Parents who have one child with autism have a risk two to six times greater than previously thought of having a second baby with the developmental disorder, according to researchers from the Hebrew University of Jerusalem.Autism spectrum disorders (ASD) affect the brain’s normal development of social and communication skills.The exact cause is still unknown, but defective genes are believed to be involved. What is described as “revolutionary research” by HU psychology department Prof. Nurit Yirmiya and her team was published on Monday in the prestigious journal Pediatrics.In this study – the most comprehensive conducted to date – the risk of a child developing ASD in a family with one sibling who already has ASD was examined using the largest sample collected so far and state-of-the-art diagnostic methods.The sample was made up of 664 infants, all of whom had at least one older biological sibling with a verified diagnosis of ASD (6 percent had more than one sibling diagnosed with autism). Infant siblings were included in the story at a very young age, before signs of autism are readily apparent and parents typically have concerns about development.The babies’ average age was eight months, and two-thirds of them were enrolled in the study before six months of age.Infant siblings were followed until the age of 36 months, when symptoms of ASD are present and reliably identified.The main finding of the study was that recurrence risk among siblings is 18.7%, which is substantially higher than the results of previous studies, which found a 3% to 10% risk of recurrence. Furthermore, the researchers report even higher recurrence rates for infants who fell into certain subcategories – for male infants, a recurrence rate of 26.2% was found, compared to 9.1% for female infants.Remarkably, this means that almost one in three brothers of children with autism will eventually develop ASD. In addition, the recurrence risk for infants (boys and girls together) with more than one older affected sibling was 32.2% – almost twice the rate of those having just one older sibling with ASD.Twelve research groups from North America took part in the study, and all the participants were part of the Autism Speaks Baby Siblings Research Consortium, an international network of funded studies examining infants in families of children with autism. Yirmiya is part of this network and one of the pioneering researchers in this field.“The higher recurrence rates found in this study in comparison with previous reports resulted from the fact that previous studies were conducted in the 1980s and 1990s, when different and more narrow diagnostic criteria were used to evaluate children with autism, and there is an overall increase in the number of children who are diagnosed with ASD,” Yirmiya said.“In addition, previous studies used different assessment methods. For example, in some studies, children were not assessed directly, and other studies did not control for Estoppage [the tendency of parents to limit reproduction after the birth of a child with a disability].”She explained why this study was stronger than previous ones on the subject.“Only families that had a younger sibling participated; gold standard diagnostic methods as well as comprehensive developmental assessments were used and administered by experts; and this is by far the largest sample to date, pooled across sites so it is a geographically diverse sample. The improved methodology assures that the results of the current study reflect the true population recurrence rates better than previous reports,” Yirmiya explained.“It should be emphasized that this study is not an epidemiological one but rather a community- based sample of families who voluntarily chose to participate.The fact that families volunteered to participate may explain the higher rates found in the current study.”The results suggest the development of young siblings of a child with ASD should be monitored in a rigorous, careful, structured and routine fashion, the HU psychologist said.“The study also indicates that sibling status constitutes a significant risk factor for ASD.”She advised professionals in the field to work actively and cooperatively with parents to monitor specific developmental milestones and make appropriate intervention plans as needed.