What plays a bigger role in autism, genetics or environment? Scientists don't agree on the answer, but the debate just got more attention with the arrival of a new study involving twins. When you look at "extreme" autism symptoms, genetics plays almost the only role, according to that study, led by clinical psychologist Thomas W. Frazier II of the Cleveland Clinic.1

That conclusion in favor of genetics differs markedly from a 2011 twin study that found that the environment was a significant influence. In medical research, the environment usually refers to everything from the air you breathe to your parents' ages when you were conceived and your prenatal conditions. The earlier study found that fraternal twins, who shared the same environment before birth but have different DNA, were more likely to both have an autism spectrum disorder (ASD) than siblings who weren't twins. That study look at 192 pairs of twins in California.2

Dr. Frazier had questions about the California study, so he decided to launch his own autism twins study, the largest of its kind so far. His team analyzed information about 568 pairs of identical and fraternal twins in the Interactive Autism Network (IAN) database.1

Identical twins began as one fertilized egg, so they have virtually all the same genes. Fraternal twins share about half of their genes and are as genetically similar as any two siblings. But because twins shared a womb, they are more likely to have had the same environmental influences before birth than non-twin siblings.

Many studies analyze whether a set of twins both have an autism diagnosis or not. Dr. Frazier's team did too. If one identical twin has autism spectrum disorder (ASD), the other twin has a 76 percent chance of also being diagnosed with it. The numbers are lower for fraternal twins. The percentage of fraternal twins who each share an ASD diagnosis is 34 percent for same-sex twins and 18 percent for boy-girl pairs, Dr. Frazier said. (Girls are less likely to be diagnosed with autism than boys). Those statistics are similar to other twin studies, including the California one.2, 3

Where the new study differs is that it goes on to analyze the severity of autism symptoms. To do so, researchers used the twins' scores on questionnaires measuring social communication skills, and repetitive behaviors and obsessive interests. People with autism typically have particular problems in those areas of communication and behavior.

Some researchers have assumed that autism is an extreme version of normal behaviors, just as Attention Deficit Disorder might be an extreme version of a normal tendency for someone's attention to wander. Dr. Frazier said.

Dr. Frazier's team, however, came to a different conclusion. "Without making assumptions, we found evidence that autism represents a set of behaviors outside the norm that are strongly genetically determined, with the genetic component likely being major gene effects, which could involve different genes across people," he said.

Using a mathematical analysis of data, his team found that identical twins were much more likely to have similar levels of autistic symptoms than fraternal twins. Those researchers also concluded that:

High levels of autism symptoms are genetic in origin. Less severe symptoms are not as likely to be inherited.

A shared environment played no "significant" role in the development of extreme autism symptoms in twins.

Problems in two separate areas – social communication skills and repetitive behaviors – are driven by the same gene or genes.

This study has not settled the long-running debate over genetic versus environmental factors in autism. Dr. Frazier said the environment may be causing changes in genes that contribute to autism. "But it will be easier to find the environmental trigger to autism if we know the genes. If you really believe the environment is involved in autism, then you should be keen on genetic research," he said.