Delusions are a common symptom of many illnesses and mental abnormalities and the unique, defining feature of one type of psychiatric condition, the delusional disorders. As described in the American Psychiatric Association's diagnostic manual, these disorders consist of delusions persisting for at least a month with no known organic cause and no other obvious mental or emotional disorder or medical condition. The delusions are not bizarre, that is, not totally implausible by the standards of the culture.

The Five Types of Delusional Disorder
Five types of delusional disorder are listed in the manual. Most common are delusions of persecution -- the belief that you are under attack, often the target of a conspiracy. You are being harassed, threatened, cheated, spied on, ridiculed, followed, poisoned. Delusions of jealousy have a lover's infidelity as their theme. Erotomania (also known as de Clerembault's syndrome) is the delusion that someone is in love with you -- usually someone of higher social status, often a stranger, and sometimes a celebrity (see Harvard Mental Health Letter, June 1987). People with delusions of grandeur may believe that they are unrecognized geniuses, can banish poverty and ensure world peace, or have been chosen by God as messengers to humanity. The body is the center of somatic delusions: you believe that you are emitting a horrible smell or have an intestinal parasite, or that some body part is ugly or deformed or not functioning properly.

The reasoning of people with delusional disorders is not necessarily flawed in any obvious way, and the resulting system of beliefs may be elaborately plausible and even superficially convincing. They rarely have hallucinations, and their personalities do not change. Although the consequences of their delusions can be serious -- social isolation, rejection, stalking, harassment, and violence -- they usually come to the attention of lawyers or plastic surgeons before they seek the help of mental health professionals.

What is a Delusion?
Although we often speak as though everyone can recognize a pathological delusion, there is no simple definition or identifying sign. In the glossary of the APA's diagnostic manual, a delusion is defined as a false personal belief based on an incorrect inference about external reality, rejected by others in one's own culture, that is firmly maintained in contradiction to the beliefs of others and despite obvious and irrefutable proof or evidence to the contrary. Critics of this definition say that the reasoning involved in a delusion is not necessarily wrong, and the reality at which it is directed may be internal. Many ideas that turn out to be correct are first maintained despite nearly universal skepticism. In daily life we do not act like scientists, continually testing the correspondence of our thoughts with reality. Many beliefs -- most religious doctrines, for example -- cannot be tested at all. Many others are acquired by incorrect inferences, indoctrination, and custom. Delusions may be temporary rather than fixed, and many ordinary beliefs are stubbornly maintained despite contrary evidence. A delusion can even make itself come true; for example, a belief in malicious witchcraft may cause a depression or psychosis that ends in death.

Delusions vs. Obsessions
As the diagnostic manual acknowledges, a delusion may also be difficult to distinguish from an obsessional thought that is no longer resisted and becomes an "overvalued idea." In one study, nearly 50% of patients with obsessive-compulsive disorder were certain or nearly certain that if they did not perform their compulsive rituals, some harm would come to them. The line between obsession and delusion is also difficult to draw in body dysmorphic disorder, an irrational anxiety about imagined ugliness, and hypochondriasis, the interpretation of minor physical symptoms as signs of serious illness. In the end, social acceptability usually determines what is treated as a delusion rather than merely an idiosyncratic, unpersuasive, superstitious, or overvalued false idea.

Delusions arise from experiences, external or internal, that are felt to be unusual, significant, and urgently in need of explanation. A strange feeling, perception, or persistent thought occurs, and the person affected needs to make sense of it -- above all, to understand why it is happening to him or her and not to others. A delusional interpretation, however disturbing it may be and however damaging in the long run, provides immediate relief, just as performing a compulsive ritual brings relief from an obsessional thought. Unless the delusion is associated with a general deterioration of brain functions, as in schizophrenia or dementia, it is compatible with adequate and even superior thinking. Once the delusion is well established, apparently contrary evidence can easily be ignored or accommodated.

The raw material is supplied by many kinds of experience: hallucinatory odors, visions, and voices, an unexplained feeling or absence of feeling, or a sensory defect, emotional state, or social situation that makes communication difficult and promotes suspicion. The stranger the experience, the more bizarre the explanation. Schizophrenic patients hear voices issuing commands or sense that their thoughts are somehow not their own, and they try to still their anxiety by identifying the external power that is controlling their minds. The experiences that form the basis of delusional disorders are more common and the resulting delusions less obviously implausible.