There's a new study out in JAMA (free full text here) on the incidence of autism in the US population. Before getting to what the article actually says, it's worth seeing what the media are saying it says. The New York Times headlines it "Study Shows Increase in Autism", and Yahoo runs it as "Study Confirms Marked Rise in Autism."

The AP ("CDC Study Finds Autism To Be Less Rare") and Reuters ("Atlanta Study Finds Rise in Autism Diagnoses" do better. That's because it's very hard to tell if there's a real rise taking place or not. The numbers are going up, but the interpretation isn't as easy as it sounds. To quote the authors:

Debate continues about whether the overall prevalence of autism has increased or whether past rates underestimated true prevalence. This debate is difficult to resolve retrospectively.

It's difficult for these reasons:

In the United States, the increase in the number of individuals receiving services for autism may be attributed to several factors. Changes in diagnostic criteria have expanded the concept of autism to a spectrum of disorders. Heightened public awareness of autism also has had an effect, due in large part to efforts of parent and advocacy groups, availability of more medical and educational resources, increased media coverage of affected children and families, and more training and information for physicians, psychologists, and other service providers. Also, in 1991, the US Department of Education added autism as a category for special education services, possibly leading to increases in the number of children classified with autism because of the availability of these services. The mandate for early intervention services for children with DDs, including autism, also has contributed to greater attention being placed on autism. At the same time, studies are suggesting that some children with autism respond well to early, intense educational intervention. The combined influence of these factors has probably contributed to the identification of more individuals with autism. However, it remains unclear whether specific environmental, immunologic, genetic, or unidentified factors also have contributed to these higher reported prevalence rates.

I think that's a very fair statement. Some environmental factor might be at work, if the increase is a real one. If so, tracking it down is going to be a major undertaking, because instead of one smoking gun, there might be several - insufficient by themselves, but adding up to something. These things are very hard to unravel, because it's almost impossible to get statistically meaningful samples that represent the range of variables that you're trying to check.

As for clues to any environmental causes, one footnote from the article that I have access to is from 2000, in Environmental Health Perspectives (Medline abstract here.)

Epidemiologic studies indicate that the number of cases of autism is increasing dramatically each year. It is not clear whether this is due to a real increase in the disease or whether this is an artifact of ascertainment. A new theory regarding the etiology of autism suggests that it may be a disease of very early fetal development (approximately day 20-24 of gestation). This theory has initiated new lines of investigation into developmental genes. Environmental exposures during pregnancy could cause or contribute to autism based on the neurobiology of these genes.

I find this idea somewhat more plausible than the thimerosal hypothesis, or any other environmental factor acting in the first years after birth. For what it's worth, it makes more sense to me that any causative agent would be something that's present in very small amounts, which would overall have greater leverage to cause broad-based harm earlier in brain development. That's an Occam's razor approach, which doesn't always cut the right way, but that's how I'd call it now - if there's an environmental cause at all, and if there is indeed a rise in autism. And we're still not sure about either one.