Environmental factors may be more important than genes in determining whether a child develops autism, according to a controversial new analysis of the disorder in twins.

That finding runs counter to decades of prior research, which has generally found that genetic inheritance is the biggest determinant of a child's risk of autism. The authors of the new study, published online Monday by the journal Archives of General Psychiatry, came to their conclusion after studying 192 pairs of identical and fraternal twins in which at least one twin met clinical criteria for the neurodevelopment disorder.

But the authors' conclusion that environmental influences -- perhaps chemical exposures, infections, diet or stress levels -- could be so influential was roundly criticized by other autism experts.

"I think they're really on shaky ground to say that," said Dr. Paul Law, director of the Interactive Autism Network at the Kennedy Krieger Institute in Baltimore.

"It's a massive claim," said Angelica Ronald, a behavior geneticist at Birkbeck University of London. "It flies in the face of the previous data. I don't see why the results have come out the way they have."

The study authors acknowledged that their calculations were subject to a very wide margin of error and thus could be incorrect. Still, they said that the analysis highlights the need for more research into environmental factors that may contribute to autism.

"Genetics don't explain it," said coauthor Neil Risch, a genetic epidemiologist at UC San Francisco. "They're part of the story, but only part of the story."

Other scientists were highly skeptical.

"Their data is so similar to everybody else's, and yet they come up with another conclusion," said Robert Plomin, a behavioral geneticist at King's College London. "I don't know how this happened."

Scientists have all but given up on finding a smoking gun for most autism cases. Instead, they are looking for multiple risk factors that each have small effects. But the smaller the risk, the more difficult it is to find.

A case in point is another study -- also published online Monday by Archives of General Psychiatry -- that reported a link between anti-depressant use by pregnant women and autism in their children.

(EDITORS: STORY CAN END HERE) Researchers identified 298 children in the Kaiser Permanente health system in Northern California who had been diagnosed with various forms of autism. Then they looked up the prescription records of their mothers and found that 6.7 percent had taken selective serotonin reuptake inhibitors, or SSRIs, while pregnant.

Among a control group of mothers of 1,507 children without an autism diagnosis, 3.3 percent had been on the medications. That led the researchers to conclude that SSRIs taken during pregnancy -- especially during the first trimester, an important time for brain development -- could modestly increase the risk of autism.

Other scientists said it is possible that the depressed mothers may have had some underlying biological condition that both caused their depression and made their children more likely to develop autism. And the authors of the study themselves said the findings should be read cautiously.

"We can't determine causation from one study," said study leader Lisa Croen, an epidemiologist who heads Kaiser's Autism Research Program in Oakland.

She said that the potential risk from SSRIs should be weighed against the risk to the mother of not taking the drugs.