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 on 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.”

As recently as a few decades ago, psychiatrists thought autism was caused by a lack of maternal warmth. And while 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 faster than our genes can evolve.