Psychotic experiences in the general population: a twenty-year prospective community study

Recent work suggested that psychosis might be expressed at subclinical levels. However, the determinants of subclinical psychotic symptoms, the degree of continuity over the life span, and the impact on functioning remain unclear. Thus we analyzed the prevalence, determinants, patterns and impact of subclinical psychotic symptoms in a community cohort over a 20-year period.

The Zurich Study - a longitudinal community study - started in 1979 with a sample of 591 participants aged 20/21 years. Follow-up interviews were conducted at age 23, 28, 30, 35 and 41. Symptoms were assessed with a semi-structured interview and the SCL90-R. In this analysis, items of the SCL90-R symptom dimensions "paranoid ideation" and "psychoticism" were examined.

Two distinct symptom dimensions of subclinical psychosis became evident, one representing schizophrenia nuclear symptoms, the other representing schizotypal signs. Cannabis use in adolescence was associated specifically with schizophrenia nuclear symptoms, whereas childhood adversity as well as chronic physical or mental disorders in parents contributed to schizotypal signs. Individuals with a persistently high level of either of the two identified symptom dimensions over 20 years experienced significant deficiencies in social achievement and functioning.

Expression of psychotic symptoms in populations is continuous and characterized by differing levels of severity and persistence. A small group of individuals displays persistence of subclinical psychotic symptoms over a period of 20 years. The causes of and pathways to clinical psychotic disorder can be studied long before the disorder becomes clinically relevant.