Current estimates of the heritability of autism spectrum disorders (ASD) derived from existing clinical twin studies1, 2, 3, 4 are potentially confounded by a number of factors, including low sample size, inconsistency in case definition and the role of de novo mutation (currently estimated to contribute to some 20% of cases), and the possibility that heritable causes of ASD in a twin could result in environmentally engendered affectation of a non-identical co-twin in utero, especially if mediated by humoral or immune mechanisms. To minimize these confounds, we compared autism recurrence in half siblings versus that in full siblings, using data (6 June 2011) from over five thousand families enrolled in the Interactive Autism Network (IAN), a national volunteer register for ASD, detailed characteristics of which have been previously described by our group. Recurrence rate among full siblings was observed to be approximately twice that among half siblings, providing strong evidence of genetic transmission of ASD.
The recurrence risk is the chance that a sibling has autism if another sibling does. The authors found the recurrence risk to be twice as high for full siblings, which is consistent with a primarily genetic model of autism risk.
This follows on a number of recent twin studies ( here , here and here ) have shown a lower concordance than previously estimated and the baby siblings study which showed a 19% recurrence risk for autism.
I wonder at how many half-siblings there are in the cohort. A small subpopulation could mean large error bars.