Why has the incidence of schizophrenia in Danish psychiatric institutions decreased since 1970?

From The Danish Psychiatric Register five cohorts of all first-admitted patients to Danish psychiatric institutions from the years 1970, 1973, 1976, 1979, and 1980 have been followed, and trends in age-standardized rates of schizophrenia investigated. The first-admission rates have decreased in both sexes, significantly in males. In addition, the cumulated schizophrenia rates for the cohorts, including those of patients diagnosed as schizophrenics only at a later admission within periods of 2 and 5 years, decreased. This decrease is significant at 2 years of observation in both sexes and at 5 years in females. The decreasing first-admission rates might be explained partly by changes in diagnostic habits. The differential diagnoses of schizophrenia (paranoid states (ICD-8:297), paranoid reactive psychoses (298.3), unspecified psychoses (298.9 and 299), and borderline states (ICD-8 Danish version: 301.83] are increasingly used as first-admission diagnosis for patients later to be diagnosed as schizophrenic, possibly owing to a tendency to avoid the diagnosis of schizophrenia, when treatment of a patient is possible. A method of calculating the hospital incidence of schizophrenia approaching the real incidence better than the incidence of first-admission diagnoses is suggested.