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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