Studying twins helps researchers tease out the causes of autism.

RODALE NEWS, EMMAUS, PA—A large study released this week found that environmental factors are more responsible for autism than previously believed. In fact, environmental exposures are now believed to trigger autism more commonly than genetic factors. Researchers did not investigate which particular environmental exposures are linked to autism, although that is an emerging focus among autism experts. This latest research studied identical and fraternal sets of twins. Twins are important in autism research, because studying them helps researchers separate genetic and environmental factors that may affect autism. Identical twins have the same genes, while fraternal twins share just 50 percent of the same genes, but share the same environment in the womb.

Autism experts not involved in this particular study say the research is well done, and could help further turn the tide of autism research toward recognizing that environmental chemicals and certain drugs could be causes of autism. "It's very credible stuff," says pediatrician and epidemiologist Phil Landrigan, MD, professor and chair of the department of community and preventive medicine at Mount Sinai School of Medicine in New York City. "As new information comes in, it's becoming very clear that there's environmental contribution."

THE DETAILS: The study, published in Archives of General Psychiatry, studied nearly 200 sets of twins, some identical and some fraternal. At least one child in each set of twins had been diagnosed with autism. The researchers determined that fewer than 40 percent of the cases were caused strictly by genetics. Shared environmental factors, including exposures in the womb, were implicated in nearly 60 percent of the cases. A separate study published in the same journal issue found a doubled risk of autism in children whose mothers took SSRI antidepressants a year before giving birth.