DEFINITION

• 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[1]