A delusion is a fixed belief that is either false, fanciful, or derived from deception. In psychiatry, it is defined to be a belief that is pathological (the result of an illness or illness process) and is held despite evidence to the contrary. As a pathology, it is distinct from a belief based on false or incomplete information, dogma, stupidity, apperception, illusion, or other effects of perception.
Delusions typically occur in the context of neurological or mental illness, although they are not tied to any particular disease and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders and particularly in schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.
Although non-specific concepts of madness have been around for several thousand years, the psychiatrist and philosopher Karl Jaspers was the first to define the three main criteria for a belief to be considered delusional in his 1917 book General Psychopathology. These criteria are:
- certainty (held with absolute conviction)
- incorrigibility (not changeable by compelling counterargument or proof to the contrary)
- impossibility or falsity of content (implausible, bizarre or patently untrue)
These criteria still continue in modern psychiatric diagnosis. The most recent Diagnostic and Statistical Manual of Mental Disorders defines a delusion as:
There is controversy over this definition, as 'despite what almost everybody else believes' implies that a person who believes something most others do not is a candidate for delusional thought. Furthermore, while the above three criteria are usually attributed to Jaspers, he himself described them as only 'vague' and merely 'external'. He also wrote that, since the genuine or 'internal' 'criteria for delusion proper lie in the primary experience of delusion and in the change of the personality [and not in the above three loosely descriptive criteria], we can see that a delusion may be correct in content without ceasing to be a delusion, for instance - that there is a world-war.'.
Furthermore, when a false belief involves a value judgment, it is only considered as a delusion if it is so extreme as to defy credibility. Since the delusional conviction occurs on a continuum, it can be inferred from an individual's behavior many times. A delusion and an overvalued idea tend to confuse. The latter implies that the individual has an unreasonable belief or idea but does not hold it as firmly as when a delusion takes place.
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