In a controversial new study, scientists say environmental factors may play a bigger role in the development of autism than genetics.

The findings published in the July issue of the Archives of General Psychiatry come from the largest-ever study of twins and autism.

Researchers analyzed data on 192 pairs of twins in which at least one twin had the developmental disorder and found that genetics accounted for 38 percent of the children’s risk for autism while 58 percent of the risk stemmed from environmental factors.

They reached this conclusion by comparing the experiences of 54 sets of identical twins and 138 sets of fraternal twins. Not surprisingly, among identical twins — who share 100 percent of their DNA — if one had autism the other twin had a 70 percent chance of also having the disorder.

Fraternal twins, however, share just half their DNA but had an autism diagnosis in 35 percent of cases, which is significantly higher than the likelihood of two typical siblings sharing the diagnosis.

“The high concordance among individuals who share only half their genes relative to those who share all of their genes implies a bigger role for shared environmental factors,” said the study’s lead author Joachim Hallmayer of Stanford University.

The findings are counter to recent research which has focused more heavily on genetic causes and many autism experts were quick to question the results.

“It’s a massive claim,” Angelica Ronald, a behavior geneticist at Birkbeck University of London who was not part of the study, told The Los Angeles Times. “It flies in the face of the previous data. I don’t see why the results have come out the way they have.”

Others, however, are hailing the findings as a “game changer.”

“We now have strong evidence that, on top of genetic heritability, a shared prenatal environment may have a greater than previously realized role in the development of autism in twins,” wrote Clara Lajonchere, vice president of clinical programs at Autism Speaks, in a blog post Tuesday about the study, which the organization provided funding for.

Meanwhile, in a separate study also appearing in this month’s Archives of General Psychiatry, researchers reported a link between the use of antidepressants during pregnancy and the development of autism.

The risk was low and occurred most during the first trimester, but scientists cautioned that further research is needed and said the consequences of not treating mental health disorders during pregnancy must be considered.

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