What is "true-believer syndrome"

n. A phenomenon in which witnesses to a staged event continue to believe in its authenticity even after it has been debunked.

True-believer syndrome is an informal or rhetorical term used by M. Lamar Keene in his 1976 book The Psychic Mafia. Keene used the term to refer to people who continued to believe in a paranormal event or phenomenon even after it had been proven to have been staged. Keene considered it to be a cognitive disorder, and regarded it as being a key factor in the success of many psychic mediums.

The term "true believer" was used earlier by Eric Hoffer in his 1951 book The True Believer to describe the psychological roots of fanatical groups.