Age-period-cohort analysis of the incidence of schizophrenia in Scotland.
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.
Takei
N
N
Department of Psychological Medicine, Institute of Psychiatry, London.
Lewis
G
G
Sham
P C
PC
Murray
R M
RM
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D015984:Causality; D002648:Child; D015988:Cohort Effect; D015331:Cohort Studies; D016001:Confidence Intervals; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D015994:Incidence; D016013:Likelihood Functions; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D010343:Patient Admission / Q000706:statistics & numerical data / Q000639:trends; D012189:Retrospective Studies; D012559:Schizophrenia / Q000453:epidemiology*; D012606:Scotland / Q000453:epidemiology; D017678:Sex Distribution; D013997:Time Factors