Recent and comprehensive meta-analyses of scientific studies point to an association with a deterioration in aspects of IQ in psychotic patients, in particular perceptual reasoning, although, the between-group differences were small.

[10][11][12] According to German psychiatrist Emil Kraepelin, patients with delusional disorder remain coherent, sensible and reasonable.

The cause of delusional disorder is unknown,[8] but genetic, biochemical, and environmental factors may play a significant role in its development.

[better source needed] Some people with delusional disorders may have an imbalance in neurotransmitters, the chemicals that send and receive messages to the brain.

[18] There does seem to be some familial component, and immigration (generally for persecutory reasons),[8] drug abuse, excessive stress,[19] being married, being employed, low socioeconomic status, celibacy among men, and widowhood among women may also be risk factors.

[8] Interviews are important tools to obtain information about the patient's life situation and history to help make a diagnosis.

In case of non-bizarre delusions, Psych Central[22][better source needed] notes, "All of these situations could be true or possible, but the person suffering from this disorder knows them not to be (e.g., through fact-checking, third-person confirmation, etc.)."

[23] There is a certain amount of evidence that alternative treatment-regimes (beyond conventional attempted treatment with antipsychotics) may include clomipramine for people with the somatic subtype of paranoia.

[24][25] There is a dearth of well-published studies investigating the effectiveness of trimipramine; another derivative of tricyclic-antidepressant imipramine and one which has modest anti-psychotic properties weakly analogous to those of clozapine; in delusional disorder per-se.

However, trimipramine was compared to a combination of amitriptyline and haloperidol in a double-blinded trial involving patients with severe, psychotic depression (specifically with customary delusional features) and appeared favourable in its treatment.

The combination of pharmacotherapy with cognitive therapy integrates treating the possible underlying biological problems and decreasing the symptoms with psychotherapy as well.

In other cases, a belief may be incorrectly deemed delusional by a doctor or psychiatrist who subjectively concludes that a patient's assertions are unlikely, bizarre, or held with excessive conviction.

Psychiatrists rarely have the time or resources to check the validity of a person's claims leading some true beliefs to be erroneously classified as delusional.

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 that might make a belief otherwise interpretable.

[35][36] An Indian movie Anantaram (Thereafter) directed by Adoor Gopalakrishnan also portrays the complex nature of delusions.