By Stephanie Stahl

PHILADELPHIA (CBS)– New research says a person’s likelihood of having autism is largely based on their inherited genes.

Scientists still aren’t sure exactly what causes autism spectrum disorders. Many feel it’s probably some combination of family history and environmental factors like being exposed to toxins.

The new research says genetics are a much bigger factor than previously thought.

“They showed by this thought experiment, this calculation experiment, that a lot of autism–83 percent–is due to some type of genetic cause,” said Dr. Charis Engc, with the Cleveland Clinic.

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In the largest study of its kind, Harvard researchers analyzed records of more  3.1 million children and found a strange pattern of recurrence based on gender.

Having an older female child with autism increased the risk for younger siblings also having it, especially for boys.

“Taking a family health-history would be important, because if there’s already someone with autism spectrum disorder, and the study is correct, than the likelihood is higher. It’s not a lot higher, so people should not panic, but it just gives an indication,” said Eng.

Researchers say they have not identified specific genes that cause autism and at this point they can’t explain how or why gender would be involved.

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Autism affects about 1 in 68 children in the United States, four times more boys than girls.

Doctors say early diagnosis and intervention is important to help offset symptoms that often include difficulty with social and communication skills.

Parents say they usually start seeing signs of autism around age 2.

Stephanie Stahl