An infant’s environment may be just as important as genetics in determining the risk of developing autism, according to a new study.
The study, from Stanford and the University of California-San Francisco, looked at 192 pairs of twins, both identical and fraternal, and used a mathematical model to project that environmental factors accounted for as much as 55 percent of the risk of developing the disorder.
Over the past decade, autism research has focused heavily on the potential genetic causes of the condition. But the new study suggests that a child’s environment plays a much more significant role than scientists once thought.
"We're not trying to say there isn't a genetic component—quite the opposite,” Neil Risch, the director of the UCSF Institute for Human Genetics and the designer of the study, told the San Francisco Chronicle. “But for most individuals with autism spectrum disorder, it's not simply a genetic cause."
Twins are helpful to researchers studying heritable disorders like autism because they often highlight the roles genetics and the environment play in a child’s development. Identical twins share nearly 100 percent of their genes, and fraternal twins share about half their genes. If raised together, both kinds of twins are usually exposed to similar environments during childhood.
If autism were entirely genetic, the presence of the disorder in one identical twin would nearly guarantee its presence in the other twin. But the results of the study showed that if one identical twin had autism, the chance of the other twin having the disorder was about 60 percent to 70 percent.
Fraternal twins, meanwhile, shared an autism diagnosis between 20 percent and 30 percent of the time, a number higher than the researchers had anticipated. Fraternal twins share no more DNA than any other siblings, but they do share the same womb, meaning that could play a role.
Dr. John Constantino, a professor of psychiatry at the Washington University School of Medicine in St. Louis, who wasn't involved in the new research, tells the Associated Press that that finding "puts a spotlight on pregnancy as a time when environmental factors might exert their effects." The possible non-genetic factors identified by the study include parental age, low birth weight, multiple births and maternal infections during pregnancy.
A second study also points to the medications a pregnant woman takes as a possible environmental trigger. The study showed that mothers who took antidepressants during the year before birth, and particularly in the pregnancy's first trimester, were more likely to give birth to autistic children. Although scientists stress that it is too early to advise pregnant woman not to take antidepressants.
Scientists emphasize that autism is very likely the result of a combination of genetic and environmental factors, and they made clear that future research should attempt to pinpoint the disorder's environmental triggers.