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Autism only moderately follows genetic lines, with environmental influences weighing heavily in development of the disorder, researchers found in a twin study.

Comparing fraternal and identical twins for autism incidence, shared environment appeared to explain 55% of the variance in strictly-defined autism and 58% in autism spectrum disorder, Joachim Hallmayer, MD, of Stanford University, and colleagues reported.

Genetic heritability appeared to explain 37% of the variability for autism and 38% for autism spectrum disorder, the group noted online in the Archives of General Psychiatry.

Genetics may have gotten heavier emphasis in autism than it warranted, the researchers cautioned.

Prior studies appear to have seriously underestimated concordance in fraternal twins compared with the current findings, which would overestimate the influence of genetic factors on susceptibility, they noted.

This has skewed the research focus toward finding the genetic factors underpinning autism rather than potential environmental triggers or causes, they wrote in the paper.

More research into prenatal and early postnatal environmental risk factors, such as parental age, low birth weight, multiple births, and maternal infections during pregnancy, as well as how such factors influence genetic susceptibility, "are likely to enhance our understanding of autism," the group explained.

Their California Autism Twins Study included 202 pairs born between 1987 and 2004 in which one or both twins had autism spectrum disorder as identified through records of a state department that coordinates services for people with autism and other developmental disorders.

Among these 404 twins, 242 met criteria for autism spectrum disorder, with 171 classified as autism by a narrow definition.

After excluding 10 pairs without any confirmed autism spectrum disorder, 54 of the remaining pairs were identical twins and 138 were fraternal (dizygotic).

Concordance for autism status within pairs was:

• Among males, 58% in 40 identical twin pairs (95% confidence interval 0.42-0.74) and 21% in 31 fraternal pairs (95% CI 0.09 to 0.43)
• Among females, 60% in seven identical twin pairs (95% CI 0.28 to 0.90) and 27% for 10 fraternal pairs (95% CI 0.09 to 0.69).

Concordance for autism spectrum disorder status within pairs was:

• Among males, 77% in 45 identical twin pairs (95% CI 0.65 to 0.86) and 31% in 45 fraternal pairs (95% CI 0.16 to 0.46)
• Among females, 50% in nine identical pairs (95% CI 0.16 to 0.84) and 36% in 13 fraternal pairs (95% CI 0.11 to 0.60)

The results should be readily generalizable, the researchers suggested, pointing to the ethnically and demographically diverse California population represented in the study.

Prior twin studies included only individuals with Northern European ancestry.

The genetic component of autism spectrum disorder may have been overestimated if identical twins share relevant environmental exposures to a greater degree than fraternal twins or if the genetic and environmental components of the disease interact, Hallmayer's group cautioned.

But varying estimates about prevalence of the developmental disorder did little to change the relative contribution of genetics and environment determined by the study, they noted, calling the results "quite robust."

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