Infants have higher risk of developing autism if they have siblings with the disorder, a U.S. study found Monday.
The risk that an infant with an older sibling with autism also will develop the disorder, previously estimated at 3-10 percent, is 19 percent, according to the study by researchers at the University of California Davis.
For male infants the risk was even higher, at 26 percent, and for infants with more than one older sibling with autism the risk of recurrence jumped to 32 percent, according to the study.
Autism, a complex disorder that affects a child's ability to think, learn, communicate and interact socially, occurs in one out of 110 children born in the United States, according to the US Center for Disease Control.
Males account for 80 percent of diagnosed cases, researchers said.
"This is the largest study of the siblings of children with autism ever conducted," said Sally Ozonoff, professor of psychiatry and behavioral sciences at UC Davis's MIND Institute and the study's lead author.
"There is no previous study that identified a risk of recurrence that is this high."
The study involved 664 infants whose average age was eight months, with two-thirds recruited younger than six months. The researchers followed their development until 36 months, when they were tested for autism.
Of the 664 infants, 132 met the criteria for an "autism spectrum disorder," with 54 diagnosed with autistic disorder and 78 diagnosed with a milder form of the disease.
The study found that in families with one older child with autism the rate of recurrence was 20.1 percent. Only 37 infants had more than one sibling with autism, but for them the recurrence rate was 32.2 percent.
The study was published online Monday and will appear in print in the September issue of the journal "Pediatrics," the researchers said.