title: Birth order and risk for schizophrenia: a 31-year follow-up of the Northern Finland 1966 Birth Cohort.
abstract: OBJECTIVE
  The Northern Finland 1966 Birth Cohort was studied in order to investigate the association between birth order and schizophrenia.
  
  
  METHODS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Psychiatry, University of Oulu, Finland.
authors.forename: L
authors.initials: L
authors.lastname: Kemppainen
authors.forename: J
authors.initials: J
authors.lastname: Veijola
authors.forename: J
authors.initials: J
authors.lastname: Jokelainen
authors.forename: A L
authors.initials: AL
authors.lastname: Hartikainen
authors.forename: M R
authors.initials: MR
authors.lastname: Järvelin
authors.forename: P
authors.initials: P
authors.lastname: Jones
authors.forename: T
authors.initials: T
authors.lastname: Croudace
authors.forename: M
authors.initials: M
authors.lastname: Isohanni
mesh_terms: D000328:Adult; D001722:Birth Order; D015331:Cohort Studies; D005260:Female; D005387:Finland / Q000453:epidemiology; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D012307:Risk Factors; D012559:Schizophrenia / Q000453:epidemiology / Q000209:etiology*; D012737:Sex Factors