Conspiracy Theories

The Secret Origins of 'Conspiracy Theory'

A new book shows how a phrase made its way from the crime pages to our political arguments—and picked up a passel of meanings along the way.

Jesse Walker | 6.23.2026 8:00 AM

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(Princeton University Press/Carl Oglesby Papers)

The Hidden History of Conspiracy Theory, by Andrew McKenzie-McHarg, Princeton University Press, 674 pages, $42.95

"Homer conceived the power of the gods in such a way that whatever happened on the plain before Troy was only a reflection of the various conspiracies on Olympus," Karl Popper told an Oxford lecture hall in 1948. "The conspiracy theory of society," the philosopher added, simply replaces those deities with "sinister pressure groups."

That passage, which Popper then revised and incorporated into the second edition of The Open Society and Its Enemies, is sometimes cited as the moment the expression "conspiracy theories" entered the language. But in The Hidden History of Conspiracy Theory, the most original study of the subject to come along in years, the Australian historian Andrew McKenzie-McHarg argues that this is both too late and too early for that honor—too late because the phrase is actually much older than that, too early because Popper's term was subtly different from the one people usually use today.

In the 19th century, McKenzie-McHarg shows, the words "conspiracy theory" were typically deployed in a forensic context. In the aftermath of a crime, a detective or lawyer or reporter might speak in entirely neutral terms about a conspiracy theory of what happened, meaning that more than one perp might have been involved. When the phrase came up in a political context, it was still being used that way: After President James Garfield was shot, the St. Louis Daily Globe-Democrat used the term while editorializing about the speculations around the assassination. The plural form, "conspiracy theories," was vanishingly rare.

Writers sometimes referred to the conspiracy theory of something: the conspiracy theory of a murder, for example. So when Popper spoke about the more cosmic "conspiracy theory of society"—or when the legal scholar Franz Neumann critiqued the "conspiracy theory of history," or others in the mid-20th century spoke about the "conspiracy theory of government" or "conspiracy theory of politics"—they were just extending that linguistic formula.

Even before then, the phrase "conspiracy" was sometimes used to stigmatize ideas. (In 1935, a former congressman mocked the "conspiracy complex" of people who blame world events on hidden forces.) But the phrase "conspiracy theory" did not, by itself, carry a stigma. Neumann himself was involved with the Nuremberg prosecutions of Nazi war criminals, where jurists used the term in an entirely non-pejorative manner: They were discussing whether to charge the defendants with conspiracy.

The phrase "conspiracy theory," in the sense that people use it today, became popular in the wake of commentaries like Popper's and Neumann's. There is, naturally, a conspiracy theory about how that happened: A widespread rumor claims that the CIA deliberately injected the phrase into the media to discredit dissident investigators. McKenzie-McHarg debunks that dubious tale, instead tracing a gradual process where people dropped the last two words of "conspiracy theory of history" or "conspiracy theory of society" and began to refer to "conspiracy theories" as a genre of stories. (Indeed, Neumann did that himself at one point in his talk, a minor milestone in the evolution........

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