Environmental factors may be more important than previously thought in determining whether a child develops autism, says a study of twins published online in July by the Archives of General Psychiatry. The findings shake up prior assumptions that genetics were largely the culprit behind the neurodevelopmental disorder, now estimated to affect 1% of the population. The study, believed to be the largest of its kind, looked at 192 pairs of identical and fraternal twins, in which at least one child in each pair had strict autism or a milder autism spectrum disorder. Researchers drew on records from the California Dept. of Developmental Services. They found that shared environmental factors -- prenatal and early postnatal experiences common to both twins, such as parental age, low birth weight or multiple births -- accounted for 55% of strict autism and 58% of more broadly defined ASD cases. Genetic heritability accounted for 37% of autism and 38% of ASD cases. By contrast, earlier studies found that autism could be attributed to genetic heritability 90% of the time, leading many to focus only on genetic causes. The latest research "clearly shows that genetics do play a role, but they are not the whole story," said Joachim Hallmayer, MD, lead study author and an associate professor of psychiatry and behavioral sciences at Stanford University School of Medicine in Palo Alto, Calif. "Autism is not a simple disorder, and we have to better understand both the genetic and environmental factors and the interaction between them." Dr. Hallmayer acknowledged that the findings were subject to a wide margin of error, suggesting that a shared environment may not necessarily be more important than genetics, as the numbers appear to indicate. Nevertheless, it remains a substantially influential factor, he said. Underscoring that finding was the fact that concordance rates -- both twins having the disorder -- among fraternal twins, who share only half of their genes, were much higher than in prior studies, Dr. Hallmayer said. The California database also enabled researchers to look at a more diverse population. The analysis is the first to use the latest autism diagnostic techniques, which require not only interviews with the parents, but also direct observation of the child. "These new findings are in line with other recent observations supporting both environmental and genetic contributions to ASD, with the environmental factors likely prenatal and the genetic factors highly complex," said National Institute of Mental Health Director Thomas R. Insel, MD. Researchers have yet to find a "smoking gun" in terms of environmental causes of autism, Dr. Hallmayer said. But a separate Archives of General Psychiatry study, also released online in July, may provide yet another hint. Researchers linked prenatal exposure to certain antidepressant medications known as selective serotonin reuptake inhibitors, especially during the first trimester, to a modest increase in the risk of developing an autism spectrum disorder. They found that mothers of children with ASD were twice as likely to have at least one antidepressant prescription in the year prior to delivery. Those women were more than twice as likely to have a child later diagnosed with autism, compared with women with no antidepressant prescription during the study period. But authors cautioned that the findings were only preliminary and that additional studies are needed. The full and original article can be found at: http://www.ama-assn.org/amednews/2011/07/18/prse0722.htm