CHICAGO -- A new study suggests that nearly 1 in 5 children with an autistic older sibling will develop the disorder, too -- a rate much higher than previously thought.
Researchers followed 664 infants who had at least one older brother or sister with autism. Overall, 132 infants or about 19 percent ended up with an autism diagnosis, too, by their third birthdays. Previous smaller or less diverse studies reported a prevalence of 3 percent to 14 percent.
"We were all a bit surprised and taken aback about how high it is," said lead author Sally Ozonoff, a psychiatry and behavioral sciences professor with the Mind Institute at the University of California, Davis.
The study involved 12 U.S. and Canadian sites and was published online today in Pediatrics.
Ozonoff noted that 80 percent of siblings studied did not develop autism and that the prevalence rate was an average. It may be different for each family, depending on other risk factors they may face.
Autism has no known cause, but experts believe that genetics and external influences are involved. Research is examining whether these could include infections, pollution and other noninherited problems.
Ozonoff noted that siblings are exposed to similar outside influences, which could partly explain the results.