Another study, also released Monday, found that among twins, the environment plays a bigger role in the development of autism than genetics, a surprising finding in light of previous research.
Three past studies on twins have estimated about 90 percent of the risk of autism is attributable to genes. But researchers led by Dr. Joachim Hallmayer, an associate professor at the Stanford School of Medicine in Palo Alto, Calif., compared 192 sets of twins -- cases in which one had an autism spectrum disorder and the other did not. They looked at both identical and fraternal twins.
"The results suggest that environmental factors common to twins explain about 58 percent of the liability to autism," the authors wrote. "Although genetic factors also play an important role, they are of substantially lower magnitude than estimates from prior twin studies of autism."
But exactly what those environmental factors are, Hallmayer said, is "the multimillion dollar question."
Experts who were not involved with this study say that this research adds to the growing body of literature that suggests non-genetic factors strongly influence the development of autism.
"This seems particularly important as the reports of an increasing incidence of autism would be more difficult to reconcile with an almost purely genetic cause," said Dr. David Beversdorf, an associate professor at the University of Missouri in Columbia. Over the past 30 years, the number of children with autism has increased from about 4 in 10,000 to about 40 in 10,000.
These environmental stressors seem to have the greatest impact while in utero.
"[They] are having an impact during the time that the brain is undergoing its greatest and fastest growth spurt and evolution and has cells at different levels of maturity and immaturity and, therefore, is potentially more sensitive to certain stressors," said Dr. Max Wiznitzer, associate professor of pediatric neurology at Case Western Reserve University School of Medicine in Cleveland. "That is why, in my opinion, the 'environment' is the womb."
While more and more research is contributing to the understanding of where autism fits in the "nature vs. nurture" debate, there is still a lot left to learn about precisely what genetic and environmental factors are and whether or not they interact.
"This is an important area for future research, but our knowledge today does not yet permit assertions as to environmental causes and ways to lessen risk," said Beversdorf.