Psychiatrists frequently talk about ‘schizophrenic spectrum’ or ‘depressive spectrum’ disorders, implying a group of illnesses with significant phenomenological or psychopathological relationships to each other. Illnesses in the spectrum need not themselves be schizophrenia or major mood disorder, but may well be cognates or precursors.

Delusional disorder, as at present featured in ICD10 and DSMIV, is both an illness and a category, since only one delusional disorder is described – an updated version of the venerable paranoia. But when paranoia was a well-accepted diagnosis in the late nineteenth and early twentieth centuries, it was frequently seen as a member of a group of paranoid illnesses and, in Chapter 7, the concept of a ‘paranoid spectrum’ is discussed. In addition to paranoia, paraphrenia and paranoid schizophrenia were members of that grouping.

Nowadays, paraphrenia is neglected and paranoid schizophrenia, although clearly distinct in many ways from the rest of schizophrenia, is included with the latter.