If parents have a child with some form of autism, there is a 19% chance that their next child will have autism also, according to a study that was published in the journal Pediatrics. Researchers have long known that autism runs in families.  Previous estimates of the risk of recurrence ranged from 3 to 10 percent. These estimates were based on small numbers of families and with a more narrow definition of autism than the way that autism is defined today.

In the new study, researchers in 12 locations across the U.S. and Canada followed 664 infants who had at least one older sibling with autism.  The infants were an average of 8 months old when enrolled in the study which is before the signs of autism are usually apparent. At age 3, each child was evaluated. Among the boys, 26% were found to have some form of autism. The rate for girls was 9%. The rates were the same regardless of the sex of the older sibling and the severity of the older sibling's autism. Having more than one older sibling with autism further increased the chance of diagnosis to 32%.

Scientists have collected DNA samples from many of the children in the study to look for genetic differences between the sibling pairs who have autism and those who did not. The aim is to develop a clearer understanding of the genes involved in autism and potentially develop tests that would measure risks in individual families.  The study provided further evidence for the strong role of genetics in autism.