Jacqueline Stenson MSNBC contributor

Once a rare diagnosis, it seems there’s now an epidemic of autism sweeping the nation. Many of us know a child with the disorder, and concerned parents are searching for suspicious signs even in young babies. But while more kids are being labeled with autism, whether the condition is truly more common among today’s children than past generations of youngsters is largely unclear.

There’s no question that autism diagnoses are increasing, but it's unknown how much of that is due to greater awareness of the disorder by doctors and the public, a broader definition of it, a true increase in incidence or other factors.

“There is a chance we’re seeing a true rise, but right now I don’t think anybody can answer that question for sure,” says Dr. Chris Johnson, a professor of pediatrics at the University of Texas Health Sciences Center at San Antonio and co-chair of the American Academy of Pediatrics Autism Expert Panel.

Parents who believe the disorder is increasing due to some modern threat that is damaging the brains of children have pointed the finger at childhood vaccinations and the mercury-containing preservative thimerosal that was once widely used in many of them. There are also suspicions about lead or other toxins in the environment, diet, viruses and medications. Indeed, some experts say it's possible that exposures in utero or in early childhood may play a role.

Frustrated parents struggling to cope with a disorder that seemed to appear virtually overnight understandably want answers. But clear insights are hard to come by.

Cases skyrocketing
Studies done in the 1960s indicated that autism was quite rare, affecting only about one person in every 2,000 to 2,500, according to the Centers for Disease Control and Prevention. Other research in 1970 put the figure at one case per 10,000, Johnson says.

Precisely how many people have autism today is unknown. But estimates suggest there are five to six cases of autism spectrum disorders (ASDs) per 1,000 people, says Johnson. That roughly equates to as many as one case out of every 166 people.

It’s important to note that today’s figures apply to the whole category of ASDs, which includes autism as well as related conditions like Asperger Syndrome and Pervasive Developmental Disorder Not Otherwise Specified. Children with these disorders have varying degrees of impaired communication and social interaction.

Diagnostic criteria changed dramatically in 1987, broadening the number of people who could be considered to have ASDs. In decades earlier, only those with severe autistic characteristics would be diagnosed with autism; others might have been categorized as mentally retarded, for example. So making comparisons across decades is difficult.

“The frequency of the diagnosis has clearly increased but that doesn’t tell you beans,” emphasizes Dr. William Barbaresi, a pediatrician at the Mayo Clinic in Rochester, Minn.

To get a better picture of autism and its potential causes, Barbaresi and colleagues examined new cases of autism in Olmsted County, Minn., from 1976 to 1997.

Using data on every child living in the county during those years, the researchers used modern diagnostic criteria to conclude that the incidence of autism specifically rose dramatically, from 5.5 cases per 100,000 children from 1980 to 1983, to 44.9 cases from 1995 to 1997.

A sharp increase started between 1988 and 1991, a period during which broader diagnostic criteria for autism were newly in use and increased awareness of the disorder occurred, Barbaresi says.

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