Subtle motor, emotional, cognitive and behavioral abnormalities are often present in apparently healthy children and adolescents who later develop schizophrenia. This suggests that some aspects of causation are established long before psychosis is manifest. We aim to develop a descriptive model of the factors contributing to the development of schizophrenia. Our main focus is on genetic factors, pregnancy and delivery complications, early development and scholastic performance. This is done by reviewing the Northern Finland 1966 Birth Cohort, its scientific activities (publications and work in progress) and selected literature.

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 (®rst-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 ®rst-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 speci®c birth order status is an independent risk factor for schizophrenia. Theoretical explanations may arise from biological factors unidenti®ed here and/or psychological stressors linked with these positions.

It is possible that the mother's GMP status and the large family size associated with this are causal factors in the development of other psychoses than schizophrenia, alcoholism and depression among adult offspring.

These results support the hypothesis that growing up as an only child is associated with violent criminality among male subjects.

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