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Autism is undeniably influenced by genes, but a new study suggests that environmental factors may also contribute significantly — more than researchers previously thought — to the developmental disorder. In fact, environmental factors may play at least as big a role as genes in causing autism.

Dr. Joachim Hallmayer, a psychiatrist at the Stanford University School of Medicine, and his team report online in the Archives of General Psychiatry that shared environmental influences may account for as much as 55% of autism risk, while less than 40% can be attributed to genes.

The study modeled risk, but did not specify which environmental factors were at play. But other research has implicated increasing maternal and paternal age, low birth weight, multiple pregnancies and any medications or infections to which an expectant mom is exposed during pregnancy.

Autism, which affects at least 1% of children, is a complex disorder, so it’s no surprise that both environmental and genetic factors contribute to its development. But in recent years, experts have focused intensively on the genetic components of autism; with the availability of more sophisticated tools to analyze genetic changes and development of disease, researchers have identified important clues about autism’s roots in DNA.

But the rise in autism spectrum disorders has occurred too quickly to be explained fully by genes. And scientists know that genetic changes don’t occur in a vacuum. Such aberrations, combined with non-genetic factors, may offer a fuller picture of what causes the disorder.

To determine how much either factor may contribute to autism, Hallmayer’s group analyzed identical and fraternal twins, in which either one or both were diagnosed with autism or an autism spectrum disorder. Identical twins share identical genetic makeup, while fraternal twins are only as genetically similar as any two siblings. So by comparing the prevalence of autism between the two groups, the scientists were able to determine with relative assurance how much genes and shared environment contributed to the twins’ conditions.

The study found that the likelihood of both twins being affected by autism was higher among identical than fraternal twins. That suggests that genetics plays a key role in the disorder. But importantly, the chance of both twins being affected by autism was not low among fraternal twins, which is counter to what would be expected if genetics were the dominant factor.

The study also found that autism rates among both identical and fraternal twins were higher than in the general population. That further suggests that environmental factors, probably shared by the twins as early as in the womb, contribute significantly to causing the disorder. “The fact that both groups have elevated rates suggests that something is making the two groups of twins similar to each other,” says Neil Risch, director of the Institute for Human Genetics at University of California San Francisco and senior author on the paper. “Whether it occurs in utero, during childbirth or soon thereafter, we can’t differentiate. But it suggests that something environmental is causing the twins to be alike.”

Risch notes that the results do not discount genetic factors by any means. “It’s not either-or in terms of genetics or environment,” he says. “We’re not saying autism isn’t genetic, because the huge majority of twins don’t have autism. Obviously something is priming the risk, and it looks like that may be a genetic predisposition. So a genetic base and environmental factors together may explain autism better.”

The risk in twins with a genetic vulnerability may be triggered by being a multiple, for instance; something about the more crowded uterine environment may contribute to a greater chance of developing the disorder, Risch notes.

The good news is that as researchers better understand the environmental factors that are responsible for autism, the more some of these factors may be modified to help lower the risk of the disorder. A fuller picture of the spectrum of both genetic and non-genetic contributors to autism may also help lead to more effective ways to treat it.