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Autism Risks: Genes May Not Play Biggest Role

Up to now, genetics were thought to account for 90 percent of a child's risk for autism, but a new Stanford University School of Medicine study suggests environmental factors could play a much larger role than previously thought.

The largest study of its kind, the research focused on autism in 192 pairs of twins — 54 identical, 138 fraternal. The surprise came when Stanford researchers found a greater number of fraternal twins shared autism than identical twins. Fraternal twins share only half their genes with each other, thus, when both fraternal twins are autistic, it suggests factors other than genetics are at work.

In fact, "About half of what we see is due to environmental factors, and half of what we see is due to genetic factors," Dr. Joachim Hallmayer tells Guy Raz, host of weekends on All Things Considered. Hallmayer is the lead author of the study.

Scientists have long suspected that there's been an environmental contribution to autism, Hallmayer says, and previous studies have shown that it may occur in the early stages of pregnancy. He says there's also been evidence pointing to the age of the father and mother as an environmental factor. Of all of the risk factors, however, "being male is the most significant risk factor we know of."

Because there are multiple forms of autism, Hallmayer stresses the importance of conducting further study on autism and the factors that contribute to it.

"I think we have to really be very careful to look at both environmental [and] genetic factors, look at the interaction, and then really drill down," Hallmayer says. "I'm very convinced that there will be no one-size-fits-all."

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