There are limited amount of studies comparing time trends of incidence and risk factors of psychosis.

To compare time trends of incidence of psychosis in two population samples.

To study 1) onset age and cumulative incidence of psychoses in two Northern Finland Birth Cohorts (NFBC), 2) changes in type of diagnosis and risk factors.

The NFBC 1966 (N=12,058) and NFBC 1986 (N=9,432) are prospective cohorts of the two provinces of Finland with the live born children followed since pregnancy. The data for psychosis and risk factors were collected from variety of nationwide registers and earlier collected data of the NFBCs. The follow-up time was in both cohorts in average 26.5 years.

Proportion of all psychoses was higher in NFBC 1986 than in the NFBC 1966 (1.81% vs 1.0%). There were more affective psychoses in NFBC 1986 (0.5% vs 0.1%), but incidence of schizophrenia was the same (0.4%) in both cohorts. The age of onset was lower in NFBC 1986 than in NFBC 1966 and majority of this cases were females. Only parental psychosis was a significant risk factor predicting psychosis (Hazard Ratios >3.0) in both cohorts.

In conclusion, two birth cohorts within 20 years covering altogether about 40 years showed changes in terms of incidence, age of onset, and type of psychosis.