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A delusion is commonly defined as a fixed false belief and is used in everyday language to describe a belief that is either false, fanciful or derived from deception. In psychiatry, the definition is necessarily more precise and implies that the belief is pathological (the result of an illness or illness process).

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.

[edit] Psychiatric definition

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 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 live on in modern psychiatric diagnosis. In the most recent Diagnostic and Statistical Manual of Mental Disorders, a delusion is defined as:

A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everybody else believes and despite what constitutes incontrovertible and obvious proof or evidence to the contrary. The belief is not one ordinarily accepted by other members of the person's culture or subculture (e.g., it is not an article of religious faith).

[edit] Diagnostic issues

The modern definition and Jaspers' original criteria have been criticised, as counter-examples can be shown for every defining feature.

Studies on psychiatric patients have shown that delusions can be seen to vary in intensity and conviction over time which suggests that certainty and incorrigibility are not necessary components of a delusional belief.^[1]

Delusions do not necessarily have to be false or 'incorrect inferences about external reality'.^[2] Some religious or spiritual beliefs (such as 'I believe in the existence of God') including those diagnosed as delusional, by their nature may not be falsifiable, and hence cannot be described as false or incorrect.^[3]

In other situations the delusion may turn out to be true belief.^[4] For example, delusional jealousy, where a person believes that their partner is being unfaithful (and may even follow them into the bathroom believing them to be seeing their lover even during the briefest of partings) may result in the faithful partner being driven to infidelity by the constant and unreasonable strain put on them by their delusional spouse. In this case the delusion does not cease to be a delusion because the content later turns out to be true.

In other cases, the delusion may be assumed to be false by a doctor or psychiatrist assessing the belief, 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 person’s claims leading to some true beliefs to be erroneously classified as delusional.^[5] 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's 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 which might make a belief otherwise interpretable.

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. Similarly, Thomas Kuhn argued in The Structure of Scientific Revolutions that scientists can hold strong beliefs in scientific theories despite considerable apparent discrepancies with experimental evidence.^[6]

These factors have led the psychiatrist Anthony David to note that "there is no acceptable (rather than accepted) definition of a delusion".^[7] In practice psychiatrists tend to diagnose a belief as delusional if it is either patently bizarre, causing significant distress, or excessively pre-occupies the patient, especially if the person is subsequently unswayed in belief by counter-evidence or reasonable arguments.