Genes may be more responsible for autism than previously thought, researchers say, with the developmental disorder traced to heritability in the vast majority of cases.

In a study of all twins born in England and Wales between 1994 and 1996, researchers found that autism is rooted in genetics 74 to 98 percent of the time.

“Our main finding was that the heritability of ASD was high. These results further demonstrate the importance of genetic effects on ASD, despite the dramatic increase in prevalence of the disorder over the last 20 years,” said Beata Tick of King’s College London, the lead author of the study published this month in the journal JAMA Psychiatry.

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For the study, researchers looked at data on more than 6,000 twin pairs, some identical and some fraternal. Extensive in-home evaluations including diagnostic interviews and play-based assessments were conducted on a portion of the twins, with 181 kids ultimately receiving an autism diagnosis.

They found that if one twin had autism, the risk that the other would also be on the spectrum was “significantly higher” in identical twins who share DNA as compared to fraternal twins.

What’s more, researchers found that genetic risk factors for autism are also intertwined with genes that cause people who are otherwise typically developing to exhibit less extreme versions of behavior often associated with those on the spectrum.

“Our findings add weight to the view that ASD represents the extreme manifestation of autistic skills and behaviors seen in the general population,” said Patrick Bolton of King’s College London who worked on the study.