For decades, researchers and parents have searched for clues about what causes autism spectrum disorder (ASD), which now affects about 1 out of every 59 children in the United States
Now, a sweeping new study looking at more more than 2 million people from five countries (Denmark, Finland, Sweden, Israel and Australia) found that about 80% of an individual's risk of developing autism comes from inherited genetic factors and not from "environmental"
"Everywhere we looked, in five different samples, what we saw was that genetic factors were most important," study author Sven Sandin, a statistician and epidemiologist with the Karolinska Institutet in Sweden, told HuffPost in an email.
That might sound surprising to mothers and fathers who for years have read the story after stating possible risk factors from the air pollution to the overwhelmingly discredited (but lasting) vaccine hypothesis. And, indeed, experts say the media and public have been disproportionately focused on the role of modifiable contributors to autism, when genetics are the major driving force
Genes are the biggest risk factor, but there are still lots of questions about which ones.
In a new study published in JAMA Psychiatry on Wednesday, Sandin and his co-authors used models to analyze population data from five countries that included more than 2 million people, more than 22,000 of whom had been diagnosed with autism. Looking at the outcomes among family members and weighing them against factors such as shared environments and their specific genetic connections led to their conclusion that inherited genes account for about 80% of the risk of autism in children with disorder
That's pretty much in line with similar recent studies that have suggested genes are the major contributing factor to ASD. "What is immense about this study is the number of children across the world who were [included] ̵
Of course, researchers have known for decades that genetics contribute to ASD. But now that they are grappling with just how significant a factor heredity is, there is mounting pressure to determine which specific genes contribute in which specific ways
And there is now a question mark
"There is a lot of work that still needs to be done, "Sandin told HuffPost. "We still do not know which specific genes contribute to risk. Also, there are many potential environmental factors that could be related to ASD either directly or acting together with genes. We have, so far, only been scratching the surface. "
That does not mean modifiable factors
" If I was going to walk away after reading this study and talk to a family … I'd say, 'OK, we still do not know the other 20%. It is fairly controversial, what makes up the modifiable part, "Swanson said.
Although they are often called" environmental "influences in the scientific literature, these factors go beyond the possible effects of chemical exposures on a person's likelihood of developing autism. Researchers are exploring the possible role that everything from diet to infection during pregnancy might play. For now, the studies are ongoing. "There is no much that autism researchers and pediatricians can say in the way," If you avoid XYZ, you will reduce your child's risk of developing autism. "
In an editorial that accompanied the new study, a team of three autism and psychiatry experts said the media has focused disproportionately on those environmental factors, perhaps because they are modifiable. An understandable pull exists to focus on the things that parents might be able to control to a certain extent. But that may have contributed to an incorrectly skewed sense of how much nature versus nurture contributes to autism risk
"We have gone wrong to pointing fingers at things that do not cause autism," Swanson said. "Look at where we are right now with the vaccine rubella vaccine and look at where we are with outbreaks because there has been erroneous, falsified, misinformation that for decades led parents to believe vaccines were causing autism spectrum disorders." 
Expectant parents who know that they have a family history of autism may want to consider talking with a genetic counselor if they have concerns, Swanson said.
Being aware of family history and how it might contribute to childhood autism risk can also help parents and pediatricians be mindful of early signs of autism as they arise. Research clearly shows that early interventions, which can begin as early as age 2, can help improve physical, emotional, and communication skills
"On some level, I feel like we should feel comforted by [these findings]. Because it's almost like autism is explained … it's not because of what a parent is doing right or wrong, "Swanson said. "It's largely based on a child's genetic makeup."