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 July 4 in Archives of General Psychiatry.
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, may be 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 Interview-Revised, a structured parent interview, and the Autism Diagnostic Observation Schedule, a standardized play and interview scale.
Of the 404 twins, 242 met criteria for autism spectrum disorder, including 171 who met the stricter criteria for autism. A subset of 192 twin pairs was included in the final analysis: 54 monozygotic and 138 dizygotic pairs.
For autism, probandwise concordance rates for monozygotic twins were similar for 40 male pairs (58%) and 7 female pairs (60%), and were comparable with rates reported previously.
However, probandwise concordance rates for dizygotic pairs were higher than previously reported, at 21% for 31 male pairs and 27% for 10 female pairs.
For ASD, probandwise concordance for monozygotic twins was 77% for 45 male pairs and 50% for 9 female pairs; however, concordance rates for dizygotic twins were 31% for 45 male pairs and 36% for 13 female pairs.
Among the female dizygotic twins of 76 boys with ASD, the probandwise concordance rate was 5.3%. But the rate for the male dizygotic twins of six girls with ASD was nearly 10 times greater, at 50%.
"These dizygotic concordance rates are higher than previously reported and have a significant impact on the heritability analysis," the investigators said.
For autism, genetic factors accounted for an estimated 37% and a shared environment accounted for an estimated 55% of susceptibility to autism. For ASD, genetic factors accounted for an estimated 38% and a shared environment accounted for 58%.
Dr. Hallmayer and his associates noted that their study population was highly diverse in ethnicity, socioeconomic status, and other demographic factors. That means that the results should be readily generalizable to other populations.
That diversity also may explain some of the differences between the findings of the latest study and those of previous twin studies, "which were based exclusively on individuals from Northern Europe," they added.
The National Institute of Mental Health and Autism Speaks supported the study. No financial conflicts of interest were reported.