NEW YORK (Reuters Health) - When one identical twin develops the developmental disorder autism, the risk of the other developing it is high -- substantially higher than it is for fraternal twins, a new study confirms.

The study, which gathered information from 277 twin pairs in which at least one had an autistic disorder, found that when one identical twin developed an autistic disorder, the other one also did 88 percent of the time.

That compared with 31 percent among fraternal twins. Unlike identical twins, fraternal twins are no more genetically similar than non-twin siblings.

What’s more, researchers found, identical twins also had greater similarities in the form of autism that they developed, their level of day-to-day functioning and the risk of intellectual impairment.

The findings, reported in the Archives of Pediatrics & Adolescent Medicine, confirm the importance of genes in autism development.

Autism spectrum disorders (ASDs) include several developmental brain disorders that hinder a person’s ability to communicate and interact socially. ASDs range from the more-severe cases of “classic” autism to Asperger’s syndrome -- where a person has normal intelligence and verbal skills, but difficulty socializing and understanding subtler forms of communication, like body language and vocal tone.

“Autism research has been guided by one important observation for the past several decades - that autism has a large genetic component,” Dr. Paul Law, of the Kennedy Krieger Institute in Baltimore, told Reuters Health in an email. “That observation was made through twin studies.”

This new study confirms those findings using a much larger sample of twins, according to Law. The data come from an online registry called the Interactive Autism Network, which Kennedy Krieger set up two years ago to connect parents of children with autism with researchers.

Of the twin pairs in the current study, 67 were identical and 210 were fraternal. Among identical twins, all females had been diagnosed with an ASD, whereas the “concordance” was 86 percent among males.

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.

SOURCE: Archives of Pediatrics & Adolescent Medicine, October 2009.