By Lisa Rapaport

(Reuters Health) – Most of the variation in risk for autism may be explained by genetics rather than differences in environmental factors like lifestyle or community characteristics or certain pregnancy outcomes, a study in five countries suggests.

Autism spectrum disorder has long been thought to involve inherited genetic traits, and some previous research also suggest that some non-inherited traits and maternal characteristics may also play a role. For the current study, researchers examined data on births from 1998 to 2007 in Denmark, Finland, Sweden, Israel, and Western Australia.

Overall, about 80% of the variation in risk for autism was linked to inherited genetic traits, although this ranged from 51% in Finland to almost 87% in Israel.

“The results show that genetic factors are most important (about 80% of the variation in risk) but that the environment also play a role,” said Sven Sandin, senior author of the study and a researcher at the Karolinska Institute in Stockholm, Sweden

Previous studies have found having a sibling diagnose with autism increases the risk by a factor of 10 to 14, Sandin said by email. And having a cousin with autism doubles the risk.

But knowing genetics influence the risk doesn’t necessarily help parents prevent autism in their own families, Sandin said.

“Since our results do not tell us anything about specific risk factors or specific genes, there is not much parents can learn to influence the risk of developing autism,” Sandin said.

The current study included more than 2 million children from more than 680,000 families who were followed until they reached 16 years old. Slightly more than 22,000 kids were diagnosed with autism.

Nonshared environmental factors – circumstances that are different for different siblings – explained up to about 27% of the variation in autism risk, the study found.

So-called maternal effects – such as a preterm delivery or certain medical issues during pregnancy – didn’t appear to explain variation in autism risk, researchers report in JAMA Psychiatry.

The study wasn’t a controlled experiment designed to prove whether or how genetics or other factors might directly cause autism. And, even though it was large, it didn’t contain many kids with autism.

“These results do not change what we do now in terms of autism prevention or treatment, but they do suggest that we should think about genetic testing, both using the technology that we have available now and using approaches that may improve over the next decade,” said Dr. Jeremy Veenstra-VanderWeele, coauthor of an editorial accompanying the study and a psychiatrist at the Center for Autism and the Developing Brain at NewYork-Presbyterian Hospital in White Plains.

“It is possible that a better understanding of genetic risk will allow us to predict risk before diagnosis in the future, potentially allowing us to intervene before a diagnosis has been made,” Veenstra-VanderWeele said by email. “At this point, however, these results are important to drive new research, rather than to change what we do in the clinic at this point in time.”

SOURCE: JAMA Psychiatry, online July 17, 2019.