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 song, or a joke, not deliberately engaging in plagiarism but rather experiencing a memory as if it were a new inspiration.
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 Nietzsche's Also Sprach 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.
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.
Research has distinguished between two kinds of cryptomnesia, though they are often studied together. The distinction between these two types of plagiarism is in the underlying memory bias responsible—specifically, is it the thought that is forgotten, or the thinker? The first type of bias is one of familiarity. The plagiarizer regenerates an idea that was presented earlier, but believes the idea to be an original creation. The idea that is reproduced could be another's idea, or one's own from a previous time. B. F. Skinner describes his own experience of self-plagiarism:
- "One of the most disheartening experiences of old age is discovering that a point you just made—so significant, so beautifully expressed—was made by you in something you published long ago."
The second type of cryptomnesia results from an error of authorship whereby the ideas of others are remembered as one's own. In this case, the plagiarizer correctly recognizes that the idea is from an earlier time, but falsely remembers having been the origin for the idea (or, having lost the specific memory of encountering it in print or conversation, assumes that it "came to" the plagiarizer as an original idea). Various terms have been coined to distinguish these two forms of plagiarism — occurrence forgetting vs. source forgetting and generation errors vs. recognition errors. The two types of cryptomnesia appear to be independent: no relationship has been found between error rates and the two types are precipitated by different causes.
Cryptomnesia is more likely to occur when the ability to properly monitor sources is impaired. For example, people are more likely to falsely claim ideas as their own when they were under high cognitive load at the time they first considered the idea. Plagiarism increases when people are away from the original source of the idea, and decreases when participants are specifically instructed to pay attention to the origin of their ideas. False claims are also more prevalent for ideas originally suggested by persons of the same sex, presumably because the perceptual similarity of the self to a same-sex person exacerbates source confusion. In other studies it has been found that the timing of the idea is also important: if another person produces an idea immediately before the self produces an idea, the other's idea is more likely to be claimed as one's own, ostensibly because the person is too busy preparing for their own turn to properly monitor source information.
As explained by Carl Jung, in Man and His Symbols, "An author may be writing steadily to a preconceived plan, working out an argument or developing the line of a story, when he suddenly runs off at a tangent. Perhaps a fresh idea has occurred to him, or a different image, or a whole new sub-plot. If you ask him what prompted the digression, he will not be able to tell you. He may not even have noticed the change, though he has now produced material that is entirely fresh and apparently unknown to him before. Yet it can sometimes be shown convincingly that what he has written bears a striking similarity to the work of another author — a work that he believes he has never seen."
- "The ability to reach a rich vein of such material [of the unconscious] and to translate it effectively into philosophy, literature, music or scientific discovery is one of the hallmarks of what is commonly called genius." — Carl Jung, Man and His Symbols.
- "We can find clear proof of this fact in the history of science itself. For example, the French mathematician Poincaré and the chemist Kekulé owed important scientific discoveries (as they themselves admit) to sudden pictorial 'revelations' from the unconscious. The so-called 'mystical' experience of the French philosopher Descartes involved a similar sudden revelation in which he saw in a flash the 'order of all sciences.' The British author Robert Louis Stevenson had spent years looking for a story that would fit his 'strong sense of man's double being,' when the plot of Dr. Jekyll and Mr. Hyde was suddenly revealed to him in a dream." — Carl Jung Man and His Symbols
Jorge Luis Borges's story, "Pierre Menard, Author of the Quixote," is a meta-fictive enactment of cryptomnesia. This work is written in the form of a review or literary critical piece about (the non-existent) Pierre Menard. It begins with a brief introduction and a listing of all of Menard's work:
Borges's "review" describes this 20th-century French writer (Menard) who has made an effort to go further than mere "translation" of Don Quixote, but to immerse himself so thoroughly as to be able to actually "re-create" it, line for line, in the original 16th century Spanish. Thus, Pierre Menard is often used to raise questions and discussion about the nature of accurate translation. Or, in this case, the hermeneutics of cryptomnesia.
Jung gives the following example in Man and His Symbols. Friedrich Nietzsche's book Thus Spoke Zarathustra includes an almost word for word account of an incident also included in a book published about 1835, half a century before Nietzsche wrote. This is considered to be neither purposeful plagiarism nor pure coincidence: Nietzsche's sister confirmed that he had indeed read the original account when he was 11 years old; and Nietzsche's youthful intellectual prowess, his later cognitive degeneration due to neurosyphilis, and his accompanying psychological deterioration (specifically, his increasing grandiosity as manifested in his later behavior and writings) together strengthen the likelihood that he happened to commit the passage to memory upon initially reading it and later, after having lost his memory of encountering it, assumed that his own mind had created it.
In some cases, the line between cryptomnesia and zeitgeist may be somewhat hazy. Readers of Lord Byron's closet drama Manfred noted a strong resemblance to Goethe's Faust. In a review published in 1820, Goethe wrote, "Byron's tragedy, Manfred, was to me a wonderful phenomenon, and one that closely touched me. This singular intellectual poet has taken my Faustus to himself, and extracted from it the strangest nourishment for his hypochondriac humour. He has made use of the impelling principles in his own way, for his own purposes, so that no one of them remains the same; and it is particularly on this account that I cannot enough admire his genius." Byron was apparently thankful for the compliment; however, he claimed that he had never read Faustus.
Helen Keller compromised her and her teacher's credibility with an incident of cryptomnesia which was misapprehended as plagiarism. The Frost King, which Keller wrote out of buried memories of a fairytale read to her four years previously, left Keller a nervous wreck, and unable to write fiction for the rest of her life.
...I am now upon a painful chapter. No doubt the parrot once belonged to Robinson Crusoe. No doubt the skeleton is conveyed from Poe. I think little of these, they are trifles and details; and no man can hope to have a monopoly of skeletons or make a corner in talking birds. The stockade, I am told, is from Masterman Ready. It may be, I care not a jot. These useful writers had fulfilled the poet's saying: departing, they had left behind them Footprints on the sands of time, Footprints which perhaps another — and I was the other! It is my debt to Washington Irving that exercises my conscience, and justly so, for I believe plagiarism was rarely carried farther. I chanced to pick up the Tales of a Traveller some years ago with a view to an anthology of prose narrative, and the book flew up and struck me: Billy Bones, his chest, the company in the parlour, the whole inner spirit, and a good deal of the material detail of my first chapters — all were there, all were the property of Washington Irving. But I had no guess of it then as I sat writing by the fireside, in what seemed the spring-tides of a somewhat pedestrian inspiration; nor yet day by day, after lunch, as I read aloud my morning's work to the family. It seemed to me original as sin; it seemed to belong to me like my right eye...
The precedent in United States copyright law, as of 1976, has been to treat alleged cryptomnesia no differently from deliberate plagiarism. The seminal case is Bright Tunes Music v. Harrisongs Music, where the publisher of "He's So Fine," written and composed by Ronald Mack, demonstrated to the court that George Harrison borrowed substantial portions of his song "My Sweet Lord" from "He's So Fine." The Court imposed damages despite a claim that the copying was subconscious. The ruling was upheld by the Second Circuit in ABKCO Music v. Harrisongs Music, and the case Three Boys Music v. Michael Bolton, upheld by the Ninth Circuit, affirmed the principle. (Losing his case embittered Harrison enough to write, compose, and perform "This Song," whose lyrics include the harsh line, "(It) don't infringe on anyone's copyright.")
In Interpretation and Overinterpretation, Umberto Eco describes the rediscovery of an antique book among his large collection, which was eerily similar to the pivotal object in his novel The Name of the Rose.
I had bought that book in my youth, skimmed through it, realized that it was exceptionally soiled, and put it somewhere and forgot it. But by a sort of internal camera I had photographed those pages, and for decades the image of those poisonous leaves lay in the most remote part of my soul, as in a grave, until the moment it emerged again (I do not know for what reason) and I believed I had invented it.
- Taylor, F..K. (1965).Cryptomnesia and plagiarism. British Journal of Psychiatry, 111, 1111–1118.
- Théodore Flournoy, From India to the Planet Mars: A Case of Multiple Personality with Imaginary Languages (1901/1994) quoted at www.answers.com/topic/cryptomnesia-psychoanalysis
- Jung, Carl Gustav. (1902). On the psychology and pathology of so-called occult phenomena. Coll. works, Vol. 1, London: Routledge and Kegan Paul.
- Jung, Carl Gustav. (1905). Cryptomnesia. Coll. works, Vol. 1, London: Routledge and Kegan Paul; United States Bollingen Foundation.
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- Marsh, R. L. & Landau, J. D. (1995). Item availability in cryptomnesia: Assessing its role in two paradigms of unconscious plagiarism. Journal of Experimental Psychology: Learning, Memory, and Cognition, 21, 1568–1582.
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- Skinner, B. F. (1983) Intellectual self-management in old age. American Psychologist, 38, 239–244.
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- Oppel, Frances (2005). Nietzsche On Gender. University of Virginia Press. p. 204. ISBN 0-8139-2320-4. "The book of adventure stories as "source" was discovered by Carl Jung, who describes his find, and his subsequent belief that Nietzsche had "crpytomnesia," the concealed recollection of a textual memory, to his seminar of fellow depth psychologists who gathered once a week between 1934 and 1939 to analyze Thus Spoke Zarathustra. The piece of text "secretly crept up and reproduced itself" in "Of Great Events" (Z 2). Jung recognized the story about seamen stopping on an island to hunt rabbits, having read it in his grandfather's library. He wrote Elisabeth Forster-Nietzsche, who confirmed that she and Nietzsche had read the same book in their grandfather's library, when Nietzsche was eleven. This, Jung informed his Zarathustra seminar, "shows how the unconscious layers of the mind work." He added, "the absolute parallel is of course formed by the rabbits" (1218)."
- Reprinted in Lord Byron: The Critical Heritage by Andrew Rutherford 
- Foer, Joshua. "No one has a photographic memory". Slate. Retrieved 11 March 2014.
- Stevenson, Robert Louis. "The Art of Writing". About.com:Classic Literature. Retrieved 26 July 2010.
- 420 F. Supp. 177 (S.D.N.Y. 1976), available at http://cip.law.ucla.edu/cases/case_brightharrisongs.html
- 722 F.2d 988, 221 U.S.P.Q. 490, available at http://digital-law-online.info/cases/221PQ490.htm
- 212 F.3d 477 (9th Cir. 2000), available at http://cip.law.ucla.edu/cases/case_threeboysbolton.html
- Umberto Eco; Stefan Collini; Richard Rorty, Jonathan Culler, and Christine Brooke-Rose (1992). Interpretation and overinterpretation. Cambridge University Press. p. 88. ISBN 978-0-521-42554-4. Retrieved 24 August 2011.