HealthDay News — Autism spectrum disorder (ASD) is markedly heritable, with probandwise concordance of 96 percent, but there is variation in the severity of symptomatology above the diagnostic threshold, according to a study published online Dec. 18 in Behavior Genetics.

Lauren Castelbaum, from the Washington University School of Medicine in St. Louis, and colleagues used three existing data sets involving 366 pairs of uniformly-phenotyped monozygotic (MZ) twins with and without ASD to examine twin-twin similarity for autistic trait severity.

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The researchers found that probandwise concordance was 96 percent for ASD. For pairs with ASD trait burden below versus above the threshold for clinical diagnosis, MZ trait correlations differed markedly, with R²s on the order of 0.6 and 0.1, respectively. For ASD diagnosis, categorical MZ twin discordance was rare, and more appropriately operationalized by standardized quantification of twin-twin differences.

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“Identical pairs in which one or both exceed the clinical threshold for affectation exhibit a continuous range of contrasts, the most pronounced of which qualify for categorical discordance, but this occurs rarely,” the authors write. “More often, even when there exist substantial differences between twins, both typically exhibit levels of symptom burden near or above the threshold for a diagnosis.”

One author disclosed receiving royalties for the commercial distribution of the Social Responsiveness Scale, a quantitative measure of autistic traits for ages 30 months through adulthood.

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