By Laurie Tarkan
Reposted from globeandmail.com
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 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.”
“I think we now understand that both genetic and environmental factors have to be taken seriously,” said Joachim Hallmayer, an associate professor of psychiatry and behavioral sciences at Stanford and the lead author of the new study, which is to be published in the November issue of Archives of General Psychiatry.
Other experts have cited factors like parental age, multiple pregnancies, low birth weight, and exposure to medications or maternal infection during pregnancy.
In the study, the largest of its kind among twins, researchers looked at 192 pairs of identical and fraternal twins whose cases were drawn from California databases. At least one twin in each pair had the classic form of autism, which is marked by extreme social withdrawal, communication problems and repetitive behaviors. In many cases, the other twin also had classic autism or a milder “autism spectrum” disorder like Asperger’s syndrome.
The study found that autism or autism spectrum disorders occurred in both children in 77 percent of the male identical twins and in 50 percent of the female identical twins. As expected, the rates among fraternal twins were lower: 31 percent of males and 36 percent of females.
A second article, also released early on the journal’s website, found an elevated risk of autism in children whose mothers took a popular type of antidepressant -- so-called SSRIs like Prozac, Zoloft, Celexa and Lexapro -- during the year before delivery.
Joseph Coyle, editor in chief of the psychiatry journal, called the two studies “game changers.” 
