Age at migration and the increased incidence of psychotic disorders among immigrants in the Netherlands: A test of selective migration

Context: Remarkable high rates of psychotic disorders have been reported among immigrants, mainly in the United Kingdom and in the Netherlands. It could never be definitively shown that the relationship between migration and psychotic disorders is causal individuals may migrate as a result of their (genetic) predisposition for schizophrenia. Also, it is unknown at what age migration increases the risk for psychotic disorder. Objective: To investigate whether the risk for psychotic disorders is increased in immigrants who migrated before the age of 15, compared to Dutch. For this group, migration has been a parental decision. The second objective was to determine at what age migration is most significantly related to the increased risk. Methods: Over 7 years we identified the individuals aged 15-54 making contact with a physician for a suspected psychotic disorder in The Hague, the Netherlands; administered diagnostic interviews to these individuals; and assigned DSM IV diagnoses by consensus of two psychiatrists. A comprehensive municipal registration system provided the denominator, including information on age at migration to the Netherlands. Results: Compared to Dutch, the risk for psychotic disorders was signifi- cantly increased in non-Western immigrants who migrated before the age of 15, adjusted IRR = 2.21 (95% CI, 1.74-2.80). Those who migrated between 0 and 4 years old had the most elevated risk (adjusted IRR = 2.96 (2.10- 4.17)), and the risk gradually decreased with older age at migration. Conclusions: Selective migration cannot explain the increased risk for psychotic disorders among immigrants in the Netherlands. The adverse influence of migration on risk for psychotic disorders is most prominent in early life