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Age–period–cohort analysis of the incidence of schizophrenia in Scotland

  • Noriyoshi Takei (a1), Glyn Lewis (a1), Pak C. Sham (a1) and Robin M. Murray (a1)

Studies examining a possible decline in the incidence of schizophrenia over the last two to three decades have paid little attention to the possible role of birth cohort effects. We collected data on a Scottish national sample of all schizophrenic patients, admitted for the first time between 1966 and 1990 (N = 11348; male = 6301). In an Age–Period–Cohort analysis, a full model, incorporating three factors, had a substantially better fit to the data than other models (especially, an Age–Period model), providing clear evidence of the presence of a cohort effect. After adjustment for the effects of age and period, there was a 55% reduction in the rate of schizophrenia in men and a 39% fall in the number of women over the 50-year birth period from 1923 to 1973. The marked decline in the first admission rates observed in Scotland cannot, however, be attributed entirely to this cohort effect. Rather, a greater proportion of the declining first admission rates (88%) is ascribed to the period effect (i.e. artefactual or causally related cross-sectional effects). Nevertheless, the fact that a birth-cohort effect accounts for part of the declining incidence, suggests that causal environmental factors operating early in life have been diminishing in intensity.

Corresponding author
1Address for correspondence: Dr Noriyoshi Takei, Genetics SectionDepartment of Psychological MedicineInstitute of Psychiatry and King's College HospitalDe Crespigny ParkLondon SE5 8AF.
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