In families with one autistic child there is a 19% chance that their next child will also have the condition, new research has found.
It has long been known that autism runs in families, but most previous estimates of the risk of recurrence range from 3% to 10%, with smaller numbers of families participating in the research.
In the new study, researchers in 12 locations across the US and Canada followed 664 infants who had at least one older sibling with autism.
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 gender of the older sibling and the severity of that child's case.
Having more than one older sibling with autism further increased the chance of diagnosis to 32%.
The calculations are averages, and the risk for any given family could be higher or lower, experts said.
The scientists took samples of DNA from many of the children in the study to look for genetic differences between the sibling pairs who had autism and those who did not. Their objective is to establish a better understanding of the genetic background to autism and develop tests to look at risk in individual families.