Shared Environment Tied to Autism Risk in Healthy Twin

Environmental factors account for more variance in liability to autism than genetic factors

TUESDAY, July 5 (HealthDay News) -- In twins, shared environment may have a greater impact on susceptibility to autism spectrum disorders (ASD) than genetic inheritance, according to a study published online July 4 in the Archives of General Psychiatry.

Joachim Hallmayer, M.D., from the Stanford University School of Medicine in California, and colleagues investigated the genetic heritability and effects of a shared environment in ASDs. A total of 192 twin pairs born between 1987 and 2004, in which one twin was diagnosed with ASD, were included in the analysis. Participants were assessed by structured diagnostic assessments (Autism 10 Diagnostic Interview Revised and Autism Diagnostic Observation Schedule). Concordance rates were calculated and parametric models were fitted for a narrow definition of strict autism and a broad one of ASD.

The investigators found that the probandwise concordance for strict autism in male twins was 0.58 and 0.21 for 40 monozygotic pairs and 31 dizygotic pairs, respectively. In female twins, the concordance was 0.60 and 0.27 for seven monozygotic pairs and 10 dizygotic pairs, respectively. For ASD, the probandwise concordance for male twins was 0.77 and 0.31 for 45 monozygotic pairs and 45 dizygotic pairs, respectively. For female twins the concordance was 0.50 and 0.36 for nine monozygotic pairs and 13 dizygotic pairs, respectively. Shared environmental factors account for a considerable proportion of the variance in liability, (55 percent for autism and 58 percent for ASD) in addition to moderate genetic heritability (37 percent for autism and 38 percent for ASD).

"The results suggest that environmental factors common to twins explain about 55 percent of the liability to autism. Although genetic factors also play an important role, they are of substantially lower magnitude," the authors write.

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