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- A delusion is a fixed, false, and idiosyncratic belief and is one of the trilogy of psychotic symptoms: hallucinations, delusions and thought disorder.
- "Fixed" refers to the strength of the patient’s belief. The patient is certain, and not persuaded by any arguments to the contrary.
- "False" relates to the veracity of the patient’s belief.
- "Idiosyncratic" means the belief is characteristic of the individual patient.
- Beliefs about which the truth cannot be known with reasonable certainty, such as cultural or religious beliefs should not be classified as delusions without careful consideration.
- Beliefs shared by a group of people, for example, a belief in UFOs, are not delusions.
- Several of Schneider’s first rank symptoms (FRS) of schizophrenia are delusions. These include:
- Delusions of control or being controlled
- Thought withdrawal
- Thought insertion
- Mind reading
- Delusional perception (ordinary perception given a bizarre interpretation)
- Delusions can be divided by theme (e.g. persecutory, grandiose), plausibility (bizarre, non-bizarre), relationship to mood (congruent or non-congruent) or along other dimensions.
- Most patients with delusions have more than one.
- Delusions and hallucinations frequently co-occur.
- The time course of delusions is highly variable and depends on the underlying diagnosis.
- An "overvalued idea" is a belief which narrowly fails to satisfy the definition of a delusion. One example would be an anorexic patient’s belief that he or she is fat.
- "... madmen put wrong ideas together, and so make wrong propositions, but argue and reason right from them; but idiots make very few or no propositions, and reason scarce at all." John Locke