Autism: Environmental Factors Messing With Our Kids? (STUDY)

The debate about what causes autism continues, with two new studies suggesting there might be more of an environmental component at play than previously thought.

The first, published online Monday in the Archives of General Psychiatry, suggests that at least half of what the authors call "liability to autism" might be explained by environmental factors.

Researchers considered more than 50 sets of identical twins and 130 sets of fraternal twins, in which at least one child had a diagnosis of strict autism or Autism Spectrum Disorder (ASD). Relying on both parental reports and direct observation, they then used existing twin models, which rely on the degrees of shared genetics among fraternal and identical twins to determine how much certain factors are associated with autism risk. They found that only 40 percent of the risk of autism development was owed to genetic heritability, while 55 percent was linked to environmental factors.

"The take home message is that we really have to take more seriously the environmental factors, and how these come together with the genetic factors to play a role in the etiology of autism," said Dr. Joachim Hallmayer, an associate professor of psychiatry and behavioral science at the Stanford School of Medicine. Environmental factors, he explained, could include anything from a virus to drugs taken during pregnancy.

Medication during pregnancy was the subject of a second study published in the same journal, which suggests that prenatal exposure to certain antidepressants may "modestly" increase the risk of ASD development.

To reach that conclusion, researchers looked at medical records of nearly 300 California-based children and their mothers, comparing them with a control group of more than 1,500 kids and moms.

They found a twofold increased risk of ASD associated with selective serotonin reuptake inhibitors (SSRIs) when mothers took the drugs in the year before giving birth. The earlier a woman took SSRIs, the more pronounced the effect: Researchers reported a threefold increased risk of ASD among women who took SSRIs during their first trimester of pregnancy.

"These two studies are interesting because they're kind of a paradigm shifter in finding there is a much lower genetic contribution [to autism and ASD development]," said Lisa Croen, PhD, director of the Kaiser Permanente Autism Research Program and the second study's lead author. "There is a much larger environmental component at play here than has been understood up to this point," she added. Croen was also an author on the twin pairs study.

She cautioned, however, that people need to be "extremely cautious" about the findings, particularly with regards to autism risk and antidepressant exposure. The new study is only the first to find this possible association, she said, and is not intended to scare people into dropping their treatment.

Dr. Don Mordecai, Director of Mental Health at Kaiser Permanente's San Jose Medical Center added that the findings in no way represented a "smoking gun study." At most, he suggested the study should increase caution in pregnant women who, in concert with their physicians, are weighing the relative risks of continuing taking antidepressant medications versus the risk of their children developing ASD.

"This is always a really hard decision for women," he said. "I worry about people saying 'I'm going to eliminate all these risks and stop treatment.' For women with serious depression and higher anxiety disorders, there are many risks associated with not seeking treatment, too. It's about coming to a conclusion with your physician that's best for you."

WATCH: "Unraveling The Cause Of Autism"