A new and controversial study published July 4 in the Archives of General Psychiatry says that environment may indeed trump lineage in determining whether a child might develop autism.

The authors based their findings on a study of over 190 pairs of identical and fraternal twins in which at least one twin met the definition for the developmental disorder.

Their conclusion, that exposure to chemicals, disease, diet, or possibly stress could be at the root of autism, has not been well received by other experts.

“We know that autism has a genetic background because it runs in families,” says Carol Marchetto, Ph.D. “But the specific genes have not been identified.” Marchetto is a researcher in the Laboratory of Genetics at the Salk Institute in La Jolla. She says that cause-and-effect relationships from environmental factors have been demonstrated in other diseases such as Parkinson’s, but not yet in autism.

“I don’t think one can say it’s one or the other, or both genetic and environmental effects together,” says Marchetto. “I think the jury is still out. I think the more research the better, right?”

No one knows what causes autism. It affects one in every 100 children, and roughly half of those affected will be boys. According to the National Institutes of Health, more children will be diagnosed in the coming year with autism than with AIDS, diabetes, and cancer combined. Autism is a lifelong diagnosis and has been called the fastest growing developmental disability in America. And yet, autism receives less than 5% of the research funding of many less prevalent childhood diseases.

Currently, there is no cure for autism.

For years, autism was blamed on a myriad of causes, some of them as far-fetched as bad parenting or even climate. Researchers have considered pre-natal risk factors such as mother’s age or exposure to household cleansers, food additives, or exposure to microwave radiation. Childhood vaccinations took on witch hunt proportions among public critics, even though the link to autism was never proven scientifically. New cases continued to appear long after the assumed culprit, a low-cost mercury-based preservative used in most vaccines called thimerosal had been banned in the U.S.

“It’s more like a multi-genetic disease, we believe,” says Marchetto. “There are likely a number of mutations that are hitting a number of pathways. That’s why it’s harder to pinpoint exactly what is happening.”

Dr. Marchetto was part of a recent landmark study between the Salk Institute for Biological Studies and the University of California, San Diego. Human stem cells were used to replicate autism in the lab for the first time ever in an attempt to study the origins and effects of the disease. “On top of that,” she says, “we were able to test a few drugs that would ameliorate the behavior.”

In the last two decades there has been a more than 600% increase in diagnoses, say experts. Does that mean autism is on the rise? “Some people think that the diagnostic tools are getting better, and some think [autism] is growing,” says Marchetto. “But nonetheless, one in a hundred kids is diagnosed as being within in the autism spectrum. So regardless if it is diagnostic or an actual increase, we are at a high point right now.”