Schizophrenia is characterized by two or more of the following: Bizarre delusions, hallucinations (auditory or visual), disorganized speech, grossly disorganized or catatonic behavior, and/or negative symptoms like blunted affect (affective flattening) or a general lack of desire, drive, or motivation to pursue meaningful goals (avolition). Subtypes include paranoid, disorganized, catatonic, undifferentiated, or residual types. Compared with Delusional Disorder (DD), social and occupational dysfunction is clinically significant. There must be continuous signs of the disturbance that persist for at least 6 months, including at least 1 month of bizarre delusions. On the whole, schizophrenia is marked by delusions that are “not plausible,” where DD is characterized by delusions that are conceivably possible, even if unlikely.
The essential feature of Delusional Disorder (DD) is the presence of one or more “nonbizarre” delusions that persist for at least 1 month, and which have never escalated to a constellation of symptoms that typify Schizophrenia. (American Psychiatric Association, Diagnostic and statistical manual of mental disorders, 2000, p. 323) Subtypes include ertomanic, grandiose, jealous, persecutory, somatic, mixed, and unspecified types. DD can be distinguished from Schizophrenia by the absence active phase schizophrenia symptoms. This would include prominent auditory or visual hallucinations, bizarre delusions, disorganized speech, grossly disorganized or catatonic behavior, or negative symptoms. (4th ed., text rev.; DSM-IV-TR; American Psychiatric Association, 2000, p. 327) In short, DD is comparatively mild in its symptoms when compared with schizophrenia.
In short, the primary differentiating factor between schizophrenia and DD is the word bizarre. If the delusion is plausible, even if improbable, then the diagnosis is DD. If the delusion is outlandish, or impossible, then the diagnosis is schizophrenia. “If delusions are judged to be bizarre, only this single symptom is needed to satisfy Criterion A for Schizophrenia.” (4th ed., text rev.; DSM-IV-TR; American Psychiatric Association, 2000, p. 299)
American Psychiatric Association. (2000). Diagnostic and statistical manual of mental disorders (4th ed., text rev.). Washington, DC: Author.