"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 at Davis.
The highest rates were in infants who had at least two older siblings with autism spectrum disorder - 32 percent of them also developed autism. Also, among boys with autistic siblings - 26 percent developed autism vs. 9 percent of girls. Autism is already known to be more common in boys.
The study involved 12 U.S. and international sites and was published online Monday in the journal Pediatrics.
Rates of autism in general have been rising steadily for years, and many researchers believe that much - perhaps even all - of the increase is due to more awareness and better tools for diagnosing a disorder that has always existed.
But there is no cure, and the cause is unknown. Research on siblings is seen as a key to unraveling what is behind the disorder - likely a combination of genetics and environmental factors, both of which could affect families - because of their higher rates.
Ozonoff said parents of autistic children often asked her, "How likely am I to have another child" with autism? She said her study provided a more up-to-date answer.
She noted, however, that 80 percent of siblings studied did not develop autism and that the prevalence rate was an average. It is different for each family, depending on other risk factors they may face.
Infants in the study were enrolled before they showed any signs of autism, such as poor eye contact and little social interaction.
The study is an important addition to autism research and "has critical implications for families who are deciding whether they'll have another child," said Catherine Lord, director of the Institute for Brain Development at New York-Presbyterian/Weill Cornell Medical Center. Lord was not involved in the study.
Ozonoff said the finding should prompt families and their children's doctors to be vigilant with infants whose older siblings had autism. Early diagnosis is important because experts say behavioral treatment has the best chance of working if started early.
"Pediatricians need to listen and make a very focused plan for how to monitor those things, rather than taking a wait-and-see attitude" toward children with autistic siblings, she said.
Funders for the study included the National Institutes of Health, the Canadian Institute for Health Research, and the independent group Autism Speaks.
