Several small studies conducted over the last three decades have found that it is much more common for identical twins to be diagnosed with autism than it is for fraternal twins.
That’s led to the belief that the lion’s share of the odds of developing autism is written into a person’s DNA, with a much smaller percentage coming from something in the environment.
The new study, which is published in the Archives of General Psychiatry, found that autism was surprisingly common in fraternal twins, despite the fact that they don’t share as many of the same genes as identical twins, suggesting that something in their mutual life circumstances may be playing at least as strong a role as genetics.
“There are lots of neuroscience papers that begin ‘Autism is one of the most heritable conditions in psychiatric genetics and shows over 90% heritability...’ and I don’t think people should start their papers that way anymore,” says Harold Hill Goldsmith, PhD, a professor of psychology at the University of Wisconsin at Madison.
Goldsmith is also studying autism in twins, but he was not involved in the current research.
He says that this new study, from researchers in California, is one of four new investigations of autism in twins -- there are two studies in the U.S. and two in Europe -- that appear to temper the role of genetics in the disease, though he says genes are still a key factor.
Researchers say their findings were unexpected.
“It was a surprise, definitely,” says study researcher Joachim Hallmayer, MD, an associate professor of psychiatry at Stanford University in California.
“It looks like some shared environmental factors play a role in autism, and the study really points toward factors that are early in life that affect the development of the child,” Hallmayer says.
“We have to look also at environmental factors, and from my point of view, the interaction between the genetic factors and the environmental factors,” Hallmayer says.
The study was funded by the National Institute of Mental Health and the nonprofit advocacy group Autism Speaks.
Twins and Autism Risk
For the study, researchers culled a California registry of children who receive services for developmental disabilities and identified 192 sets of twins in which at least one child had received a diagnosis of autism or other autism spectrum disorder, like Asperger’s syndrome. There were 54 pairs of identical twins in the study and 138 pairs of fraternal twins.
Researchers used standard psychological assessments to confirm the children’s diagnoses.
To calculate the relative contributions of genes and the environment to autism risk, researchers compared the number of identical and fraternal twins in which both siblings had been diagnosed with the disorder.
Because twins share nearly all aspects of family life and environment, comparing identical and fraternal siblings allowed researchers to see how autism rates differed when the only variable that was different was the number of shared genes.
As expected, identical twins were more likely to share an autism diagnosis than fraternal twins.
What was surprising, though, was that both fraternal twins were diagnosed with autism more often than could be explained by genetics alone, suggesting that common environment factors were at work.
Researchers say that based on their models, environment may account for about 58% of the risk for autism spectrum disorders, with genetics accounting for about 38%.
Independent experts, however, point out that the ranges used to compute those averages are very large. For example, the range of autism explained by environmental factors, the study shows, is anywhere between 9% and 81%. Genetics risk ranges from 8% to 84%.
“There’s a big range of confidence on those statistics,” Goldsmith says. “There’s no precision to those estimates.”
Other experts agree that it would be a mistake to think of the numbers too literally.
“I don’t think this is a revolution in thought,” says Gary W. Goldstein, MD, president and CEO of the Kennedy Krieger Institute in Baltimore.
Goldstein is on the board of Autism Speaks but was not involved in the current research.
“Having said that,” Goldstein says, “Everyone in this field understands that genetics doesn’t explain this fully and there’s going to be an environmental component to triggering vulnerable people or maybe even causing it in some people without a vulnerability gene.”