Monday, July 06, 2009

The highest autism prevalence ever?
Baron-Cohen et al. (2009) propose a world where there are 157/10,000 autistics. The authors used what they call a "novel approach" to autism epidemiology and this is worth a closer look.

The study is based on questions sent to schools about the UK SEN (Special Education Needs) register, and a survey, including a screening questionaire, sent to parents of students aged 5 to 9 years. Many schools, 66 out of 162, refused to participate. Of schools that agreed, 79 of 96, representing 8824 out of 11,635 children, provided SEN register information. Of the surveys sent to parents, 29%, representing 3373 out of 11,700 children, were responded to.

Participating schools located 83 children identified as autistic spectrum according to the SEN register. Of these, 10 children were assigned the specific diagnosis of autism (I'll call them "specific-autism"), while the rest were assigned other autistic spectrum diagnoses ("other-spectrum"). None of these diagnoses was verified by Baron-Cohen et al. (2009) in any way. These 83 SEN children constitute 94/10,000 of the proposed 157/10,000.

In the survey, parents were asked whether their child had an existing autistic spectrum diagnosis. The authors "confirmed," to their standards, through information from parents only, diagnoses for 19 children. Four others attended special schools and were assumed to be "confirmed" as autistic without any verification, for a total of 23. No information about how many of these children are specific-autism or other-spectrum is given. The 23 children were expanded into a prevalence of 99/10,000--impressively close to the SEN register figure.

The screening questionaire sent to parents was the recently re-named CAST (Childhood Autism Spectrum Test). All children reported by parents as scoring above one CAST threshold, and a minority of children scoring above a lower CAST threshold, and who were not already reported by the parent survey to be autistic, were invited for assessment. Eventually 77 children underwent assessment by ADOS, ADI-R, and clinical judgment. Eleven children were given autistic spectrum diagnoses--four specific-autism, seven other-spectrum.

So there are three sets of autistic children here: 83 SEN children, 23 diagnosis survey children, and 11 CAST children. The CAST children are considered to represent "unknown" or "undetected" autistics in schools, while the other two sets of children (SEN and diagnosis) represent equivalent samples of "known" autistics who have been located in two different ways from very nearly the same cohort.

Working from the diagnosis and CAST samples, Baron-Cohen et al. (2009) come up with a ratio of total number of cases (known plus unknown) to known cases, which yields a multiplier of 1.67. This in turn is applied to the SEN-based prevalence of 94/10,000, resulting in that grand total of 157/10,000.

And this all looks quite reasonable. Even the Neuroskeptic thinks so. But here are a few things I noticed.

The authors report no means of verifying whether the SEN and diagnosis samples are equivalent, an assumption on which the study depends. There is a dearth of demographic information, including the absence of any reported demographic measure common to both the SEN and diagnosis samples.

And for neither "known" sample (SEN and diagnosis) were any of the diagnoses verified either through direct assessment or direct access to records. Other recently reported UK epidemiology has not arrived at such a tidy agreement between prevalence of existing autism diagnoses and prevalence of autistics on the SEN register, nor at such a high prevalence using information from one or both sources.

Before they started sending out surveys, Baird et al. (2006) found that about 15% of 255 autistics with a "known" autism diagnosis (according to various health and clinical records) were not on the SEN register--under any diagnosis at all. This percentage would likely be higher if SEN autistic spectrum diagnoses only were considered.

Whether the 255 "known," pre-survey diagnoses from Baird et al. (2006) would be considered "confirmed" according to the standards of Baron-Cohen et al. (2009) is unclear to me, but in Baird et al. (2006), they add up to a prevalence of ~45/10,000. This is less than half the prevalence of what should be the comparable "known" diagnosis sample in Baron-Cohen et al. (2009).

Then there is Williams et al. (2008), where the authors had access to both medical and SEN records for an entire 14,062-child cohort. Using both kinds of records, they located a total of 86 "known" autistics for a prevalence of ~62/10,000. Of these, 36% were found in medical records only; 17% were found in SEN records only; and 47% were found in both.

Here again the "known" diagnosis total prevalence, derived from direct examination of complete records rather than selective responding to questions directed at schools or parents, is a much lower figure than found via both the SEN and diagnosis samples in Baron-Cohen et al. (2009). And Williams et al. (2008) show that diagnoses appearing in medical records, and diagnoses recorded via SEN, do not produce the same samples of autistic children--at least, not when there is direct access to these records for a complete cohort.

The original contribution of Baron-Cohen et al. (2009) is, according to the authors, the attempt to identify school-aged autistics who haven't been spotted as atypical. This is claimed to set their study apart from Baird et al. (2006). But 8 of the 77 children chosen for direct assessment through CAST screening in Baron-Cohen et al. (2009) were reported to be from "special schools" which, according to the authors, "require extensive assessment of individual special needs." Yet the authors suggest the CAST-screened sample was, in their words,

...quiet and cooperative at school and not difficult to manage and therefore teachers may not be aware that they have difficulties.
I don't know whether the CAST-screened special school children (who may or may not have been in the final 11) were on the SEN register, but doesn't this seem likely? Assuming for now (because you can only assume) that this was not under an autism diagnosis, in fact these were exactly the children screened for autism by Baird et al. (2006)--all children on the SEN register for any reason.

Further in this direction, Baron-Cohen et al. (2009) surprisingly report no way for the authors to verify whether the CAST-screened children were or were not on the SEN register, under any diagnosis--including autism. But the study hinges on the two samples, the SEN and CAST samples, being independent from each other.

One more thing to note is the actual composition of the 157/10,000. These numbers come from the reported data. Here goes:

11/10,000: "known" specific-autism children (SEN register)
23/10,000: "unknown" specific-autism children (CAST screening)
83/10,000: "known" other-spectrum children (SEN register)
40/10,000: "unknown" other-spectrum children (CAST screening)

If you add up the figures, you get the 157/10,000 total, of which 34/10,000--well within the range of previous findings--represents prevalence for the specific diagnosis of autism. The authors checked ratios of known to unknown autistic males vs females, and found no difference, showing that female autistics were not more likely than male autistics to be undetected. But the authors didn't check ratios of known to unknown autistics for specific-autism vs other-spectrum diagnoses.

If they did, they would find that for every known specific-autism child, there are about two unknown. The opposite is true for other-spectrum children, where about two are known for every unknown child. While only one-third of all specific-autism children are known, two thirds of other-spectrum children are known. Does this mean that children who meet criteria for the specific diagnosis of autism are much harder to spot, or are doing much better in school, than children who are popularly regarded as having "less severe" autistic spectrum diagnoses?

Or maybe the data are just very difficult to interpret. As usual, if I've made any factual errors, please let me know.


Baird, G., Simonoff, E., Pickles, A., Chandler, S., Loucas, T., Meldrum, D., & Charman, T. (2006). Prevalence of disorders of the autism spectrum in a population cohort of children in South Thames: the Special Needs and Autism Project (SNAP). Lancet, 368, 210-215.

Baron-Cohen, S., Scott, F., Allison, C., Williams, J., Bolton, P., Matthews, F., & Brayne, C. (2009). Prevalence of autism-spectrum conditions: UK school-based population study The British Journal of Psychiatry, 194 (6), 500-509 DOI: 10.1192/bjp.bp.108.059345

Williams, E., Thomas, K., Sidebotham, H., & Emond, A. (2008). Prevalence and characteristics of autistic spectrum disorders in the ALSPAC cohort Developmental Medicine & Child Neurology, 50 (9), 672-677 DOI: 10.1111/j.1469-8749.2008.03042.x


Stephanie Lynn Keil said...

Yes, I'm curious as to how "disabled" these apparently 1 in 64 people with an ASD are. If they are simply eccentric and can function perfectly well do they really need a diagnosis? If they can function well than according to the DSM they can't be diagnosed with an ASD because they don't meet this criteria:

"The disturbance causes clinically significant impairments in social, occupational, or other important areas of functioning."

Obviously, it's different in other places but I think that if the condition is not disabling than a person cannot be said to have an Autism Spectrum DISORDER.

And if 1 in 64 people have an ASD than where are they? Do most children grow out of this awkward phase in their teens and become normal adults? I am not seeing 1 in 64 people with autism in real life. At the college where I live I see eccentric people who function perfectly fine. Could they then be said to have an ASD even though they are not disabled by it, unlike me?

I think this will only make the diagnosis of ASD meaningless and so those that are disabled by it, like myself, will not be taken seriously.

It should only be used when the condition is disabling. If you go by this ASDs are MUCH rarer than 1 in 150, as it is here in the US.

Michelle Dawson said...

The criterion quoted,

"The disturbance causes clinically significant impairments in social, occupational, or other important areas of functioning"

is a DSM-IV-TR criterion for Asperger's Disorder. It does not appear in the criteria for Autistic Disorder or PDD-NOS.

I know many successful blind lawyers. They are definitely more successful than many sighted lawyers. Deciding that they cannot really be blind because they are so successful, as is being suggested, is not okay in my view.

Similarly I would disagree that a successful person with Down syndrome cannot really have Down syndrome, or a successful person with Tourette's can't really have Tourette's, and so on.

In the original post, I report the prevalence of autism (all autistic spectrum diagnoses) found in Williams et al. (2008), which was conducted in the UK and which I first wrote about a long time ago. The reported prevalence in this recent study, which has been ignored by autism advocates, is 1 in 162.

Marius Filip said...

"I know many successful blind lawyers. They are definitely more successful than many sighted lawyers."

They are all definitely more succesful than many sighted lawyers? There is not one single lousy guy among them?

From such a great snapshot perhaps I can safely assume that mediocrity is reserved for sighted lawyers, I guess.

I'm wondering how many lawyers being afraid of people and/or with mental retardation and/or repetitive behavior and/or severe temper tantrums and/or severe seizures and/or rigid thinking and/or ... (the list may continue) have you encountered.

It's interesting how you give an example where a disability does not affect the core of someone's professional functioning in a certain field yet jump from there to dismiss so easily the issue that Mrs. Keil was actually talking about.

Michelle Dawson said...

Let's see, I didn't state that I know every blind lawyer. Nor did I state that all the blind lawyers I know are successful. Here is what I wrote:

"I know many successful blind lawyers. They are definitely more successful than many sighted lawyers."

Mr Filip's conclusion that "mediocrity is reserved for sighted lawyers" is totally his own.

Being a lawyer requires numerous skills that blind people might be presumed to lack, including the ability to read text.

Neuroskeptic said...

Thanks for the link!

You're right that one of the problems with this study was that they didn't attempt to directly verify the existing diagnoses.

However they did classify some of the reported diagnoses as "confirmed" and others as "unconfirmed":

"Overall, 41 cases of autism-spectrum conditions were reported by
parents using the diagnosis survey: 37 from mainstream schools
and 4 from special schools (Fig. 2). All cases from special schools
were assumed confirmed since entry to a special school would
require extensive assessment of individual special needs. Of the
37 cases reported from mainstream schools, 24 (65%) parents
replied to the request for further information providing details
of the reported diagnosis. Two children had not actually received
diagnoses. Of the remaining 22 cases, 19 (16 boys, 3 girls)
included information consistent with having a diagnosis of an
autism-spectrum condition (the 3 other parent-reported cases
were too vague to assign an ICD–10 classification) (Fig. 2). The
ratio of confirmed:reported cases from the diagnosis survey was
therefore 19:24."

This doesn't strike me as an especially adaquate method, but it's better than nothing (it guards against the people who just ticked the "Autism" box instead of the "Downs Syndrome" box by mistake because they were next to each other).

Overall, I'm convinced by the claim that a signficant proportion of autism cases go undiagnosed at the primary school level. But I think we really need to know more about these kids - in particular, how "impaired" they are and whether they would benefit from treatment. If they turn out to be smartest kids in their classes (quite possible), then assuming that they need "treatment" might be unhelpful.

Michelle Dawson said...

Hi Neuroskeptic, great to see you here. And you make an excellent point re the use of the term "confirmed" in Baron Cohen et al. (2009), something I didn't make clear.

I have changed the text of my post slightly to make it clearer and more accurate in this respect. Many thanks.

In my view, all kids (and pretty much all adults) need a lot in the way of assistance, education, services, and so on.

This is true of both autistics and nonautistics, though the kinds of assistance, education, and so on that autistic children need is likely to be atypical given that, for instance, learning in autism is atypical.

The same autistic person may be extremely advanced in some areas and struggle greatly in others. Successful autistics can have a very uneven profile of abilities, against a typical baseline. Possibly autistics shouldn't spend all our time being required to do things we're lousy at.

Anemone said...

The more I think about this paper, the more confused I get. I've considered commenting twice already and given up, even though I know it's an important topic. I don't know if actually reading the paper would make things clearer or even muddier.

Every time this subject comes up, I wonder if this new and improved method of catching autistic people would catch or miss me. I also wonder about the interaction between diagnosis, neurotype, and environment. For example, would they catch more females and miss more males if they looked at people in the workplace/unemployed rather than at people in an education setting (which generally seems to favour girls over boys in its setup)? Also, which environments convince people they need a diagnosis, and are there situations that are so favourable that some who qualify for a diagnosis never want or need one?

I guess this is one of those "exploratory" subjects, that opens up new questions. At least, I hope they're looking at it like that, rather than as some definitive statement.

Sebstian said...

To me, talking about autism prevalence seems premature in that to me the question if there is a distinct difference between autism and not-autism ('cut-of') or if rather every person is to a different degree autistic or not is not answered. I tend to see a dimensional model of autism/normal which would include every person. A question of social implications would be how cut-of changes.

Joseph said...

Does this mean that children who meet criteria for the specific diagnosis of autism are much harder to spot, or are doing much better in school, than children who are popularly regarded as having "less severe" autistic spectrum diagnoses?

I think that's entirely plausible.

Here's a data point of something that is not in the same dimension, but is analogous. In Cal DDS, only about 7% of those identified as having mental retardation also have a classification of autism.

In reality that should be more like 20%. It could even be close 40%.

So they might be missing 2 out of 3, and up to 5 out of 6 of autistics in the population with intellectual disabilities.

This is at least comparable to what they miss in the general population.

Neuroskeptic said...

Michelle - "Possibly autistics shouldn't spend all our time being required to do things we're lousy at."

Hear, hear. Incidentally, I'm sure Simon Baron-Cohen would agree. He seems generally pretty in tune with such concerns, which is good given his academic prominence.

AnneC said...

To me it makes more sense to consider autism a way brains can work first and foremost.

It seems like it was unique enough of a phenomenon for Kanner, etc., to notice and distinguish from, say, global cognitive impairment and from global cognitive adeptness.

The fact that autistics tend to have what is considered by many to be "uneven" skill levels/development suggests that there's sort of an atypical-ness that goes "all the way down", so to speak.

And the question of what distinguishes that atypical-ness is to me a separate one from how much a person might be "suffering" or seriously limited at any given point in time/situation.

Hence, it makes sense to me to study populations in which autism may have been "missed" previously, etc. The idea of the science (correct me, anyone, if I'm wrong) seems to me not to make anyone "feel good" or "feel bad" but to accurately characterize phenomena worth characterizing.

laurentius rex said...

If ever there were more proof wanting about the social construction of autism, the malleability of the prevalence rates and diagnostic instruments demonstrates this.

Autism will I expect continue to increase until it no longer becomes a category and the burden of difference is spread amongst newer medical nosologies fitting the societal demands of a future we have not yet reached.

Autism only exists because there is an external reason for it to exist, a bit of an funtionalist argument I grant you, but non the less worth considering for that.

The point about disability or if you want to call it "clinical significance" is that this is not an absolute but a variable, dependent upon context whose intersubjectivity is not reliable and open to constant renegotiation.

It is a game of Fizzbin

laurentius rex said...

Or by another analogy, one can be a square peg rattling around in a sufficiently large round hole until some disturbance shakes one loose.

Any subsequent tightening of the hole might mean one no longer fits in it.

The same could perhaps happen in reverse where it the hole that expands due to the friction of the sharp edges of the square pegs upon it.

I am severely tempted to quit from the whole arena of autism as the category becomes increasingly meaningless in the crossfire of the warring parties who dispute it.

Michelle Dawson said...

In response to Joseph, there isn't yet a consistent, credible figure for proportion of autistics who score (at any given time) in the range of intellectual disability on various tests of intelligence or developmental level.

In response to Anne C., I agree that autism is a cognitive phenotype (another way of saying that autism involves atypical brain function)--this is what a lot of my work is about. But this view is not the consensus in autism research, where there is no consensus at all as to what autism is.

Also, we've written a book chapter (Mottron et al., 2008, which was mostly written circa 2005) which among other things explores the problem of seeing autistics as defective nonautistics. So far, these models have come with very powerful theories but have made poor predictions. In my view, this should be learned from.

In response to Mr Rex, prevalence for the specific diagnosis of autism has been reported consistently within a not-entirely-disgraceful range since 1996. This is keeping in mind that these are prevalence--not incidence--figures. No one should be surprised when prevalence figures differ somewhat from place to place.

Joseph said...

In response to Joseph, there isn't yet a consistent, credible figure for proportion of autistics who score (at any given time) in the range of intellectual disability on various tests of intelligence or developmental level.

To clarify, I was not referring to the true prevalence of intellectual disability among autistics. It's debatable whether autistics are tested right and so forth.

I was referring to surveys of ASD in populations previously identified as having intellectual disability. For example, La Malfa et al. (2004) finds PDD in 39.2% of a population with ID, even though only 7.8% were previously identified as such. Much earlier work with children and adults had produced similar results (Shah, Holmes & Wing, 1982; Wing & Gould, 1979).

The point is that it's not surprising that ASD is missed in people already labeled with another neurological disability.

Michelle Dawson said...

La Malfa et al. (2004) used a very short and simple screening instrument that involves no direct observation of the person being "diagnosed" and that is supposed to capture as autitic "doubtful" cases not meeting PDD-NOS criteria.

Also, La Malfa et al. (2004), who assume that 70% of autistics score in the range of intellectual disability, depended on WAIS scores, but report ranges of scores that are below the Wechsler floor. They do not mention how they managed this.

Arnel said...

Dear Michelle,
I would really be interested to know what your credentials are. What is your degree in? Where did you earn your degree? Were you really dx with autism, or did you ascertain on your own that you have/had autistic traits as a child? Do you see yourself so now? And if you are so well able to function, then how did you happen to achieve this without intervention?

I think I could entertain your ideas more sincerely if I had these questions answered. Thank you for your time.


Arnel Gonce
Mother of autistic twins

Arnel said...

Where are your credentials that show me you know what you are talking about?
It would be nice to see your educational credentials, as well as career. How about it?
Arnel Gonce,

My apologies if this is a repeat comment.

Michelle Dawson said...

In response to Arnel, I've never attended university as a student.

On this blog there are links to my research page (French) and a list of papers etc (English).

The way to verify what I wrote in this post (or any of my science-related blog posts) is to read the sources I cite as well as a good chunk at least of the related literature (in this case autism epidemiology), if you haven't already. It's always especially important to read primary sources. It is also important to familiarize yourself with standards in non-autism areas.

If Arnel spots any factual errors in my writing, then as always I would appreciate knowing.

Arnel said...

I'm still unclear as to Michelle Dawsons credentials. Writing papers, in my opinion does not make one an expert in any field. I find it fairly disturbing that someone could actually act to prevent a family from receiving funding to help their child merely based on the fact that they share the same diagnosis. I cannot verify even whether Michelle was self diagnosed or was diagnosed by a qualified individual, when she was diagnosed, as a child or an adult, and whether she herself has received any type of therapy. Obviously she is very high functioning, which is even more disturbing that she feels compelled to crusade against parents helping their children who may be less functioning than herself. I can speak from my own experience as a parent. At the age of 14, my twins, after explaining what she has done, are horrified--yes, they themselves who had four years of ABA treatment--are HORRIFIED that someone would actually have a child with autism spectrum disorder and NOT help them. They are glad I did what I did to help them, and are grateful that they can function in this world better than if I had left them untreated.

Michelle Dawson said...

In response to Arnel, see this. This too. This too. And so on.

I err on the side of believing that most people have something to say, and I don't mind if people are somewhat off-topic for a comment or two. Also, I'm sort of curious as to how far Arnel will go.

But still I'm going to suggest Arnel find another forum--there are plenty that would welcome her standards--in which to express her fact-free vilification.

Readers here deserve better, as do autistics (and nonautistics) in general.

H said...

I have Baron Cohen's paper several times as I am likely to be asked a question about it in my exam.I share many of the same concerns as Michelle about the methodology. Pleasingly, our lecturer has directed us to this blog on a number of occassions. Its nice to a different perspective is gradually being incorporated into our syllabus.
Thanks for the interesting discussion.