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.

The researchers did not say which environmental influences might be at work. But other experts said the new study, released online Monday, marked an important shift in thinking about the causes of autism, which is now thought to affect at least 1 percent of the population in the developed world.

"This is a very significant study because it confirms that genetic factors are involved in the cause of the disorder," said Dr. Peter Szatmari, a leading autism researcher who is the head of child psychiatry and behavioral neuroscience at McMaster University in Ontario. "But it shifts the focus to the possibility that environmental factors could also be really important."

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As recently as a few decades ago, psychiatrists thought autism was caused by a lack of maternal warmth. And although that notion has been discarded in favor of genetic explanations, there has been growing acceptance that genes do not tell the whole story, in part because autism rates appear to have increased far more quickly than our genes can evolve.

"I think we now understand that both genetic and environmental factors have to be taken seriously," said 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.

Other experts have cited factors such as parental age, multiple pregnancies, low birth weight and exposure to medications or maternal infection during pregnancy.

In the new study, the largest of its kind among twins, researchers looked at 192 pairs of identical and fraternal twins whose cases were drawn from California databases. At least one twin in each pair had the classic form of autism, which is marked by extreme social withdrawal, communication problems and repetitive behaviors. In many cases, the other twin also had classic autism or a milder "autism spectrum" disorder such as Asperger's syndrome.

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.

"We, like everyone else, were very surprised because we didn't expect it to be that high," said a senior author of the study.