Why Roman Emperors Were More Likely to Be Assassinated During Droughts

Great power often comes with great responsibility—and risk. Just ask the men who presided over the Roman Empire some 1,500 years ago: One-fifth of these emperors met violent ends at the hands of their subjects.

A new study published in Economics Letters offers a surprising explanation for Rome’s high rate of regicide. As Laura Geggel reports for Live Science, bouts of low rainfall resulted in inadequate harvests, leaving Roman soldiers malnourished and more inclined to mutiny.

“That mutiny, in turn, would collapse support for the emperor and make him more prone to assassination,” study co-author Cornelius Christian, an economist at Ontario’s Brock University, tells Geggel.

Christian and co-author Liam Elbourne of St. Francis Xavier University in Nova Scotia, Canada, arrived at this conclusion after comparing ancient climate data with statistics on military mutinies and emperor assassinations between 27 B.C.E. and 476 C.E.

Building on a 2011 study published in Science, the researchers tracked the level of seasonal precipitation present in ancient Gaul (now France) and Germany, regions which constituted the Roman frontier and were therefore heavily protected by military troops. According to The Economist, this data was collected via measurements of rainfall-sensitive oak-tree rings.

Christian and Elbourne realized that a 20 percent decrease in average annual rainfall led to an increase of 0.11 standard deviations in the likelihood of an emperor’s assassination the following year. The leaders most at risk were members of the Gordian dynasty, which lasted from 235 C.E. to 285 C.E. and saw 14 of 26 emperors assassinated. In addition to experiencing poor harvests and subsequently starving troops, The Economist notes that Gordian emperors faced plague, invasions and economic depression.

Vitellius, a Roman emperor assassinated in 69 C.E., serves as a prime example of the economists’ hypothesis. Appointed governor-general of Lower Germany by predecessor Emperor Galba, Vitellius was notorious for his gluttony and gambling addiction. Despite his inclinations toward vice, Donald L. Wasson writes for Ancient History Encyclopedia, the governor-general was well-liked and respected by the armies under his jurisdiction—especially since he tended to grant every favor asked of him.

Upon his ascension to the throne, however, Vitellius grew increasingly ruthless, allegedly killing or torturing subjects at “the slightest pretext” and further indulging his hedonistic lifestyle. Just months after becoming emperor, Vitellius was overthrown by Vespasian, the fourth and final man to rule over Rome in 69 C.E. The deposed emperor, Wasson reports, was captured by Vespasian’s men, and “while pleading for his life, he was dragged through the streets, tortured, killed … and thrown into the Tiber.”

According to Christian, the year of Vitellius’ death happened to be one of low rainfall on the Roman frontier. “Vitellius was an acclaimed emperor by his troops," Christian tells Geggel. "Unfortunately, low rainfall hit that year, and he was completely flabbergasted. His troops revolted, and eventually he was assassinated in Rome."

In an interview with Live Science, Brown University historian Jonathan Conant described the low rainfall hypothesis as “plausible.” He cautioned, however, that numerous factors were at play. Many political assassinations occurred during the third century C.E., a period of “massive inflation, disease outbreaks and external wars,” all of which contributed to widespread instability across the Roman Empire.

Still, Christian maintains that “usually there is a drought preceding the assassination of the emperor.”

“We're not trying to claim that rainfall is the only explanation for all these things,” he tells Geggel. “It's just one of many potential forcing variables that can cause this to happen.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's associate digital editor, history.