Migration and autism spectrum disorder: population-based study.

BACKGROUND Migration has been implicated as a risk factor for autism, but evidence is limited and inconsistent. AIMS To investigate the relationship between parental migration status and risk of autism spectrum disorder, taking into consideration the importance of region of origin, timing of migration and possible discrepancies in associations between autism subtypes. METHOD Record-linkage study within the total child population of Stockholm County between 2001 and 2007. Individuals with high- and low-functioning autism were defined as having autism spectrum disorder with and without comorbid intellectual disability, and ascertained via health and habilitation service registers. RESULTS In total, 4952 individuals with autism spectrum disorder were identified, comprising 2855 children with high-functioning autism and 2097 children with low-functioning autism. Children of migrant parents were at increased risk of low-functioning autism (odds ratio (OR) = 1.5, 95% CI 1.3-1.7); this risk was highest when parents migrated from regions with a low human development index, and peaked when migration occurred around pregnancy (OR = 2.3, 95% CI 1.7-3.0). A decreased risk of high-functioning autism was observed in children of migrant parents, regardless of area of origin or timing of migration. Parental age, income or obstetric complications did not fully explain any of these associations. CONCLUSIONS Environmental factors associated with migration may contribute to the development of autism presenting with comorbid intellectual disability, especially when acting in utero. High- and low-functioning autism may have partly different aetiologies, and should be studied separately.