ABSTRACT
Mentally subnormal people are liable to the full range of mental illnesses which occur in people of normal intelligence, but the incidence of different mental illnesses varies according to the degree of mental handicap, e.g. the manic-depressive type of psychosis occurs only in the least severely handicapped, whereas the schizophrenic group of psychoses occur through a much wider range of intelligence (Reid, 1972). However, when a case is first seen several years after the onset of a schizophrenic deterioration, it is very difficult to be sure what the level of intelligence was before the onset of the psychosis. In juvenile schizophrenia, the psychosis starts before mental development is complete and is itself a cause of both mental subnormality and subsequent mental deterioration.
