A rigorous study of nearly 400 twins has shown that
environmental factors have been underestimated, and genetics
overestimated, for their roles in autism-spectrum disorders. The
study found that the children’s environment represents more than
half of the susceptibility — 55 percent in the most severe form
of autism and 58 percent in the broad spectrum of the disorder —
while genetics is involved in 37 percent and 38 percent of the
risk, respectively. This is the largest and most rigorous twin
study to date to research the genetic component of the disorder,
and tapped expertise from the UCSF Institute for Human Genetics
and Stanford University — which jointly led the research — as
well as from Kaiser Permanente, UC Davis, the Autism Genetic
Research Exchange and the California Department of Public
Health. “Autism had been thought to be the most heritable of all
neurodevelopmental disorders, with a few small twin studies
suggesting a 90 percent link,” said UCSF geneticist Neil Risch,
PhD, director of the UCSF I