This week’s study provided more evidence for that nuanced understanding. Using data from a unique resource—the California Autism Twins Study— Hallmayer and his colleagues showed that autism concordance between monozygotic twins is lower than was previously thought. Much more important, autism concordance between dizygotic twins is much higher than previously thought. Because these twins largely share a common environment yet are far less genetically similar to each other than monozygotic twins would be, their concordance in autism symptoms suggests that these common environmental factors play a more important role than had been imagined.
The sheer craftsmanship of this study is noteworthy. The findings are important, too. Contrary to some current misreporting, the findings continue to indicate that genetics play a powerful role in autism. But other things matter, too. Some risk-factors common to fraternal twins—maybe including the simple fact that one is a twin—are important for at least some people diagnosed with these disorders. Analysis of the sort Hallmayer and his colleagues conducted, however, does not address the underlying mechanisms that lead to autism. The weakness and strength of such statistical twin studies lie in their ability to identify genetic patterns within families without directly investigating what is causing these correlations.The study’s limitations, however, have not stopped people from drawing exaggerated conclusions. One National Institute of Mental Health news website asked, “Balance Tips toward Environment as Heritability Ebbs in Autism?” But with similar accuracy, one might have concluded that “New study confirms important heritability of autism disorders.” UPI’s coverage opens with: “Parental age, low birth weight, multiple births and maternal infection during pregnancy may increase the risk of autism spectrum disorder, U.S. researchers say.” Indeed. Yet this study can’t directly address any of these issues. These are simply common influences that face fraternal twins.
Had the observed heritability patterns been much stronger, of course, that would have made it easier to dismiss claims about environmental triggers. Now we know this would be a serious mistake, though we don’t know much more about what these triggers might be. The New York Times writes: “New Study Implicates Environmental Factors in Autism.” True enough. Yet “the environment” is a big place. It includes everything from pollution exposure, to whether a woman was malnourished during pregnancy, to the shape of her placenta, to her own genetic endowments, to whether or not she breastfed or read to her children.
Friday, July 8, 2011
The Media and the Twin Study
At The New Republic, Harold Pollack writes of the twin study emphasizing the importance of "environmental" variables: