MONDAY, July 4 (HealthDay News) Contrary to current thinking, environmental factors may play a larger role than shared genes in the development of autism, a new study in twins suggests.
A second study in the same journal finds that anti-depressants during pregnancy may be one important environmental trigger.
In the first study, researchers from Stanford University identified 192 pairs of twins from a statewide California registry of children who receive services for developmental disabilities. At least one twin was diagnosed with an autism spectrum disorder, which researchers confirmed by examining and testing each child.
The study included 54 pairs of identical twins (meaning they share all of the same genes) and 138 pairs of fraternal twins (who share half of their genes).
About 42.5 percent of the male-male pairs and 43 percent of the female-female pairs of identical twins both had autism. About 12.9 percent of the male-male fraternal twins and 20 percent of the female-female fraternal twins both had autism, researchers said.
It's not surprising that the identical twins were more likely to each have autism, since they share all the same genes, explained lead study author Dr. Joachim Hallmayer, an associate professor of psychiatry and behavioral science at Stanford University. Research has suggested that genetics play a role in the development of autism.
Nevertheless, if a disorder was 100 percent due to genetics, both siblings in each pair of identical twins would have it, which is not the case with autism.
That means the shared environment -- and this could be in utero or in early life -- has to play a major role, the researchers explained.
According to their calculations, this means that genes account for 37 percent of the risk of "classic," or severe autism and 38 percent of the risk of milder autism spectrum disorders. By the s
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