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Environmental factors play a more important role in causing autism than previously assumed and, surprisingly, an even larger role than genetics, according to a new study out of UCSF and Stanford that could force a dramatic swing in the focus of research into the developmental disorder.

The study, published in Monday’s issue of the Archives of General Psychiatry, looked at 192 pairs of twins in California and, using a mathematical model, found that genetics account for about 38 percent of the risk of autism, and environmental factors account for about 62 percent.

Previous twin studies had suggested that autism was highly inheritable, with genetics accounting for roughly 90 percent of all cases worldwide. As such, much recent research into autism has focused on tracking down the genes and unlocking the complex genetic codes that are associated with autism.

“We’re not trying to say there isn’t a genetic component – quite the opposite. But for most individuals with autism spectrum disorder, it’s not simply a genetic cause,” said Neil Risch, director of the UCSF Institute for Human Genetics, who designed the study.