"I was very surprised. The environmental influence is stronger than I thought," Hallmayer said. "It doesn't mean that genes don't play a role, but they may not play as big a role as thought."
Earlier small studies on twins had suggested that genetics accounts for about 90 percent of the autism risk. Researchers pointed out, however, that unlike the Stanford study, those studies did not include standard clinical assessments for autism diagnoses.
The Stanford study is published in the July 4 online issue of Archives of General Psychiatry.
The research is interesting and a good reminder that it's important for researchers to search for environmental triggers of autism, said Dr. Gary Goldstein, president and CEO of the Kennedy Krieger Institute in Baltimore.
However, a close look at the statistics shows they might not be as powerful as it seems, Goldstein said. The statistics have a wide "confidence interval," or range of uncertainty. For the genetic influence on autism, for example, the confidence interval was 9 percent to 81 percent -- meaning there's a chance that the actual number could fall anywhere in that range, he said.
"I think everyone in the field believes that genetics are important to autism and that the environment must also be involved. But we don't know exactly what those environmental factors are, and how those factors interact with the genes," Goldstein said. "This study gives further support that we should be looking at both genes and the environment."
"Environmental" influences mean anything that isn't in the genetic code. Research has suggested that a host of possible factors, including advanced maternal or paternal age, assisted reproductive technology and artificial insemination, maternal infections during pregnancy, giving b
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