It seems every couple of weeks a new headline pops up claiming to have found the cause of autism, whether it's air pollution, a viral infection, or the wholly disproven MMR vaccine claim.
But the largest study of its kind, involving more than 2 million children across five countries, has reaffirmed that genetics is actually the biggest risk factor in the development of autism spectrum disorder (ASD).
Reporting in JAMA Psychiatry, the researchers found that autism spectrum disorder is 81 percent reliant on inherited genes, while environmental factors account for less than 20 percent of the risk. Maternal factors – such as the mother's weight, whether she has polycystic ovary syndrome, if the baby is born by caesarian section, etc. – were found to have a "nonexistent or minimal” impact on the development of ASD.
To reach these conclusions, the study looked at the medical records of more than 2 million children born in Denmark, Finland, Sweden, Israel, and Western Australia between 1998 and 2011. The international team of researchers followed the subjects until they reached age 16, by which time over 22,000 were diagnosed with ASD.
As is often the case, the reality of the condition is a lot more complex than it first seems. ASD can never be attributed to one sole cause and is most likely the result of a number of interconnected factors. This means that environmental factors can still play a highly influential role, even if researchers still aren’t certain of the most significant environmental risk factors just yet (although, once again, it is categorically not vaccines). Equally, scientists aren't fully aware of the specific gene interactions that can contribute to ASD, although there's a growing amount of research looking into this.
“Although families are often most concerned about environmental risk factors for autism, the reality is that genetic factors play a much larger role overall," Dr Andrew Adesman, director of developmental and behavioral pediatrics at Cohen Children's Medical Center in New Hyde Park, who was not involved in the study, told Health Day.
"Environmental factors also play a smaller, but important, role," he added. "This does not mean that we can completely ignore the environmental risk factors and their interaction with the genetic risk factors."
This is not the only time research has pointed towards the conclusion that autism can be largely explained by genetics. Research on twins has proved especially useful for determining the heritability of ASD, with one twin study in 2016 suggesting that the development of ASD is 64 to 91 percent down to genetics.