Shared environment may play a more substantial role in autism spectrum disorder (ASD) development than shared genes, recent study data indicate.

“Although genetic factors also play an important role, they are of substantially lower magnitude than estimates from prior twin studies of autism,” Joachim Hallmayer, MD, of Stanford University School of Medicine in Palo Alto, Calif., and colleagues wrote online first in the Journal of the American Medical Association’s Archives of General Psychiatry.

In the analysis of 404 twins born between 1987 and 2004 who participated in the California Autism Twins Study, 242 met criteria for ASD and 171 met narrow definitions for autism. The study included 54 pairs of identical twins with a confirmed ASD and 138 paris of fraternal identical twins.

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Shared environment appeared to explain 55% of the variance in strictly defined autism and 58% of those with an ASD, the researchers found; whereas genetic heritability appeared to account for variability in 37% of children with autism and 38% of children with an ASD.

Additionally, autism concordance rates within twin pairs were as follows:

  • Narrow autism in male pairs: 58% in 40 identical twin pairs (95% CI: 0.42-0.74) and 21% in 31 fraternal pairs (95% CI: 0.09-0.43)
  • Narrow autism in female pairs: 60% in seven identical twin pairs (95% CI: 0.28-0.90) and 27% for 10 fraternal pairs (95% CI: 0.09-0.69)
  • Broad-spectrum ASD in male pairs: 77% in 45 identical twin pairs (95% CI 0.65 to 0.86) and 31% in 45 fraternal pairs (95% CI: 0.16-0.46)
  • Broad-spectrum ASD in female pairs: 50% in nine identical pairs (95% CI: 0.16-0.84) and 36% in 13 fraternal pairs (95% CI: 0.11-0.60)

Findings from three previously published studies indicated a 72% ASD concordance rate among identical twins vs. 0% among fraternal twins — which may have lead to a gross overestimation of the influence of genetic factors on autism susceptibility, according to the researchers.

“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 researchers wrote. “The finding of significant influence of the shared environment, experiences that are common to both twin individuals, may be important for future research.”

Peter Szatmari, MD, of the Offord Centre for Child Studies at McMaster University in Hamilton, Ontario, Canada, called the findings “exciting news” in an accompanying editorial, and said it renewed interest in the role of “fetal programming” in autism’s etiology — a hypothesis in which environmental risk factors interact with genetic variants.

“Some of these factors may be modifiable and could possibly reduce the risk of ASD in the long term,” Szatmari wrote. These include: parental age, maternal genotype, maternal-fetal immunoreactivity, in vitro fertilization and maternal illnesses during pregnancy.

However, Szatmari warned that study weaknesses including missing data; differential misclassification of autism among identical and fraternal twin pairs; differences in the sensitivity of the autism diagnostic tools among identical and fraternal twins; and the potential that twinning could be an independent risk factor for ASD have the potential to bias Hallmayer et al’s concordance findings.

Hallmayer J et al. Arch Gen Psychiatry. 2011;doi:10.1001/archgenpsychiatry.2011.76.

Szatmari P. Arch Gen Psychiatry. 2011;doi:10.1001/archgenpsychiatry.2011.99.