Rainfall, military food shortages and the assassination of Roman emperors

Dictators maintain control of their country by maintaining control of the military. If they lose control of the military, they often lose control of their country, or worse. This 2018 article by Cornelius Christian (Brock University) and Liam Elbourne (St. Francis Xavier University), published in the journal Economics Letters (sorry, I don't see an ungated version online), looks at cases where Roman emperors lost control, and their lives (through assassination).

Christian and Elbourne first note some key facts drawn from past research:

(1) The Roman economy was largely agricultural, depending on rainfed agriculture (Harper, 2017).

(2) The bulk of the Roman army was stationed along the Western frontier, and relied heavily on local food sources (Roth, 1998; Elton, 1996)...

(3) Food transport, in Ancient Rome, was very slow (Terpstra, 2013).

Christian and Elbourne then infer that shocks to military food supply, arising from low rainfall in the frontier areas like Germania, would reduce support for the emperor, increasing the chances that the emperor is assassinated. They use data covering the period from 27 BCE to 476 CE, which covers the period from the beginning of the reign of Augustus to the fall of the Western Roman Empire. Over that period, the relationship between rainfall and the number of assassinations is captured in Figure 1 from the article:

If you squint really hard, you can probably see that there are more assassinations in times when rainfall is lower. This is most apparent in the middle of the study period, between 200 and 300 CE, when there was a century of relatively low rainfall and many assassinations. Turning to their quantitative results, Christian and Elbourne find that:

...a standard deviation decline in rainfall causes an 11.6% standard deviation increase in assassination probability... [and] a standard deviation drop in rainfall causes a 13.4% standard deviation increase in total assassinations.

Now, of course this paper doesn't definitively establish a causal relationship between rainfall and assassinations. However, it is interesting that when, instead of using rainfall from the year before, Christian and Elbourne use rainfall from the following year, the correlation with assassinations becomes statistically insignificant. They also find that rainfall is a statistically significant predictor of frontier mutinies, which is consistent with rainfall increasing disquiet in the military.

However, the parts of the story included in this article don't quite feel like they add up to a coherent whole to me. The correlations are strong, but not necessarily entirely convincing. If the mechanism through which negative rainfall shocks increase emperor assassinations is through declining military support, why don't Christian and Elbourne use an instrumental variables approach, with rainfall as an instrument for frontier mutinies, and frontier mutinies as an endogenous variable explaining assassinations? Surely they tried this approach, so there being no mention of it in the paper is perhaps a signal to us that the approach doesn't work. In that case, we should be a little sceptical of the results.

Nevertheless, it is an interesting paper, and I'm sure there will be more work in this area. As attributed to both Napoleon Bonaparte and Frederick the Great, "an army marches on its stomach". When the stomach is not full, I guess the army stops marching, and eventually turns on its leaders.