1863 – Charles Astor Bristed's Letter: The earliest known usage of "conspiracy theory" appeared in a letter by American scholar Charles Astor Bristed, published in The New York Times on January 11, 1863. In his letter, Bristed addressed claims that British aristocrats were intentionally weakening the United States during the American Civil War to advance their financial interests. https://en.wikipedia.org/wiki/Charles_Astor_Bristed
1870s-1890s: The phrase appeared sporadically in historical and political writings, often referring to explanations based on secret plots.
1870 – The Journal of Mental Science: An 1870 issue of The Journal of Mental Science included a report mentioning the term "conspiracy theory" in the context of psychological discussions.
1902 – The American Historical Review (Vol. 8) criticized a simplistic interpretation of history, stating: "A 'conspiracy theory' of history seems to be the last resort of those who have lost faith in the intelligence of their fellow men."
1909 – The American Historical Review: A 1909 article in The American Historical Review used the term "conspiracy theory" while discussing historical interpretations, indicating its presence in academic discourse. https://en.wikipedia.org/wiki/Conspiracy_theory
1914 – The New York Times referenced a conspiracy theory about political events in Europe leading up to World War I.
1920 – The term appeared in discussions of the Bolshevik Revolution, often referring to fears about communist infiltration.
1920 – The New York Times published an article referencing "conspiracy theory" in the context of European political events.
1938 – The American Sociological Review used conspiracy theory in discussing economic and political paranoia: "The conspiracy theory of the depression persists despite evidence to the contrary."
1940s – Historian Charles A. Beard discussed a "conspiracy theory" of how U.S. entry into World War II was orchestrated. Pearl Harbor and the “Back Door to War” Theory Facts | Britannica
1945 – Philosopher Karl Popper, in The Open Society and Its Enemies, described the conspiracy theory of society: Arguing that many people mistakenly believe history is driven by secret plots rather than complex social forces. The Open Society and Its Enemies - Wikipedia
1956 – Richard Hofstadter, a historian, examined conspiracy theories in The Age of Reform, linking them to American populist movements. Richard Hofstadter: Anti-Intellectualism in American Life, The Paranoid Style in American Politics, Uncollected Essays 1956–1965 - TheHumanist.com
These examples confirm that the term was widely used before the CIA’s 1967 memo. The agency may have influenced its modern usage, but it did not create it. The claim that the CIA "invented" the term conspiracy theory after the Warren Commission is a common misconception.