Environmental factors are more important than previously thought in leading to autism, as big a factor as genes, according to the largest analysis to date to look at how the brain disorder runs in families.
Sven Sandin, who worked on the study at King’s College London and Sweden’s Karolinska Institute, said it was prompted “by a very basic question which parents often ask: ‘If I have a child with autism, what is the risk my next child will too?’”
The findings, published in the Journal of the American Medical Association, suggest heritability is only half the story, with the other 50 per cent explained by environmental factors such as birth complications, socio-economic status, or parental health and lifestyle.
The study also found that children with a brother or sister with autism are 10 times more likely to develop the condition, three times if they have a half-brother or half-sister with autism, and twice as likely if they have a cousin with autism.
“At an individual level, the risk of autism increases according to how close you are genetically to other relatives with autism,” said Sandin. “We can now provide accurate information about autism risk, which can comfort and guide parents and clinicians in their decisions.”
People with autism have varying levels of impairment across three common areas: social interaction and understanding, repetitive behaviour and interests, and language and communication. The exact causes are unknown, but evidence has shown it is likely to include a range of genetic and environmental risk factors.
For this latest study, researchers used Swedish national health registers and analyzed anonymous data from all two million children born in Sweden between 1982 and 2006, 14,516 of whom had a diagnosis of autism.
The study involved two separate measures of autism risk – heritability, which is the proportion of risk in the population that can be attributed to genetic factors, and relative recurrent risk, which measures individual risk for people who have a relative with autism.
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