The heritability of autism is well-established but not ironclad. The current debate continues to center on how large a role genetics plays in the progress of this developmental disorder characterized by difficulties with social interaction and communication, and by restricted and repetitive behavior.

The consensus among mainstream autism researchers, however, is that genetic factors predominate. Genetic factors might also be the most significant cause for Autism Spectrum Disorder (ASD).

ASD is consist of a range of mental disorders of the neurodevelopmental type. It includes autism and Asperger syndrome.

But despite the strong links to heritability, it’s been found that most cases of ASD occur sporadically with no recent evidence of family history. This quirk has led some scientists to argue spontaneous de novo mutations in the father's sperm or mother's egg contribute to the likelihood of developing autism.

The debate over the real causes of autism has gotten lively but now seems to have been settled on the side of genetics.

The largest ASD study of its kind involving more than 2 million people across five countries found that (ASD) is 80 percent reliant on inherited genes. The corollary to this is that environmental causes (such as mutations) are responsible for just 20 percent of the risk.

The study was based on the fact that the origins and development of ASD remain unresolved. Its authors noted no individual-level study has provided estimates of additive genetic, maternal, and environmental effects in ASD across several countries.

The objective of the study was to estimate the additive genetic, maternal, and environmental effects in ASD.

The study was a population-based, multinational cohort study including full birth cohorts of children from Denmark, Finland, Sweden, Israel and Western Australia born between January 1, 1998 and December 31, 2011. Participants were followed up to 16 years of age. Data was analyzed from September 23, 2016 to February 4, 2018.

The analytic sample included 2,001,631 individuals, of whom 1,027,546 (51.3 percent) were male. Of the total participants, 22,156 were diagnosed with ASD.

Data showed that 80.8 percent of the participants’ risk of developing the condition was due to genetics. The balance of the risk was due to as-yet-unidentified environmental causes. Only a miniscule risk, about 1 percent, was due to maternal factors.

Autism Rising
Autism is "difficult to track," given the lack of medical tests to diagnose the disorder, according to Michael Kogan, author of the new report. Kelly Sikkema/Unsplash

Researchers noted the new numbers almost corroborate those from prior, smaller studies on ASD, bolstering the validity of the new and more extensive study.

The new study "provides stronger evidence that autism is mostly due to genetic, and not environmental, factors,” said Dr. Andrew Adesman, director of developmental and behavioral pediatrics at Cohen Children's Medical Center in New Hyde Park, N.Y.

"Although families are often most concerned about environmental risk factors for autism, the reality is that genetic factors play a much larger role overall," he said.

Dr. Adesman noted environmental factors also play a smaller but important role in the development of ASD, so "this does not mean that we can completely ignore the environmental risk factors and their interaction with the genetic risk factors."