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Autism Spectrum Disorders Health Center

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Environment Plays Bigger Role in Autism Than Thought

Study in Twins Finds a Shared Environment Influences the Development of Autism More Than Shared Genes

Twins and Autism Risk

For the study, researchers culled a California registry of children who receive services for developmental disabilities and identified 192 sets of twins in which at least one child had received a diagnosis of autism or other autism spectrum disorder, like Asperger’s syndrome. There were 54 pairs of identical twins in the study and 138 pairs of fraternal twins.

Researchers used standard psychological assessments to confirm the children’s diagnoses.

To calculate the relative contributions of genes and the environment to autism risk, researchers compared the number of identical and fraternal twins in which both siblings had been diagnosed with the disorder.

Because twins share nearly all aspects of family life and environment, comparing identical and fraternal siblings allowed researchers to see how autism rates differed when the only variable that was different was the number of shared genes.

As expected, identical twins were more likely to share an autism diagnosis than fraternal twins.

What was surprising, though, was that both fraternal twins were diagnosed with autism more often than could be explained by genetics alone, suggesting that common environment factors were at work.

Researchers say that based on their models, environment may account for about 58% of the risk for autism spectrum disorders, with genetics accounting for about 38%.

Independent experts, however, point out that the ranges used to compute those averages are very large. For example, the range of autism explained by environmental factors, the study shows, is anywhere between 9% and 81%. Genetics risk ranges from 8% to 84%.

“There’s a big range of confidence on those statistics,” Goldsmith says. “There’s no precision to those estimates.”

Other experts agree that it would be a mistake to think of the numbers too literally.

“I don’t think this is a revolution in thought,” says Gary W. Goldstein, MD, president and CEO of the Kennedy Krieger Institute in Baltimore.

Goldstein is on the board of Autism Speaks but was not involved in the current research.

“Having said that,” Goldstein says, “Everyone in this field understands that genetics doesn’t explain this fully and there’s going to be an environmental component to triggering vulnerable people or maybe even causing it in some people without a vulnerability gene.”

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