Autism, a brain disorder that interferes with communication and social skills, affected an estimated one in 110 American 8-year-olds in 2006, according to a federal study released Friday.
The U.S. Centers for Disease Control and Prevention looked at the medical diagnoses of 307,790 children who were age 8 in 2006. They found 2,757, or 0.9 percent, had been diagnosed with autism.
The CDC team found that autism cases were four to five times higher among boys than girls, with 1 in 70 boys and 1 in 315 girls identified.
The overall ratio is far higher than previous estimates that the incurable family of conditions affected 1 in 150 U.S. children. A few decades ago autism was thought to be rare.
The average increase since 2002 for boys was 60 percent while the average increase for girls was 48 percent, the study said.
The CDC also found autism was far more common among non-Hispanic white children, but found increases in autism among all racial groups included in the review.
Researchers reported a 55 percent increase in autism for white children, 41 for black children and 90 percent among Hispanic children. Estimates of increases for Asian/Pacific Islander children ranged from 1.0 percent to 16.2 percent, the CDC said.
The CDC's Catherine Rice, who led the study, said there was no single factor behind the rising numbers of the brain disease.
Some of the increases are due to better detection, particularly among children who may not have come to attention in the past -- including girls, Hispanic children and children without cognitive impairment, Rice told reporters.
But she said something may be happening to make autism more common as well.
The researchers said most children with autism showed symptoms before age three, but identification is often not made until later. The CDC study found the average age of diagnosis was 4-1/2 years.
The study analyzed data collected from 11 Autism and Developmental Disabilities Monitoring Network sites -- in Alabama, Arizona, Colorado, Florida, Georgia, Maryland, Missouri, North Carolina, Pennsylvania, South Carolina, and Wisconsin.
There is no cure for autism, a spectrum of diseases ranging from severe and profound inability to communicate and mental retardation to relatively mild symptoms. But experts believe intensive, early treatment can help many children with the disorder.
Before the 1980s, the term 'autism' was used primarily to refer to autistic disorder and was thought to be rare, affecting approximately one in every 2,000 children, the researchers wrote.
Parents and advocates have been alarmed by the rise in numbers since then, although some of the increase could be attributed to changes in diagnosis and classification.
The advocacy group Autism Speaks called on the U.S. government to step up its efforts to develop treatments.
We need meaningful action now that acknowledges the scope of this problem and allocates the resources necessary to take the fight against autism to a new level, co-founder Bob Wright said in a statement.
Autism research is due for a large infusion of money from President Barack Obama's $5 billion plan to boost U.S. medical and scientific research.
There's a multi-pronged approach going on because we know that there is no single cause for autism. We're not going to find the one answer, the CDC's Rice said.
Autism researchers are looking at a broad range of potential environmental factors, including household products, medical treatments, diet, food supplements and infections. Some recent studies have found strong evidence of several genetic causes for autism.
Pfizer Inc, the world's largest drugmaker, announced in July that it has begun developing treatments for autism.