Doctors have long known that autism can run in families.
Studies have suggested that up to 10% of younger siblings of autistic kids will develop the condition.
A new study, however, shows the risk is twice that high. In a study of 664 children -- the largest of its kind -- nearly 20% of younger siblings of autistic children were also diagnosed with autism.
That means these younger siblings have a risk of autism roughly 20 times greater than children in the general population, says Sally Ozonoff of the University of California-Davis, author of the study, in today's Pediatrics. About 1 in 110 U.S. children have an autism-spectrum disorder, says the Centers for Disease Control and Prevention.
The risk is even greater in families with two or more autistic children: Almost one-third of their younger siblings also will develop the condition, the study says.
"I was taken aback by that number," says Thomas Insel, director of the National Institute of Mental Health, part of the National Institutes of Health. NIH and advocacy group Autism Speaks helped pay for the study.
Autism was even more common in the younger brothers of autistic children, the study says. In general, about 80% of autistic kids are boys.
In this study, risk of autism in younger brothers was nearly three times greater than in younger sisters: 26% of brothers of autistic kids also had the disorder, vs. 9% of sisters. Scientists don't know why autism is more common in boys, but they're investigating, says Alycia Halladay, director of environmental research at Autism Speaks.
Given their higher risk, younger siblings of autistic kids should get careful screening, Halladay says. Studies show intensive behavioral therapy is most effective when begun as early as possible.
The findings also could help provide genetic counseling to parents of autistic children, Ozonoff says. But the study, by itself, doesn't say much about whether genes cause autism, Ozonoff says. Siblings share both DNA and environment, but the findings can't tell scientists which plays a greater role.
Researchers have found that a number of factors relating to prenatal environment can influence autism risk, such as maternal infections or use of antidepressants during pregnancy, Insel says.
He says scientists have identified genes that cause 15% to 16% of autism cases and probably will find more. Autism is now considered to be not one but many disorders, with different causes that result in similar symptoms, he adds.
"It's turning out to be not just one gene or two genes, or even 10 genes" involved in autism, Insel says, "but lots of genes."
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