Cryptomnesia occurs when a forgotten memory returns without it being recognized as such by the subject, who believes it is something new and original. It is a memory bias whereby a person may falsely recall generating a thought, an idea, a tune, a name, or a joke; they are not deliberately engaging in plagiarism, but are experiencing a memory as if it were a new inspiration.

Cryptomnesia was first documented in 1874. The earliest case involved medium Stainton Moses, who thought he was communicating with spirits but unknowingly repeated details he had previously read in a newspaper. The term was coined by psychiatrist Théodore Flournoy while studying medium Hélène Smith, highlighting how forgotten memories can resurface distorted by imagination. Psychologists like Carl Jung, Sigmund Freud, and Jacques Lacan further explored the concept, linking it to subconscious memory retrieval, creativity, and self-misrecognition. Jung analyzed Friedrich Nietzsche’s writings, suggesting his use of previously encountered material was likely unintentional.

Experimental research has demonstrated cryptomnesia’s prevalence. In one of the first studies, participants took turns generating examples within categories and were later asked to produce new, unique ones. Between 3% and 9% inadvertently repeated others’ ideas or misattributed them to themselves, a finding replicated in word puzzles, brainstorming sessions, and similar tasks. Cryptomnesia is more likely when cognitive load impairs source monitoring — people are less able to recall where an idea originated and may unconsciously claim it as their own. Jung noted that this process often drives creativity: authors may write something they believe to be original, only to later discover its similarity to previously encountered works.

Numerous famous cases illustrate cryptomnesia’s impact. Nietzsche unknowingly reproduced passages from a book he had read in his youth, while Lord Byron’s Manfred showed striking parallels to Goethe’s Faust, which Byron claimed never to have read. Helen Keller inadvertently borrowed from a story read to her years earlier, leading to accusations of plagiarism that deeply affected her. Musicians like George Harrison faced legal consequences for subconscious copying, as seen in the “My Sweet Lord” copyright case, while Aerosmith’s Steven Tyler once failed to recognize his own band’s song on the radio. Authors such as Robert Louis Stevenson, Colleen McCullough, and Umberto Eco have all reported similar experiences.

Cryptomnesia was first documented in 1874, involving the medium Stainton Moses, who during a séance believed himself to be in spiritual contact with two brothers from India who had recently been killed. Despite the apparent communication, he was unable to ascertain any details which had not already been given in newspaper coverage of the story the week before. Researchers concluded that Moses had read the story but forgotten that he had read it, instead mistaking the partial memory for a message from the spirit world.

The word was first used by the psychiatrist Théodore Flournoy, in reference to the case of medium Hélène Smith (Catherine-Élise Müller) to suggest the high incidence in psychism of "latent memories on the part of the medium that come out, sometimes greatly disfigured by a subliminal work of imagination or reasoning, as so often happens in our ordinary dreams."

Carl Gustav Jung treated the subject in his thesis "On the Psychology and Pathology of So-Called Occult Phenomena" (1902), and in an article, "Cryptomnesia" (1905), suggested the phenomenon in Friedrich Nietzsche's Thus Spoke Zarathustra. The idea was studied or mentioned by Géza Dukes, Sándor Ferenczi and Wilhelm Stekel as well as by Sigmund Freud in speaking of the originality of his inventions. Jacques Lacan illustrated the adaptability of the concept in his formulation of the ego ideal (the "ego" as Other) in refashioning the "case" of Marguerite Pantaine (Case of Aimée). Her experiences of self-"misrecognition" provided a structure for Lacan's key subsequent theories of The Symbolic and the mirror stage.

In the first empirical study of cryptomnesia, people in a group took turns generating category examples (e.g., kinds of birds: parrot, canary, etc.). They were later asked to create new exemplars in the same categories that were not previously produced, and also to recall which words they had personally generated. People inadvertently plagiarized about 3–9% of the time either by regenerating another person's thought or falsely recalling someone's thought as their own. Similar effects have been replicated using other tasks such as word search puzzles and in brainstorming sessions.