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Birth order and risk for schizophrenia: a 31-year follow-up of the Northern Finland 1966 Birth Cohort

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Objective: The Northern Finland 1966 Birth Cohort was studied in order to investigate the association between birth order and schizophrenia.

Method: Four categories of birth order status (first-born, last-born, only child and other status) were formed and linked to data on psychiatric morbidity. Effects were adjusted for wantedness of pregnancy, perinatal complications, maternal age at delivery, family type and number of siblings.

Results: The risk for schizophrenia was elevated among male first-borns (ratio 1.5; 95% CI 1.0–2.2) and female last-borns (ratio 1.3; 95% CI 0.9–1.9). The risk was lower than expected among male last-borns (ratio 0.7; 95% CI 0.5–0.9) and females belonging to other status (ratio 0.6; 95% CI 0.3–0.9).

Conclusion: These results suggest that specific birth order status is an independent risk factor for schizophrenia. Theoretical explanations may arise from biological factors unidentified here and/or psychological stressors linked with these positions.

Keywords: birth order; cohort study; schizophrenia

Document Type: Original Article

Affiliations: 1: Department of Psychiatry, University of Oulu, Finland 2: Department of Obstetrics and Gynecology, University of Oulu, Finland 3: Department of Public Health Science and General Practice, University of Oulu, Finland and the Department of Epidemiology and Public Health, Imperial College School of Medicine London, UK, 4: Division of Psychiatry, University of Nottingham, UK

Publication date: August 1, 2001


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