The existence of data from a 20-year-old (1976 to 1978) first-admission chart-review study allowed a preliminary look at the issue of the changing occurrence of schizophrenia (23). From this cohort, all patients with a DSM-III diagnosis of schizophrenia were selected. A similar cohort of patients identified from the KPRLS, admitted from 1996 to 1998 with a diagnosis of schizophrenia and with no KPRLS-documented  contact since 1984 and no reported previous psychiatric admissions, was compared with the earlier sample.

The main study examines changing rates of schizophrenia from 1986 to 1996 in this region. To calculate population rates from the KPRLS, only the 2 counties closest to Kingston are used, since all residents from these counties receive their care in Kingston. Although incidence rates are a far better indicator of the occurrence of a disorder than are prevalence rates, the number of newly diagnosed schizophrenia cases in this area is too small for meaningful analysis. For this reason, treated prevalence rates rather than incidence rates are reported in this study. Data from the KPRLS in the 3 census years of 1986, 1991, and 1996 were used to calculate annual treated prevalence rates for inpatient contacts and for outpatient or emergency contacts for the population in the 2 counties (140 000 over age 15 years) served by these hospitals. In each of the 3 years, the cohorts were compared on age, sex, diagnosis, and treatment setting and chi-square analysis was performed using the SAS Institute Statistical Analysis Package.

Results

Data from the 1976–78 first-admission study were compared with the KPRLS data for 1996–1998. The number of first-admissions with a diagnosis of schizophrenia decreased from 171 to 100, a difference of 42%. Despite  completely different methodologies and very small numbers, this large difference suggested the possibility of a decrease in the incidence of schizophrenia in this region over 20 years.

Annual treated prevalence rates for schizophrenia by service-contact type are shown in Table 1. There was a significant decrease of more than 50% in inpatient prevalence rates from 1986 to 1996 with no statistically significant corresponding change in outpatient contact rates.

Over the same period, prevalence rates for major affective disorder showed a nonsignificant increase for inpatients and a significant increase for outpatients (Table 2). For all other diagnoses, the inpatient rates remained the same or decreased.

Patients with major affective disorder were divided by diagnostic subgroup and sex for comparison with the schizophrenia population (Table 3). The increase in treated prevalence for major affective disorder is accounted for by the large, significant increase in major depression for inpatients and outpatients of both sexes. Prevalence rates for bipolar disorder showed a nonsignificant decrease for male and female inpatients and male outpatients and a significant decrease for female outpatients. Table 3 also shows that the significant decrease in inpatient schizophrenia prevalence rates applies to both men and women and is not accounted for by a corresponding increase in outpatient rates for either sex.

Discussion

The controversy about the reported falling rates of schizophrenia over the last 40 years has not been resolved. Many authors have discussed the methodological problems in this type of research resulting from confounding factors. Several review articles have outlined these important issues, which include changes in population characteristics, service delivery, and diagnostic criteria and practices (24–27). The many methodological differences among studies also make comparisons difficult.

Most authors have discussed the differential rates of schizophrenia in ethnic groups and social classes and the effect of changes in these factors on the incidence of schizophrenia. The increased rates in some British studies have been attributed to the large number of Afro-Caribbean immigrants, whose rates of schizophrenia in both first and second generations are much higher than in the rest of the population (10,11). The fall in Scottish rates may be related to an out-migration of preschizophrenic individuals to English cities (1). Although a similar decline in rates of schizophrenia has been reported in Denmark, the emigration rates have been stable and low (4). In Finland, there has been little immigration, and emigration is primarily to Sweden; of these emigrants, many who develop psychiatric illness are known to return to Finland for extended treatment (5). For these reasons, the authors concluded that migration is unlikely to be a major confounding factor in the observed decline in rates (5). Increasing rates of schizophrenia reported in inner-city Salford could be attributed to the fact that the 25% decrease in the general population may reflect a selective migration of healthy individuals (9). Changes in the socioeconomic status and ethnic composition of the population was not a factor in the current study because Kingston is a very stable community with little immigration or emigration.