The estimate released on Thursday by the Centers for Disease Control and Prevention represents an overall increase of about 25 percent since the last analysis in 2006 and a near-doubling of the rate reported in 2002.

Among boys, the rate of autism spectrum disorders is one in 54, almost five times that of girls, in whom the rate is one in 252.

"One thing the data tells us with certainty - there are many children and families who need help," CDC Director Thomas Frieden said at a press conference.

The reported spike in the prevalence of autism and related disorders raised questions about whether it is real or an artifact of greater awareness that has led parents, teachers, and even health-care providers to see symptoms of autism in children who would not have received the diagnosis a generation ago.

If it is real, that suggests that some change in the environment might be responsible. In recent years suspicion has focused on everything from mercury, a known neurotoxin, in air and food, to the increasing age of new mothers and fathers.

There is a good possibility that much of the reported increase in the prevalence of autism is illusory, however. When asked about this during the news conference, CDC's Frieden pointed out that "doctors have gotten better at diagnosing the condition and communities have gotten better at providing services, so I think we can say it is possible that the increase is the result of better detection."

Advocates for people with autism nevertheless seized on the new data to call for more research to identify the causes of autism-spectrum disorder and for more services for those affected by it.

"This is a national emergency and it's time for a national strategy," said Mark Roithmayr, president of the research and advocacy group Autism Speaks. He called for a "national training service corps" of therapists, caregivers, teachers and others who are trained to help children with autism.

"Inevitably when these statistics come out, the question is, what is driving the increase?" said Roithmayr. Better diagnoses, broader diagnostic criteria and higher awareness, he estimated, account for about half the reported increase.

EXAMINING RECORDS

The new analysis from the CDC comes from the Autism and Developmental Disabilities Monitoring Network, which currently operates at sites in 14 states.

To determine whether a child has autism or a related disorder, what CDC calls "clinician reviewers" examined the medical and school records of 337,093 eight-year-olds in those states in 2008 and conducted screening. Children whose records included either an explicit notation of autism-spectrum disorder or descriptions of behavior consistent with it were counted as falling on the autism spectrum.

The prevalence of autism in the states monitored by CDC varied widely, from a high of one in 47 in Utah to one in 210 in Alabama. Experts said that variation likely reflected differences in awareness of the disorder among parents, teachers and even physicians, as well as differences in the availability of services, rather than any true "hot spots" of autism.

Autism spectrum disorders are marked by a suite of symptoms, all arising from atypical brain development that results in problems with socialization, communication, and behavior.

Although the disorder can be mild or severe, in general children with autism have difficulty communicating and making friends. Many find it painful to look other people in the eyes, which impairs their ability to understand what others are thinking and feeling.

There is no brain-imaging test for autism, let alone a blood test or other rigorously objective diagnostic. Instead, physicians determine whether someone fits the criteria laid out in the American Psychiatric Association's Diagnostic and Statistical Manual, or DSM.

The manual has undergone significant changes over the years, including in the diagnostic criteria for autism. In its current version, someone must fit at least eight of 16 criteria, including symptoms involving social interaction, communication, and repetitive or restricted behaviors and interests.

The previous version was stricter, describing one diagnostic criterion as "a pervasive lack of responsiveness to other people." In the current manual, that became "a lack of spontaneous seeking to share .... achievements with other people" and friendships that appear less sophisticated than the norm for a child's age.

The earlier manual also required "gross deficits in language development" and "peculiar speech patterns" for a diagnosis, while the current one lists difficulty "sustain(ing) a conversation" or "lack of varied . . . social imitative play."

Morton Ann Gernsbacher, a professor of psychology and autism researcher at the University of Wisconsin, Madison, and others have cited these changes to question the reality of the reported autism increase.

ENVIRONMENTAL FACTORS

Scientists had long estimated that 90 percent of autism risk was genetic and 10 percent reflected environmental factors. But a 2011 study of twins by scientists at Stanford University concluded that genes account for 38 percent of autism risk and environmental factors 62 percent.

Exactly what those factors are, however, remains the subject of intense research, with two large studies funded by the National Institutes of Health examining everything from what the mother of a child with autism ate during her pregnancy to what cleaners were in the house and what pollutants were in the dust.

"There is not a clear front-runner" among possible environmental causes of autism, said Craig Newschaffer, chair of the Department of Epidemiology and Biostatistics at Drexel University School of Public Health and lead investigator of one of the NIH-sponsored studies.

There is, however, what he called "good evidence" that any environmental culprit is present during the second or third trimester, the peak of synapse formation. Scientists believe that faulty brain wiring underlies autism.

They have also focused on factors that have changed in the last two decades, including pregnant women's use of certain antidepressants, increasing parental age and the rise in pre-term births and low-birth weight babies, said Newschaffer.

Even as experts disagree on whether the reported increase in the prevalence is real and what causes the disorder, there is a clear consensus that "the earlier a child is diagnosed the more he will benefit from interventions," Dr. Coleen Boyle, director of CDC's National Center on Birth Defects and Developmental Disabilities said during the news conference.

Unfortunately, the nation has made slim progress on that front. In 2006, the median age of diagnosis was four-and-a-half years. In 2008 it was four years - an age when experts say is too late for interventions to do all the good they would if begun earlier.

The American Academy of Pediatrics recommends that children be screened for autism at 18 months and again at 24 months. Parents should look for symptoms such as failing to point, not making eye contact, and being slow to develop language, said APA's Dr. Susan Hyman.

"It is critical to act quickly if there is a concern about autism," Frieden said.

Research funded by Autism Speaks found that autism costs the United States $126 billion annually. That reflects the cost of healthcare, special education and other services, as well as loss of productivity, underemployment and unemployment among adults with autism.

(Reporting by Sharon Begley; Editing by Michele Gershberg and Philip Barbara)

• A Permanent Economic Emergency