Fallacy of a "changepoint" in the autism increase

This is my comment on the 2010 paper by Michael E McDonald and John P Paul. "Timing of increased autistic disorder cumulative incidence" Environ. Sci. Technol. 44, 2112-2118.

The authors reckon that they can usefully analyse the autism increase curve by making an approximation of it in terms of two straight lines. They then point out that the junction of the two lines, the "changepoint", at which they suggest the autism increase began, is about 1988-9.

In reality, just about any curve of roughly exponential increase form can have a couple of straight lines imposed on it such as to passably plausibly account for the entire data set. Especially if you set the time axis long enough so the increase will look like an abrupt event rather than a gradual one.

Nice correlations can be found for each line with its corresponding part of the data, and impressively high significance levels pointed out. It does not however follow that the increase is usefully understood in terms of such pairs of lines.

The paper of McDonald and Paul features remarkably small graphs of the increase, which tend to give the impression that there was no increase before their "changepoint". And they use a whopping 50-year timespan. It would be better to have larger (taller) graphs around the critical period so we can examine the end of the "level" section more closely.

I will put here this superb graph of the US IDEA data that others have kindly prepared. (Right-click it to open an enlargement in a separate window.)
(Graph provided by Thoughtful House Center for Children,
Graphing IDEA Professional 2010, Thoughtful House, Austin, TX,
Accessed at http://www.thoughtfulhouse.org/disabilities/ on November 19, 2010.)

It should be quite obvious from this graph that there was not some abrupt changepoint around 1988, and not around any other year either.

One can also see that the increase was already beginning by 1980. This is nicely in line with the update review of the antiinnatia theory in which I state that the cause of the increase was the introduction of non-gamma-2 dental amalgams in the 1970s. (Among other evidence, the world's most famous dentist, Hal Huggins, said they became the new "state of the art" in 1975-6.)

Furthermore there is a conspicuous de-steepening of the gradient at 1992-3. I expect that this was due to some minor improvement of the amalgam usage protocol, such as avoiding for pregnant women, a slightly less toxic formulation, or improved suction systems.

I would not have commented on this "changepoint" paper except that it has been cited by Andrew Wakefield and some Age of Autism people as supposedly showing that the start of increase coincided with some changes in vaccination usage. No it didn't.

Autism did not begin only in the 1930s

In two weeks time there will be published a new book:
The Age of Autism: Mercury, Medicine, and a Man-made Epidemic
By Dan Olmsted, Mark Blaxill

I've not read it (yet) but I can anticipate that I would agree with the authors that autism has increased and involved mercury but would be unpersuaded by their reckoning that vaccinations have been a major factor in the increase.

Here I shall just comment on this first sentence of a preview extract circulated by Safeminds:

"We believe that autism was newly discovered in the 1930s for the simple reason that it was new."

But this first sentence can be shown to be mistaken. Dr Down of Down Syndrome fame had already in the 19th century given good descriptions of both infantile autism and regressive autism.
Reference: Down, J.L. Mental Affections of Childhood and Youth, 1887 originally, re-issued as Classics in Developmental Medicine, No. 5, 1990 Mac Keith Press, London

It was discussed in the 2006 Awares conference. This is a link to the paper by Darrold Treffert.

I also suggest that the concept of the "holy fool" such as portrayed in the play and opera Boris Godunov corresponds with mild autism: a person who speaks the truth that others cannot. And bear in mind that the rare autistic individuals would be likely to have had a hard time surviving in earlier ages; even mildly autisticky people might have had a hard time sufficient to prevent them producing children.

Furthermore, there is the important consideration that a rare condition such as autism pre-1970 would only come to the attention of a person once there is a sufficient level of cosmopolitanism, due to urbanisation and transport, enabling one person to "survey" a sufficiently large sample of people. Down made his observation in 19th century England after the railways were established in the world's first modernised country. Thereafter, the US and Austria started to catch up industrially and so autism came to attention there too. That's not to say that the personal links to mercury stated in the AoA book cannot be also part of the story.