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A new study of twins suggests that environmental factors, including conditions in the womb, may be at least as important as genes in causing autism.
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“I think we now understand that both genetic and environmental factors have to be taken seriously,” said Dr. Joachim Hallmayer, an associate professor of psychiatry and behavioral sciences at Stanford and the lead author of the new study, which is to be published in the November issue of Archives of General Psychiatry.
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Identical twins share 100 percent of their genes; fraternal twins share 50 percent of their genes. So comparing autism rates in both types of twins can enable researchers to measure the importance of genes versus shared environment.

The study found that autism or autism spectrum disorders occurred in both children in 77 percent of the male identical twins and in 50 percent of the female identical twins. As expected, the rates among fraternal twins were lower: 31 percent of males and 36 percent of females.

But surprisingly, mathematical modeling suggested that only 38 percent of the cases could be attributed to genetic factors, compared with the 90 percent suggested by previous studies.

And more surprising still, shared environmental factors appeared to be at work in 58 percent of the cases.
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Clara Lajonchere, an author of the twin study and vice president of clinical programs for the research and advocacy organization Autism Speaks, said that “much more emphasis is going to be put on looking at prenatal and perinatal factors with respect to autism susceptibility.”

She added, “We need to not just study the environmental factors, but the relation between the genes and the environment.”