Harvard Mental Health Letter

Autism spectrum disorders revisited

The conventional wisdom has always been that the autism spectrum disorders (ASDs) develop mostly because of genetic rather than environmental factors. Indeed, the ASDs are usually considered among the most "heritable" psychiatric disorders, with studies in twins suggesting that genetic factors account for at least 90% of the risk of developing an ASD — much more than the genetic risk of depression, anxiety, or other psychiatric conditions. Now the largest population study in twins so far has turned the accepted wisdom on its head by suggesting that environmental factors may be more important in the development of ASDs than previously realized. Several other studies have identified possible environmental culprits: the use of selective serotonin reuptake inhibitors (SSRIs) or lack of folic acid early in pregnancy, and a variety of complications near or shortly after giving birth. All of the studies need to be replicated by independent teams — and it's clear that genetic risk still matters — but leading researchers are rethinking what causes ASDs and how to prevent them. The genetic study in particular is a "game changer," says Dr. Joseph Coyle, the Eben S. Draper Chair of Psychiatry and Neuroscience at Harvard Medical School and editor in chief of Archives of General Psychiatry, which published the paper. "For the first time, we have credible evidence that environmental factors may be as important as genetic factors." "This new research is a reminder of just how complex the autism spectrum disorders are," says Dr. Leonard A. Rappaport, the Mary Deming Scott Professor of Pediatrics at Harvard Medical School and chief of the Division of Developmental Medicine at Children's Hospital Boston. "It's likely that multiple genetic vulnerabilities are interacting with multiple environmental factors all at the same time. The picture that is emerging is still blurry, and it's full of moving targets."
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