Cryp­tom­ne­sia, accord­ing to Wikipedia, is “a mem­ory bias whereby a per­son falsely recalls gen­er­at­ing a thought, an idea, a song, or a joke, when the thought was actu­ally gen­er­ated by some­one else”.

Newsweek has an arti­cle dis­cussing this phe­nom­e­non; includ­ing what appear to be gen­uine cases of cryp­tom­ne­sia and the novel tests being con­ducted by psy­chol­o­gists to uncover them.

When peo­ple engage in cre­ative activ­ity, they are so involved in gen­er­at­ing or com­ing up with some­thing new or novel that they fail to pro­tect against what they pre­vi­ously experienced. […]

It’s eas­ier to remem­ber infor­ma­tion than it is to remem­ber its source. Under the right con­di­tions, this quirk can even evoke false memories. […]

But mis­at­tribut­ing mem­o­ries from one source to another, whether from imag­i­na­tion to real­ity or from a friend to one­self, is only one of the psy­cho­log­i­cal quirks behind uncon­scious pla­gia­rism. Another is implicit mem­ory, which Dan Schac­ter, a psy­chol­o­gist at Har­vard, called, “the fact that we can some­times remem­ber infor­ma­tion with­out know­ing that we’re remem­ber­ing it.”

All of the famous cases of cryp­tom­ne­sia are men­tioned (George Har­ri­son, Niet­zsche), but one: Richard Nixon’s wartime expe­ri­ences that were later traced to Hol­ly­wood movies.

via Mind Hacks