Delusion refers to a strongly held belief despite evidence that the belief is false.

Delusion usually occurs as a result of a neurological problem or a mental illness. However, delusion is not associated with any one disease in particular and has been found to manifest as a feature of various different physical and mental illnesses. Delusion is a typical clinical feature in psychotic disorders such as schizophrenia, bipolar disorder, and paraphrenia.

According to the most recent version of the Diagnostic and Statistical Manual of Mental Disorders (DSM IV), delusion is defined as: “A false belief based on incorrect inference about external reality that is firmly sustained despite what almost everyone else believes and despite what constitutes the inconvertible and obvious proof or evidence to the contrary.”

This definition does not extend to beliefs that are generally accepted by members of the person’s culture, as part of their faith, for example.

History of delusion

Psychiatrist and philosopher Karl Jaspers first described the three main criteria that need to be met for a belief to be diagnosed as delusional. These were outlined in his 1917 work called “General Psychopathology” and include the following:

• The false belief is held with absolute certainty or conviction
• The belief remains unchanged despite proof that it is not true
• The belief is false, implausible, bizarre or impossible

An example of a bizarre delusion would be a belief that is strange and implausible such as a person believing aliens have removed part of their brain. An example of a non-bizarre delusion would refer to something that could potentially occur such as the person being poisoned or followed.

For an individual to be diagnosed as delusional, the belief cannot be occurring as a result of using drugs or a general medication and the person must be free of any history of schizophrenia.