A study of twins in California downgrades the role genes play in autism, but the work doesn’t hold water with some autism researchers.
Previous studies indicated that autism spectrum disorders are due mainly to genes. But the new study suggests that environment — including conditions in the womb, age of parents and other factors — may account for a greater fraction of the risk of developing autism spectrum disorders.
“People are, more and more, recognizing that autism is a complex disorder that would be hard to explain with genes alone,” says study coauthor Joachim Hallmayer, a psychiatric geneticist at Stanford University.
But some researchers question whether the new estimates accurately reflect the contributions of genes and environment to autism. “When somebody gets a totally different answer from what anyone else has seen, you need to see it a few more times before you believe it,” says Susan Folstein, a child psychiatrist at the University of Miami in Florida.
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Other studies estimated genetic heritability of autism to be as high as 90 percent, meaning that genetic factors account for the vast majority of variables contributing to the development of autism. But the new study suggests that genetic heritability accounts for less than half of the variation in risk of classical autism and other autism spectrum disorders, such as Asperger syndrome. Shared environmental factors are responsible for 55 percent of autism and 58 percent of autism spectrum disorders, researchers report online July 4 in the Archives of General Psychiatry.
But the amount of uncertainty in the new calculations makes the researchers’ interpretation suspect, Folstein says. The study’s estimate of the environmental influence on autism ranges between 9 and 81 percent, and the genetic contribution ranges from 8 to 84 percent. With that amount of overlap between the estimates, “it’s hard to put much confidence in their claims that they are different,” Folstein says, whose 1977 twin study found that autism has a large genetic component.
Before that study, autism was often blamed on bad parenting and cold, withdrawn “refrigerator mothers.” Folstein fears that the new study will cause a resurgence in that attitude. “We just lost the battle again. It’s ‘all the mother’s fault,’ ” she says.
Hallmayer and his colleagues contacted families with twins born in California between 1987 and 2004 in which at least one twin had autism or an autism spectrum disorder. The team found that when one identical twin boy had an autism spectrum disorder, 77 percent of the time the twin brother also did. Since identical twins share the same genes, the rate was not surprising.
Where the new study differs from previous ones was among fraternal twins — children born together but who are not more genetically similar than siblings born at different times. In fraternal twin boys, if one twin had an autism spectrum disorder, 31 percent of the time both did. Previous studies had pegged the percentage of fraternal twins in which both had autism — known as the concordance rate — at zero, “which made no sense,” says Thomas Insel, director of the U.S. National Institute of Mental Health. The higher concordance rate among fraternal twins in the new study is probably closer to the truth and indicates that the environment the twins share is important in determining who gets autism, he says.
But the concordance rate is exactly what has Folstein and other researchers questioning the study. A total of 1,156 pairs of twins fit the criteria for the study, but the researchers ended up assessing only 202 pairs. Although the study is large by twin-study standards, the low participation rate could be a problem. “That is too low of a response rate to marketing survey research, much less an epidemiological study,” says Edwin Cook, a child psychiatrist at the University of Illinois at Chicago.
Families in which both twins have autism may be more likely to participate in such research than families in which only one twin is affected. A bias toward families with two affected twins would throw off the calculations and make the environmental contribution seem bigger than it actually is. And since other families declined to participate, there’s no way to determine if the study sample accurately reflects the larger twin population of California, Cook says.