Delusional Disorder is one of the category of psychotic disorders. Not surprisingly, it is marked by the prevalence of delusions. Delusions may be non-bizarre or bizarre. Delusions in general are persistent, unyielding beliefs that are not true. These beliefs must persist for more than a month to be considered delusional, and must not be attributable to another disorder such as Schizophrenia, symptoms of which include delusions. Beliefs must also not be attributable to substance use. Persons with Delusional Disorder may function fairly well in society, hold a job, etc. Treatment includes medication and psychotherapy.
Non-bizarre delusions are false, persisting beliefs that while in some cases may be possible, are not true. A common example would be that their significant other is cheating. While possible, to meet the criteria, the individual would have investigated, and repeatedly rechecked the possibility, found it to be untrue, yet persist in the belief. A belief that he/she has a serious health condition that has been definitively ruled out is another common example.
Bizarre delusions are, again not surprisingly, beyond the realm of possibility in the eyes of the vast majority. One example of a bizarre delusion cited in “Psych Central” is that a stranger has removed all their internal organs, closing the wound without leaving a scar.
Delusions may also be categorized based on the nature of the belief. Likely the more well-known delusions include those of grandeur, in which the person believes themselves to be of grossly inflated power, notoriety, etc. (royalty, for example) as well as those of persecution in which the person believes that someone or some group is plotting against them. Other types include somatic delusions, in which he or she would focus on a health issue as described above. I erotomanic delusions, he/she believes someone of higher social status to be in love with them. The example above regarding the cheating lover is a jealous type of delusion. There are also mixed types (combination of types) and unspecified.
Relevance to National Paranormal Society is transparent. A person who lives with Delusional Disorder truly believes their delusion. If this belief is that they are being persecuted by a spirit for example, which as an investigator I personally have encountered, it is extremely difficult to convince this person, despite a lack of evidence, that this is not happening to them. In the case I encountered, the client continues to request I re-investigate despite having done so numerous times with no findings.
A person may claim that they were abducted by aliens and used for experimentation. To a growing number of people, this is in the realm of possibility. In these cases, it may be necessary to gain as much knowledge of the client as possible before drawing any conclusions.