Decline in the incidence of schizophrenia in Finnish cohorts born from 1954 to 1965

Background: The declining incidence of schizophrenia observed in several co untries is believed by many to merely reflect methodological problems in th e studies performed. We report the first nationwide historical cohort study of changes in the incidence of schizophrenia, in which many of the previou s methodological problems were overcome. Methods: We used the Finnish Population Register to identify everyone born in Finland from 1954 to 1965. These persons were followed up from their 16t h to their 26th birthdays, and all cases of schizophrenia (International Cl assification of Diseases, Eighth Revision and International Classification of Diseases, Ninth Revision code 295) that emerged were identified from the National Hospital Discharge Register, the Pension Register, and the Free M edicine Register. Persons for whom an age of onset could be defined were in cluded in the analyses (n = 5645). We used the Poisson regression model to estimate the effects of age, sex, birth cohort, period of diagnosis, and se ason of birth on the incidence of schizophrenia. The relative importance of cohort and period were assessed using an age-period-cohort model. Results: The incidence declined significantly in each successive cohort, fr om 0.79 to 0.53 per 1000 among males and from 0.58 to 0.41 per 1000 among f emales. The effects of cohort and period on the change were both significan t. Conclusions: The incidence of schizophrenia has declined in Finland. This w as partly caused by confounding factors, as reflected in the significant pe riod effect. The significant birth cohort effect suggests that the intensit y or frequency of one or more risk factors for schizophrenia has decreased.