In other cases, the belief may be mistakenly assumed to be false by a doctor or psychiatrist assessing it, just because it seems to be unlikely, bizarre or held with excessive conviction. Psychiatrists rarely have the time or resources to check the validity of a persons claims leading to some true beliefs to be erroneously classified as delusional. This is known as the Martha Mitchell effect, after the wife of the attorney general who alleged that illegal activity was taking place in the White House. At the time, her claims were thought to be signs of mental illness, and only after the Watergate scandal broke was she proved right (and hence sane). Similar factors have led to criticisms of Jaspers definition of true delusions as being ultimately un-understandable. Critics (such as R. D. Laing) have argued that this leads to the diagnosis of delusions being based on the subjective understanding of a particular psychiatrist, who may not have access to all the information that might make a belief otherwise interpretable. R. D. Laings hypothesis has been applied to some forms of projective therapy to "fix" a delusional system so that it cannot be altered by the patient. Psychiatric researchers at Yale University, Ohio State University and the Community Mental Health Center of Middle Georgia have used novels and motion picture films as the focus. Texts, plots and cinematography are discussed and the delusions approached tangentially.
This use of fiction to decrease the malleability of a delusion was employed in a joint project by science-fiction author Philip Jose Farmer and Yale psychiatrist A. James Giannini. They wrote the novel Red Orcs Rage, which, recursively, deals with delusional adolescents who are treated with a form of projective therapy. In this novels fictional setting other novels written by Farmer are discussed and the characters are symbolically integrated into the delusions of fictional patients. This particular novel was then applied to real-life clinical settings.Another difficulty with the diagnosis of delusions is that almost all of these features can be found in "normal" beliefs. Many religious beliefs hold exactly the same features, yet are not universally considered delusional. For instance, if a person was holding a true belief then they will of course persist with it. This can cause the disorder to be misdiagnosed by psychiatrists. These factors have led the psychiatrist Anthony David to note that "there is no acceptable (rather than accepted) definition of a delusion." In practice, psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupying the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments. Joseph Pierre, M.D. states that one factor that helps differentiate delusions from other kinds of beliefs is that anomalous subjective experiences are often used to justify delusional beliefs.