In prior posts I have argued in favor of the idea that an autism epidemic has not occurred at all. In this post I will put everything in one place, include additional data, and clarify a few points. As always, readers are invited to scrutinize the data, my interpretations of the same, and post comments and/or rebuttals. In particular, all epidemic-causing-trigger proponents are explicitly invited to do so.
Proponents of the idea of an autism epidemic often rely on data from the California Department of Developmental Services (CDDS). But as you will see, this same data contains a wealth of information that can be used to disprove that such an epidemic ever took place.
There is a problem of group inequivalence in equating number of diagnosed cases with actual prevalence and number of new diagnosed cases with actual incidence. What this means is that the characteristics of diagnosed autistics over time have changed, so comparing the number of diagnosed individuals from one quarter to the next is like comparing apples and oranges (or oranges to sheep).
|Quarter||Epilepsy||Profound MR||Severe Behaviors||Lack of MR|
Table 1 not only shows that it is not possible to make an accurate determination about prevalence and incidence changes, it also shows that a significant 'broadening criteria/awareness' phenomenon must be in effect. This is undeniable given this data. The following figure [courtesy of CDDS] also illustrates the point. Notice that the autism curve and the autism without MR curve run almost parallel to one another.
Broadening criteria does not imply that autistics diagnosed today who would not have been diagnosed in prior years are completely neurotypical. It simply means that the proportion of certain characteristics must be closer to those of the general population. Judging from the size of the autistic population and drops in these numbers it would appear that newly diagnosed autistics today are, in average, considerably less autistic than those at the beginning of the 'epidemic'. This completely undermines any epidemic-causing trigger theory.
[Note: For an updated regional analysis, see Regional Differences and Quarterly Growth Due to Two Factors.]
In a previous post on regional differences I argued that they cannot be explained by environmental factors because of group inequivalence. In particular, the Central Valley (Fresno) Regional Center has the lowest prevalence of diagnosed cases of autism, whereas the Westside (West LA) Regional Center has the highest prevalence. Even so, there is no difference in the prevalence of mental retardation in these Regional Centers (using epilepsy as a baseline, as it is assumed to be uniformly diagnosed). This is illustrated in Table 2.
|Regional Center||Autism Index||PMR+SMR Index||PMR Index||Epilepsy Index||Severe Behavior Index|
[Note: A diagnosis of 'severe behaviors' likely suffers from subjectivity as well, and is probably not, by itself, sufficient to fulfill CDDS eligibility criteria. In any case, a purely behavioral mini-epidemic could be ruled out by separating a Central Valley-equivalent group in Westside and evaluating differences in behavior.]
Group inequivalence is clear, but it is also clear that there is probably no difference in actual prevalence or incidence of autism between these two Regional Centers, whereas the difference in apparent prevalence is about 500%. This difference is equivalent to the state-wide difference between Q4 1993 and Q4 2004 (almost the entire 'autism epidemic').
As Paul Choate of CDDS Data Extraction notes, most of the 'epidemic' is confined to several Regional Centers in the greater Los Angeles area, as illustrated by the following figure [courtesy of CDDS]:
The epilepsy argument
Autism has been linked to a seizure liability, and the CDDS data itself shows that the prevalence of epilepsy among autistics is considerably higher to that of the general population. It follows that an environmental trigger capable of producing an epidemic of autism would also result in an epidemic of epilepsy. Surprisingly, we find in the data that the prevalence of epilepsy moves upward at about the same pace as the population in the state of California. Currently, annual growth in number of clients with epilepsy is about 0.6%.
The mental retardation argument
An environmental trigger that results in brain injury or insult should be expected to increase the probability of mental retardation, and thus result in an epidemic of all types of mental retardation. Instead we find that the number of clients with mental retardation is growing at a pace slower than that of the population in the state of California. Cases of profound mental retardation are actually on the decline. (This is explained by misdiagnoses of autism as mental retardation).
The numerical evidence in favor of the notion that an autism epidemic has not occurred at all is clearly overwhelming. Those who claim an epidemic has occurred are apparently not aware of the problem of group inequivalence. This is compounded by fundamental errors in the use of concepts such as incidence and prevalence. In light of these observations, I recommend that all scientific papers that claim to show an epidemic has occurred be retracted immediately.