Why is a child born in northwest Los Angeles four times as likely to be diagnosed with autism as a child born elsewhere in California?

Medical experts have pondered for years why autism rates have soared nationwide, and why the disorder appears to be much more prevalent in certain communities than in others. Now, some recent studies that zero in on California may shed some light on these baffling questions. Researchers from Columbia University, in a study published in the current Journal of Health & Place, identified an area including West Hollywood, Beverly Hills and some less posh neighborhoods that accounted for 3% of the state's new cases of autism every year from 1993 to 2001, even though it had only 1% of the population.

Another recent study, from the University of California, Davis, published in Autism Research, also found high rates of autism in children born around Los Angeles, as well as nine other California locations. Autism, usually diagnosed before a child is 3 years old, is a developmental disorder characterized by impaired social interaction and communication and repetitive behavior.

Both of the California-based studies suggest that local environmental or social factors are driving the high autism-diagnosis rates. And they conclude that childhood vaccinations—which some people fear is a factor behind rising autism—are not to blame. Otherwise, diagnoses of the disorder would be more evenly dispersed, they say.

The studies also disagree on some points. According to the UC Davis study, greater concentrations of autism occur in communities where parents are highly educated, which could mean they have more awareness of autism and access to treatment. By contrast, the Columbia researchers discount the role of educational levels. They believe that social influences, such as shared information about diagnoses, doctors and services, are largely responsible for the high rates they found in parts of Los Angeles.

In Los Angeles itself, residents have a variety of explanations for the high autism rates, ranging from a family's affluence and the activity of autism-advocacy groups to past air and water pollution.

James McCracken, a child psychiatrist at the UCLA Center for Autism Research and Treatment, says families often have to fight with state bureaucracies to be deemed eligible for services, and some spend thousands of dollars for private evaluations. "You can see the possibility for inequity according to social advantage or cultural background," he says.

But Moira Giammatteo, a San Fernando Valley mother with a 12-year-old autistic son, doesn't believe that affluent, educated parents are gaming the system. "It's not like people think, 'get this label and you can get services.' Nobody wants this diagnosis; most parents are in denial," she says.

Some of the increase in autism rates in past decades is due to changing definitions. Until the early 1990s, diagnoses of autism were rare and included only children with low I.Q.s, who were deeply withdrawn and had very minimal language skills. In 1994, diagnosticians adopted the term autism spectrum disorder (ASD), which also includes children with impaired social skills but not necessarily severe intellectual disabilities or language delays.

On average, one in 110 American 8-year-olds had an autism spectrum disorder in 2006, an increase of 57% since 2002, according to a December report from the Centers for Disease Control and Prevention. Some parts of the U.S. are seeing much higher rates than others: Metropolitan Phoenix, for example, has twice the prevalence as northern Alabama.

Whether those differences reflect actual higher risk in different regions, differences in awareness among local residents, or simply variations in record keeping is something the CDC is trying to untangle.

"We still don't know what causes autism, and we don't know a lot of the underlying factors, so we can't rule out the possibility that there are differences in the distribution of risk factors." says Jon Bai, a CDC epidemiologist.

Theories abound to explain the steep increase that has occurred in recent years. Some experts attribute it to genetic changes within families. But others say genetic changes wouldn't occur so quickly and instead they blame environmental toxins or childhood vaccinations.

Another possible explanation: Greater awareness of the disorder, and programs in some parts of the country that can help children regain skills, may make parents more willing to have their children diagnosed.

"But awareness can only go so far" to explain the rising levels of autism, says Dr. Baio. "We are still identifying more children with autism, in all levels of severity, than ever before, which is why this continues to be a perplexing and urgent concern."
Around the U.S.

Nine out of every 1,000 8-year-olds were diagnosed with autism spectrum disorder as of 2006. But rates vary widely in the survey areas, located in 11 states, that the CDC monitors. (Prevalence per 1,000):

* Alabama 6.0
* Arizona 12.1
* Colorado 7.5
* Florida 4.2
* Georgia 10.2
* Maryland 9.2
* Missouri 12.1
* North Carolina 10.4
* Pennsylvania 8.4
* South Carolina 8.6
* Wisconsin 7.6

Source: CDC's Autism and Developmental Disabilities Monitoring Network

In California, children with autism or ASD must be "substantially developmentally disabled" to qualify for services from the state's Department of Developmental Services (DDS). The two recent studies used data from the DDS in their research. The studies looked at where the children with autism were born, not where they were diagnosed, so that their findings wouldn't be skewed by families moving into the areas.

As part of their work, the Columbia researchers constructed a "SimCity map of California," referring to the city-building simulation game, says Peter Bearman, the lead investigator. They assembled data pinpointing not just where children with autism were born and diagnosed but also all the parks, day-care centers, doctors' offices, autism-advocacy groups and other gathering places.

The result: significantly higher occurrences of autism in a large area of Los Angeles stretching from Santa Monica in the west to beyond Burbank in the east, and from El Segundo in the south to the San Fernando Valley in the north. The epicenter of the autism cluster: areas around Hollywood.

Dr. Bearman says he believes social influences are the leading cause for the high autism rates in Los Angeles, although the researchers continue to examine environmental issues.

Other studies have shown that older parents run a greater risk of having an autistic child. But when the Columbia researchers adjusted the Los Angeles cluster to factor out parental age, the higher levels remained. Dr. Bearman says he believes the high levels will also remain after the data are adjusted for education levels, socio-economic status and other demographic characteristics in future studies.

In addition to parts of Los Angeles, the UC Davis study located clusters around Santa Ana, San Diego, San Francisco, Santa Clara, Fresno and Stockton where children had at least a 70% greater chance of being diagnosed with autism than in surrounding areas.

The Davis researchers believe their findings can be explained solely by parents' educational levels—by adjusting the data for educational levels, the discrepancies in autism rates virtually disappeared.

"There are many ways that you can see that a highly educated person will be more successful at getting the diagnosis," says Karla Van Meter, the study's lead author. "If I'm more educated, I might have different expectations for my child," she says.

Corrections & Amplifications
An earlier version of this article incorrectly spelled Jon Baio's surname.

Write to Melinda Beck at HealthJournal@wsj.com