A major register study from Karolinska Institutet shows that children born to certain groups of immigrants had an increased risk of developing autism with intellectual disability. The study includes all children in Stockholm County from 2001 to 2007, and brings the question of the heredity of autism to the fore.
"This is an intriguing discovery, in which we can see strong links between a certain kind of autism and the time of the mother's immigration to Sweden," says principal investigator Cecilia Magnusson, Associate Professor of epidemiology at Karolinska Institutet. "The study is important, as it shows that autism isn't governed only by genetic causes but by environmental factors too."
The study, which is published in the scientific periodical The British Journal of Psychiatry shows that children of immigrant parents, particularly from countries of low human development, are disproportionately likely to develop autism with intellectual disability, a connection that appears to be related to the timing of migration rather than complications in childbirth. Children, whose mothers migrated just before or during pregnancy, ran the highest risk of all.
"At this juncture we can but speculate about the causes, but our assumption is that environmental factors impacting on the developing fetus, such as stress, new dietary habits or infections, could lead to the development of autism with intellectual disability," says Dr. Magnusson.
At the same time, the results also show that that autism without intellectual disability (usually Asperger's syndrome) was less common in children of non-Swedish parents regardless of when the mother migrated to Sweden. According to the researchers, the latter probably can be attributed to the fact that foreign-born parents have less of a tendency to seek psychiatric care for their children than Swedish-born parents.
"We know far too little about autism, but we will continue to gather data on these children to find out more about its possible causes", says Dr. Magnusson. "We know that the incidence of autism-spectrum disorders is rising sharply in high-income countries, probably largely because the diagnostic criteria have changed, although we can't rule out a genuine increase."
Of all the children examined in Stockholm, some 5,000 had received a diagnosis according to the different registers. In all, 589,114 children were included in the study.
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Migration and autism-spectrum disorders: population-based study, British Journal of Psychiatry, online ahead of print 23 February 2012.