Delusional disorder is classified as a psychotic disorder, a disorder where a person has trouble recognizing reality. A delusion is a false belief that is based on an incorrect interpretation of reality. Delusions, like all psychotic symptoms, can occur as part of many different psychiatric disorders. But the term delusional disorder is used when delusions are the most prominent symptom.
A person with this illness holds a false belief firmly, despite clear evidence or proof to the contrary. Delusions may involve circumstances that could occur in reality even though they are unlikely (for example, the family next door plotting to kill you). Or they may be considered "bizarre" (for example, feeling controlled by an outside force or having thoughts inserted into your head). A religious or cultural belief that is accepted by other members of the person's community is not a delusion.
There are several types of delusions: persecutory, erotic, grandiose, jealous or somatic (that is, delusions about the body). People with delusional disorder usually do not have hallucinations or a major problem with mood. Unlike people with schizophrenia, they tend not to have major problems with day-to-day functioning and they do not appear odd.
When hallucinations do occur, they are part of the delusional belief. For example, someone who has the delusion that internal organs are rotting may hallucinate smells or sensations related to that delusion.
If their functioning is impaired, it is usually a direct result of the delusion. Therefore, the disorder may be detected only by observing behavior that is a consequence of the belief. For example, a person who fears being murdered may quit a job or stay home with all the shades drawn, never venturing out.
Since people with delusional disorder are aware that their beliefs are unique, they generally do not talk about them. Delusional disorder is diagnosed much less frequently than schizophrenia.