased its minting of silver coin to pay mercenaries.
What we need is an objective quantitative method to test the hypothesis of a negative correlation between silver production and warfare. Fortunately, there is such a proxy.
A hoard of Roman coins
I have literally tons of data on the incidence of coin hoards. It turns out that the overwhelming majority of the hoards we find in modern times are “emergency hoards.” These are buried stores of wealth that people hide during times of danger. And then if something terrible happened to them–they are killed, or enslaved, or driven into exile–the hoards are not recovered by the original owners. Thus, the fluctuations in the frequency of coin hoards per decade provide us with a very useful proxy for the intensity of warfare.
Some years ago Walter Scheidel and I published an article (also in PNAS) which capitalized on this relationship to resolve a long-standing debate in Roman demography. Here I plot the data from that article on coin hoard frequency and and index of war intensity in Italy, derived from textual sources:
The three periods of intense warfare that are prominent in this figure are the Second Punic War and the two rounds of civil wars during the crisis of the Roman Republic (for details, see Chapter 6 of Secular Cycles). So, what happens when we compare the hoard index of war intensity with the lead emissions index? Here’s what:
A correlation coefficient between these two curves, equaling a measly -0.09, confirms that there is no apparent relationship here.
Could it be because the hoards come from Italy, while the hypothesis is that it is warfare in Spain that should depress silver production? We are fortunate to have an Inventory of Greek Coin Hoards that covers all of Mediterranean, and beyond, published by Margaret Thompson and colleagues in 1973. Focusing on the Western Mediterranean, we see the following pattern:
These curves decline shortly after 200 BCE, because the Roman denarius becomes the main means of exchange in the Western Mediterranean (note that Italian Hoards above are largely based on the silver denarius, with a few bronze coin hoards early in the sequence).
Let’s compare the total hoards in West Mediterranean and just hoards in Spain to the lead emissions curve:
The correlation coefficients between the lead curve and either of the hoard curves is actually slightly positive. Thus, I conclude that the hypothesis of McConnell and co-authors, that warfare corresponds to downturns in lead emissions, is not supported.
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