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.