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

Objective: The Northern Finland 1966 Birth Cohort was studied in order to i nvestigate 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 morbi dity. Effects were adjusted for wantedness of pregnancy, perinatal complica tions. maternal age at delivery, family type and number of siblings. Results: The risk for schizophrenia was elevated among male first-borns (ra tio 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'%, C I 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 in dependent risk factor for schizophrenia. Theoretical explanations may arise from biological factors unidentified here and/or psychological stressors l inked with these positions.