Here is the DSM summary: Schizophrenia Spectrum and Other Psychotic Disorders

© American Psychiatric Association

http://dx.doi.org/10.1176/appi.books.9780890425596.dsm02

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Sections

Key Features That Define the Psychotic Disorders | Disorders in This Chapter | Clinician-Rated Assessment of Symptoms and Related Clinical Phenomena in Psychosis | Schizotypal (Personality) Disorder | Delusional Disorder | Brief Psychotic Disorder | Schizophreniform Disorder | Schizophrenia | Schizoaffective Disorder | Substance/Medication-Induced Psychotic Disorder | Psychotic Disorder Due to Another Medical Condition | Catatonia | Catatonia Associated With Another Mental Disorder (Catatonia Specifier) | Catatonic Disorder Due to Another Medical Condition | Unspecified Catatonia | Other Specified Schizophrenia Spectrum and Other Psychotic Disorder | Unspecified Schizophrenia Spectrum and Other Psychotic Disorder

Excerpt

Schizophrenia spectrum and other psychotic disorders include schizophrenia, other psychotic disorders, and schizotypal (personality) disorder. They are defined by abnormalities in one or more of the following five domains: delusions, hallucinations, disorganized thinking (speech), grossly disorganized or abnormal motor behavior (including catatonia), and negative symptoms.

Of course this only answers a superficial question In order to understand why, you need to dig much deeper into all the disorders, since they fit into a different part of the spectrum for different reasons.

Also keep in mind that the DSM is highly behaviorally oriented and pays little attention to the inner world of the person, since it is not technically "observable." There is a vast psychodynamic literature that sheds light on why many clinicians do two diagnoses, one for the insurance companies in order to put a code on the billing form, and another with more theoretical underpinnings.

Prof. Kemp, I am sorry I'm still confused. Are you saying that schizotypal personality disorder basically is personality disorder? and because of insurance issue so they put it in schizophrenia spectrum? could you help me please give some more information. Thank you

 Prof Bianchi, Could you please share with me some information about how does schizotypal include in schizophrenia spectrum?

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

I have little to add to the nosological references above.  I will note that to many researchers, the term "schizophrenia spectrum" has implications of genetic vulnerability and degrees of penetrance and that schizotypal PD is present in greater numbers in the first degree relatives of patients with full blown psychotic schizophrenia or schizoaffective disorders than in a control group.

Thank you for the explanation :D