A study published Monday conducted by the University of California at San Francisco and Stanford University found that environment plays a much larger role in autism than was previously thought. The study found that genetics accounts for 38 percent of the risk of autism, while environmental factors account for about 62 percent. Previously, researchers had believed that genetics accounted for roughly 90 percent of all the cases worldwide. Researchers looked at 192 pairs of twins in California, and used a mathematical model to determine the results. Twin studies are often used to distinguish between environmental and genetic influences on medical disorders, because identical twins share nearly 100 percent of their genes and fraternal twins share about 50 percent of their genes. Another study, published Monday in the Archives of General Psychiatry, looked at the use of antidepressants by mothers, and found a twofold increase of the disorder when mothers took antidepressants at some point in the year before giving birth.