The world is currently gasping for water. Australia is quickly turning into a giant dust bowl, wildfires have been ripping through Greece, and Japan’s heatwave is causing unprecedented death and discomfort. If you were a dictator, you’d be feeling pretty damn worried right now.

Canadian economic historians from Brock University and St Francis Xavier University have looked at the political assassinations of the ancient Roman Empire and attempted to cook up an equation that can help to explain when and why they occurred. Their findings are published in Economic Letters. 

Bad decisions and bad friends certainly played a role, however, rain is the decisive factor, it seems.

Being a Roman leader was a very dangerous job, in general. The Roman Empire, from 27 BCE to 476 CE, had a total of 82 emperors, 20 percent of whom were assassinated or murdered in a politically motivated attack.

The years of 235 CE to 285 CE were bloody in the northeastern empire, even by Roman standards. Over the course of these 50 years, 14 out of 26 emperors were assassinated.

The factor that appears to be consistent throughout these assassinations is that the previous year hadn’t seen much rain. In fact, a significant reduction in rainfall would increase the likelihood of an assassination by 11 percent. The researchers gathered their data from another study that collected information from over 7,000 rainfall-sensitive oak tree rings across France and eastern Germany.

The Roman economy was largely agricultural, meaning a drought could easily lead to food shortages. Soldiers, especially in the northern empire, were particularly reliant on local food sources and would be hit the hardest by starvation. A hungry soldier is an unhappy soldier, so this could lead to a massively disgruntled military. If there’s one rule for dictators, it’s don’t annoy your military, as they are perhaps your strongest pillar of power.

“Our chain of causation is as follows. When there's less rainfall, there are lower crop yields. This, in turn, causes Roman frontier troops to starve, which increases the risks of mutinies. Such troop discontent can, in turn, lead to assassinations,” study author Cornelius Christian, of Brock University, told IFLScience. "We find evidence consistent with this."

“Systemic reviews suggest that drought increases the risk of conflict, even in modern times,” Christian added. "A striking historical example is the French Revolution: a paper by Maria Waldinger of the London School of Economics shows that, across France, drought and crop failures predict more peasant uprisings in 1789. In more modern times, drought in India predicts Naxalite rebel violence."

Emperors, dictators, autocrats, and despots: you better start praying for rain.