Prevalence of mental disorders in the Zurich Cohort Study:: a twenty year prospective study

Background - In order to minimise retrospective recall in developing estimates of the prevalence of mental disorders in the general population, we conducted a prospective study of a cohort of youth from Zurich, Switzerland. Method - A 20 year prospective study of a community-based cohort aged 19-20 from Zurich Switzerland. The sample was enriched by subjects scoring high on the Symptom Checklist 90 R (Derogatis, 1977). A semi-structured diagnostic interview was administered by clinically experienced psychologists and psychiatrists. The six interviews from 1979 to 1999 assessed diagnoses and sub-threshold manifestations of major diagnostic categories (with the exception of schizophrenia) for the past twelve months, depending on the current DSM versions (DSM-III, DSM-III R, DSM-IV). Additional information on symptoms and treatment were collected for the years between the interviews. The reported prevalence rates are weighted for stratified sampling and cumulate the one-year rates of the six interviews. Results-The cumulative weighted prevalence rates for any psychiatric disorder were 48.6% excluding, and 57.7% including tobacco dependence. In addition 29.2% and 21.8%, respectively manifested sub-diagnostic syndromes. Overall there were no significant gender differences. The corresponding treatment prevalence rates were 22.4% and 31.1%, respectively for the diagnostic subjects and 6.9% and 6.1%, respectively for the sub-diagnostic groups. The total treatment prevalence rate was 37.2% of the population (males 30.0%, females 44.1%). Conclusions - Our findings reveal that psychiatric disorders are quite common in the general population. When the spectra of mental disorders are considered, nearly three quarters of the general population will have manifested at least one of the mental disorders across their lifetime. Limitations - The data are based on a relatively small sample; a single age cohort, and the study was conducted in Zurich, Switzerland. These study features may diminish the generalisability of the findings.