I think first we need to begin with understanding what “conspiracy theory” is. We talk about a number of different unrelated events under the umbrella “conspiracy theory” ranging from the attack on Pearl Harbor to the wreck at Roswell to 9/11 – wait! can’t do that yet – um… the death of Vince Foster even though these events have very different natures. Pagan, the only scholar I know of that addresses ancient “conspiracy theory,” offers one modern definition of “conspiracy theory” in her article “Toward a Model of Conspiracy Theory for Ancient Rome,” New German Critique, No. 103, (Winter, 2008) pp. 27-49:

According to Brian L. Keeley, "A conspiracy theory is a proposed explanation of some historical event (or events) in terms of the significant causal agency of a relatively small group of persons the conspirators acting in secret.... It proffers an explanation of the event in question. It proposes reasons why the event occurred."

The modern conspiracy theory is one that seeks an explanation for an event after the fact. It doesn’t cause the event or anything else. Pagan rejects this model for Rome, since, as she says, “Conspiracy demanded action, not theory.”

Instead, she sees Romans talking about conspiracies to justify taking action against someone – usually someone excluded from the normal power structure, i. e. women and slaves. To borrow her illustrative case drawn from Livy:

In 331 BCE Rome was suffering a terrible affliction of unknown cause; leading citizens were succumbing to a common illness, and practically all of them were dying. An unnamed ancilla (slave woman) approached the curule aedile, Quintus Fabius Maximus, and offered testimony if granted immunity (8.18.4). He immediately took the matter to the consuls and the Senate, both assented, and the ancilla led the authorities to women preparing poisons. The concoctions, along with about twenty women, were brought to the forum. Two of them, Cornelia and Sergia, asserted that the drugs were salutary. The ancilla challenged them to drink the cups, and after deliberating among the others, the women openly, in the sight of all, drank the poison and died (8.18.5-9). The slaves of other women betrayed their mistresses; in all, 170 were found guilty (8.18.10). According to Livy, this was the first time that a case of poisoning had ever been brought in Rome. The entire affair struck the Romans as unusual, and they attributed the matter to madness rather than criminal intent (8.18.11). So, based on a precedent from the olden times of the secession of the plebs, the people decided to elect a dictator to drive in a nail (claui figendi causa [8.18.12]). Once Quinctius performed this ritual, he resigned (8.18.13).

She notes that discovering and taking action against against the conspirators did not end the plague, although the poisoners proved themselves to be poisoners. The conspiracy, in one interpretation of the story, was in fact independent of the plague – there could be conspiracies anywhere all the time.

Pagan concludes, “Romans were more apt to think about the problems that conspiracy generated than to focus on conspiracy as a phenomenon per se.”

In her larger work, Conspiracy Narratives in Roman History (BMCR review here), Pagan looks at how Roman historians narrated conspiracies, and this might be a little more familiar in some sense: the historian uses secret information to explain an event. In some sense this is similar to the modern conspiracy theorist, who claims there is a secret reality, but the ancient narrator has access to this information – Cicero had informants who relayed Catiline’s plans, for instance, while for the modern conspiracy theorist, the secret is suppressed. The conspiracy theorist has a hypothesis that needs to be investigated, and some agent prevents this.

I’ve been scratching my head to see if, in spite of Pagan, I could find cases in which someone mentions something like a modern “conspiracy theory.” One thing occurs immediately – the death of Germanicus.

Germanicus was Tiberius’ nephew and very, very popular. When Augustus died, a number of legions mutinied against Tiberius in favor of Germanicus. Germanicus wanted nothing to do with that, but Tiberius was always suspicious of him. Tiberius sent Germanicus to the East to handle a crisis in Armenia, and a little later sent a guy Piso along as a subordinate. Tacitus says Piso was sure his job was to be a check on any ambition Germanicus might have, and that his wife Plancia had conversations with Livia, Tiberius’ mother and Germanicus’ grandmother, who out of “feminine jealously” hated Germanicus’ wife Agrippina (see women driving the conspiracy here?). So Piso gets to the East while Germanicus is moving around and goes about countermanding Germanicus’ orders, talking shit about him, and running a shoddy administration. Germanicus caught on to Piso and things became hostile between them. It got so bad that Piso was going to leave the East, but Germanicus got suddenly sick.

He died of this sickness, but before he died, according to Tacitus, they found inside the floor and walls of his house blood and human body parts, magic charms, and remains of sacrifices they identified as curses mean to kill him. Germanicus denounced Piso for his trouble making and implicated him in poisoning himself – though he specifically blamed his death on “a woman’s treachery.”

Agrippina went back to Rome (escaping Piso, who tried to stop her). Piso went to take charge of Syria, but he wasn’t recognized by Germanicus’ lieutenant. After a few engagements Piso found himself surrounded, and he surrendered on condition that he leave for Rome.

Meanwhile Agrippina was stubborn and popular enough that Tiberius referred the matter to the senate. These are the questions according to Tacitus the senate was to answer:

1. Did Piso merely celebrate Germanicus’ death, or did he murder him?

2. Did Piso incite troops to mutiny?

3. Did Piso bribe troops?

4. Did Piso make war for control of Syria?

5. Are the accusers liars?

Tacitus says Piso lost on every charge but poisoning. There’s a gap in the text, but we hear both Piso and Tiberius refused to produce some documents, and Plancia got a separate defense. The day after that Piso was found in his apartment with his throat cut. This was judged a suicide. About this Tacitus says:

I remember hearing older men speak of a document often seen in Piso’s hands. He never made it known. But his friends insisted that it contained a letter from Tiberius with instructions relating to Germanicus. If, they alleged, Piso had not been deceived by insincere promises from Sejanus [Tiberius’ right-hand man], he had intended to disclose this to the senate – thereby convicting the emperor. Moreover, his death, according to this story, was not by his own hand, but by an assassin’s.

Now, we happen to have an inscription that contains the senate’s official findings (here), and it’s useful for examining what Tacitus is doing with his narrative. I don’t want to go too far into that, only to point out the senate found Piso was a thoroughly despicable person who caused trouble corrupting troops, bribing them, and celebrating the death of Germanicus, and he was a coward for dodging the punishment he deserved. Unless I missed it (I reviewed it quickly), it doesn’t make a determination about the murder.

So that looks like a modern conspiracy theory in a way: Tiberius, the conspiracy theory goes, instructed Piso to scuttle Germanicus, and perhaps to kill him. When Piso was about to reveal this to the senate, Tiberius first dissuaded him through Sejanus with false promises, and then killed him. It explains, in retrospect, a lot of perhaps puzzling facts: Piso’s activity in the East, Tiberius’ apparent lack of sympathy for Agrippina, the mysterious document, and the “suicide,” and Tiberius can be seen as taking action to prevent the necessary evidence to prove it from coming to the attention of the public.

But look back at Pagan’s model of a Roman conspiracy theory. The “evidence” (the blood, sacrifices, etc) that revealed the “conspiracy” came to light before Germanicus died; it was a conspiracy involving a marginalized group (women: Plancia and Livia), and it caused (rather than explained) specific results – the actions of Piso, and resulted in the investigation in the senate. How the blood, etc. in Germanicus' house got there is never addressed.

So perhaps, even if we find narratives in the ancient world we recognize with the lens of modern conspiracy theory, the ancients themselves would have understood those same narratives in entirely different ways.

ALL THE EDITS: I gotta decide if I discuss authors and narrative in the past or the present tense. I couldn't keep it straight at all here.