The likelihood of a child's being given a diagnosis of autism, Asperger syndrome or a related disorder increased more than 20 percent from 2006 to 2008, according to a report released on Thursday by the Centers for Disease Control and Prevention.
The new report estimates that in 2008 one child in 88 received one of these diagnoses, known as autism spectrum disorders, by age 8, compared with about one in 110 two years earlier. The estimated rate in 2002 was about one in 155.
The frequency of autism spectrum diagnoses has been increasing for decades, but researchers cannot agree on whether the trend is a result of heightened awareness, an expanding definition of the spectrum, an actual increase in incidence or some combination of those factors. Diagnosing the condition is not an exact science. Children "on the spectrum" vary widely in their abilities and symptoms, from mute and intellectually limited at one extreme to socially awkward at the other.
Children with such diagnoses often receive extensive state-financed support services — which some experts believe may have contributed to an increase in the numbers.
Doctors working to update the Diagnostic and Statistical Manual of Mental Disorders have proposed significant changes to the definition of autism, which are due to take effect in 2013. If the changes are carried out, some experts say, they could reduce the number of children being given a diagnosis.
"One thing the data tells us with certainty: There are many children and families who need help," Dr. Thomas R. Frieden, director of the C.D.C., said in a news conference on Thursday.
C.D.C. researchers did not meet any of the children they judged to have an autism spectrum disorder. The team made the decisions based on evaluations of the children, drawn from 14 states. The estimated rates in those states varied widely, from one in 210 children in Alabama to one in 47 in Utah.
"This is a fourfold difference," Dr. Éric Fombonne, a psychiatrist at McGill University and Montreal Children's Hospital, said in an e-mail. "It means that ascertainment is unequal across states. Thus, in the next surveys, as ascertainment will most likely improve where it is currently low, average rates are bound to increase. Is there, in addition to this, a real increase in incidence? It's possible, but cannot be determined from the study design."
Over all, boys were almost five times as likely as girls to get such a diagnosis — at a rate of one in 54, compared with one in 252 for girls.
The sharpest increases appeared among Hispanic and black children, who historically have been less likely to receive an autism spectrum diagnosis than white children.