Environmental factors appear to play a larger role than hereditary factors in the development of autism, according to the California Autism Twin Study, published online.
In the study of 404 children (202 twin pairs), environmental factors accounted for about 55% of susceptibility to autism. "Although genetic factors also play an important role, they are of substantially lower magnitude than estimates from prior twin studies of autism," said Dr. Joachim Hallmayer of Stanford (Calif.) University and his associates (Arch. Gen. Psychiatry 2011 July 4 [doi:10.1001/archgenpsychiatry.2011.76]).
Potential environmental influences may include parental age, low birth weight, multiple births, and maternal infections during pregnancy, the researchers noted.
The study included identical (monozygotic) and fraternal (dizygotic) twins. Despite the genetic diversity between fraternal twins, the probability that a child would have autism or autism spectrum disorder (ASD) if his / her fraternal twin did was greater than that found in previous studies - thus suggesting that shared environment may play a greater causative role than previously suspected.
"Because of the reported high heritability of autism, a major focus of research in autism has been on finding the underlying genetic causes, with less emphasis on potential environmental triggers or causes," the investigators noted.
"Our study provides evidence that the rate of concordance in dizygotic twins may have been seriously underestimated in previous studies, and the influence of genetic factors on the susceptibility to develop autism overestimated.
"The finding of significant influence of the shared environment, experiences that are common to both twin individuals, maybe important for future research," they added.
Dr. Hallmayer and his colleagues performed what they described as "the largest population-based twin study of autism that used contemporary standards [of] diagnosis."
They identified twin pairs born in 1987-2004 in which one or both children were affected. To do so, they used a California database for the 21 regional centers that coordinate services for people with autism, mental retardation, and other developmental disabilities.
All the study subjects were rigorously assessed between 2005 and 2009 using the Autism Diagnostic In-terview-Revised, a structured parent interview, and the Autism Diagnostic Observation Schedule, a standardized play and interview scale. …