title: Testing Ødegaard's selective migration hypothesis: a longitudinal cohort study of risk factors for non-affective psychotic disorders among prospective emigrants.
abstract: BACKGROUND
  The selection hypothesis posits that the increased rates of psychosis observed among migrants are due to selective migration of people who are predisposed to develop the disorder. To test this hypothesis, we examined whether risk factors for psychosis are more prevalent among future emigrants.
  
  
  METHODS
  A cohort of 49,321 Swedish military conscripts was assessed at age 18 years on cannabis use, IQ, psychiatric diagnosis, social adjustment, history of trauma and urbanicity of place of upbringing. Through data linkage we examined whether these exposures predicted emigration out of Sweden. We also calculated the emigrants' hypothetical relative risk compared with non-emigrants for developing a non-affective psychotic disorder.
  
  
  RESULTS
  Low IQ [odds ratio (OR) 0.5, 95% confidence interval (95% CI) 0.3-0.9] and 'poor social adjustment' (OR 0.4, 95% CI 0.2-0.8) were significantly less prevalent among prospective emigrants, whereas a history of urban upbringing (OR 2.3, 95% CI 1.4-3.7) was significantly more common. Apart from a non-significant increase in cannabis use among emigrants (OR 1.6, 95% CI 0.8-3.1), there were no major group differences in any other risk factors. Compared to non-emigrants, hypothetical relative risks for developing non-affective psychotic disorder were 0.7 (95% CI 0.4-1.2) and 0.8 (95% CI 0.7-1.0), respectively, for emigrants narrowly and broadly defined.
  
  
  CONCLUSIONS
  This study adds to an increasing body of evidence opposing the selection hypothesis.
authors.affiliation: Maastricht University,School for Mental Health and Neuroscience, Maastricht,The Netherlands.
authors.forename: E
authors.initials: E
authors.lastname: van der Ven
authors.affiliation: Public Health Epidemiology, Department of Public Health Sciences,Karolinska Institutet,Stockholm,Sweden.
authors.forename: C
authors.initials: C
authors.lastname: Dalman
authors.affiliation: Public Health Epidemiology, Department of Public Health Sciences,Karolinska Institutet,Stockholm,Sweden.
authors.forename: S
authors.initials: S
authors.lastname: Wicks
authors.affiliation: Social Medicine, Department of Public Health Sciences,Karolinska Institutet, Stockholm,Sweden.
authors.forename: P
authors.initials: P
authors.lastname: Allebeck
authors.affiliation: Public Health Epidemiology, Department of Public Health Sciences,Karolinska Institutet,Stockholm,Sweden.
authors.forename: C
authors.initials: C
authors.lastname: Magnusson
authors.affiliation: Maastricht University,School for Mental Health and Neuroscience, Maastricht,The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: van Os
authors.affiliation: Maastricht University,School for Mental Health and Neuroscience, Maastricht,The Netherlands.
authors.forename: J P
authors.initials: JP
authors.lastname: Selten
mesh_terms: D000293:Adolescent; D000328:Adult; D054242:Emigrants and Immigrants / Q000706:statistics & numerical data*; D006801:Humans; D007360:Intelligence / Q000502:physiology*; D008137:Longitudinal Studies; D008297:Male; D011618:Psychotic Disorders / Q000453:epidemiology*; D012307:Risk Factors; D012917:Social Adjustment; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult
keywords: selection hypothesis