Twin study underscores role of genes in autism

The pattern was different among fraternal twins. Among pairs in which at least one was female, when one sibling developed an ASD, the other did 20 percent of the time. That figure was 40 percent when both twins were male.

The findings also go beyond confirming concordance in identical twins' odds of developing an ASD, Law pointed out.

"We show that important characteristics of ASD, such as the type of ASD, level of functioning and presence of other psychiatric disorders are more similar...among identical twins," he said. "Thus not only are they more concordant overall, but the pattern of their disease is more concordant."

The researchers also found that among identical twins, the second sibling was unlikely to be diagnosed with an ASD once a year had passed since the first sibling's diagnosis.

"Basically," Law said, "our data suggests that parents of identical twins can stop worrying after about 12 months have passed since the diagnosis of their first twin."

In contrast, he said, fraternal twins still seem to have "some degree of risk" as much as four years after the first twin is diagnosed.

While experts generally agree that genetics plays a major role in autism spectrum disorders, they also believe that environmental factors conspire with genes to make certain children vulnerable. Researchers are still trying to figure out what those environmental factors are.

(Edit:Ruby)