No one can know for sure what is going on—how much each of the following 5 possible contributing factors has actually influenced the huge jump in the rate of autism. I will give you my best educated guesses and the rationales supporting them:
•Possible Factor 1: There has been a change in the definition of autism
We know definitely that this is a partial contribution. Before DSM-IV, autism was among the most narrowly and clearly defined of disorders. The symptoms began very early in life, before age 3 years, and were a striking and devastating combination of severe language deficits, inability to form even the rudiments of social relationships, and a preoccupation with a very narrow set of rigid rituals and stereotyped behaviors.
In preparing DSM-IV, we made the decision to add a new category that described a milder (and therefore much more difficult to define and distinguish) form of autism, Asperger Disorder. This seemed necessary because some (still quite rare) children presented with more or less normal language development, but with grave difficulties in making social contact and in engaging flexibly in the expectable range of behaviors. We knew that Asperger was about 3 times more common than classic autism and expected its inclusion to triple the rate of autistic disorders to about 1 per 500 to 1000. That gets us part way up to higher rates, but not nearly to 1 per 38.
•Possible Factor 2: Previously missed cases are now being picked up
An increased awareness of the symptoms of autism and better training in its diagnosis has undoubtedly resulted in improved case finding—thus identifying people who should have been, but previously had not been, diagnosed. It is impossible to determine just how significant a contribution to skyrocketing rates comes from this improved diagnosis. My guess is that this is a factor—but not one that comes close to bringing us from 1:500 to 1:100, much less 1:38.
•Possible Factor 3: An environmental toxin is causing an outbreak of autism
This has been the most popular and most feared explanation. It is almost certainly a quite small factor, if it is a factor at all. There has been no sudden environmental change that would account for such a sudden explosion in rates. The previously most widely accepted contender, Wakefield’s vaccine theory, was proven to be the result of bad and dishonest science. That there is no plausible alternative environmental vector does not disprove the possibility that there might be one, but it certainly makes it quite remote—especially in the face of far more plausible explanations.
•Possible Factor 4: The jump in rates is due to a change in diagnostic habits as autism became an ever more popular fad diagnosis
As the spectrum of autism widened, it came to include people previously considered to be unusually creative, or normally eccentric, or to have a schizoid or schizotypal personality disorder. Autism is no longer seen as a disabling condition and a wide variety of people have discovered their inner autistic self.
Redefined and reinterpreted, autism now casts a wide net catching much milder, everyday problems on the boundary with normality. Once rare and unmistakable, autism is now used loosely to describe people who do not really satisfy the narrow criteria intended by DSM-IV. Widespread publicity, Internet support and advocacy groups, and the fact that expensive school services are provided only for those who are given a diagnosis of autism have occasioned the dramatic swing from false negative to false positive diagnosis. Of interest, an autism advocacy group that could barely contain its enthusiasm at the high rates that were reported financed the Korean study.
•Possible Factor 5: Epidemiological studies always over-report rates
There is a systematic bias in all large-scale epidemiological studies to report as mental disorder even mild presentations of symptoms that may not have any clinical significance. It is entirely plausible that 3% of the population might have a smidgen of autism- completely implausible that so many would have an autistic mental disorder. Rates in epidemiological studies should be seen as no more than a screening upper limit- not a true reflection of the rate of actual mental disorder in the population being studied.
Human nature and the true rates of neurological illness and psychiatric disorder all change very slowly. Environmental toxins are more labile, but generally don’t just pop out of nowhere to make a condition 100x more common than it was just 15 short years ago. By far the most plausible explanations for the autism “epidemic” are factors 1,4, and 5. DSM-IV gave autism purchase by introducing a milder form close to the populous boundary to normality. Then autism took flight on the wings of definitional diffusion, Internet contagion, financial incentive, and naïve interpretation of epidemiological results. The overall rate of autistic symptoms is probably the same as it always has been since time immemorial- only the naming of them has changed.
