August 21, 2019
Université de Montréal
If the current trend in
diagnostic practices holds, the definition of autism may get too blurry to be
meaningful, a research team finds.
Around the world, the number
of people diagnosed with autism is rising. In the United States, the prevalence
of the disorder has grown from 0.05% in 1966 to more than 2% today. In Quebec,
the reported prevalence is close to 2% and according to a paper issued by the
province's public health department, the prevalence in Montérégie has increased
by 24% annually since 2000.
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However, Dr. Laurent Mottron,
a professor at Université de Montréal's Department of Psychiatry and a
psychiatrist at the Hôpital en santé mentale de Rivière-des-Prairies of the
CIUSSS du Nord-de-l'Île-de-Montréal, has serious reservations about this data.
After studying meta-analyses
of autism data, his research team found that the difference between people
diagnosed with autism and the rest of the population is actually shrinking.
This study is published today
[August 21] in JAMA Psychiatry.
Less differentiation observed
Dr. Mottron worked with intern
Eya-Mist Rødgaard of the University of Copenhagen and four other researchers
from France, Denmark and Montreal to review 11 meta-analyses published between
1966 and 2019, with data drawn from nearly 23,000 people with autism.
The meta-analyses showed that
people with autism and people in the rest of the population exhibit significant
differences in seven areas: emotion recognition, theory of mind (ability to
understand that other people have their own intentions), cognitive flexibility
(ability to transition from one task to another), activity planning,
inhibition, evoked responses (the nervous system's response to sensory
stimulation) and brain volume. Together, these measurements cover the basic
psychological and neurological components of autism.
Dr. Mottron and his team
looked at the "effect size" -- the size of the differences observed
between people with autism and people without it -- and compared its
progression over the years. This measurement is a statistical tool that
quantifies the size of difference in a specific characteristic between two
groups of subjects.
They found that, in each of
the assessed areas, the measurable difference between people with autism and
people without it has decreased over the past 50 years. In fact, a
statistically significant dilution in effect size (ranging from 45% to 80%) was
noted in five of these seven areas. The only two measurements that didn't show
significant dilution were inhibition and cognitive flexibility.
"This means that, across
all disciplines, the people with or without autism who are being included in
studies are increasingly similar," said Mottron. "If this trend
holds, the objective difference between people with autism and the general
population will disappear in less than 10 years. The definition of autism may
get too blurry to be meaningful -- trivializing the condition -- because we are
increasingly applying the diagnosis to people whose differences from the
general population are less pronounced."
To verify that the trend was
unique to autism, the research team also analyzed data on similar areas from
studies on schizophrenia. They found that the prevalence of schizophrenia has
stayed the same and the difference between people with schizophrenia and those
without it is increasing.
Changes in diagnostic
practices
The diagnostic criteria for
autism haven't changed over the years that the differences have diminished.
Instead, Dr. Mottron believes that what has changed are diagnostic practices.
"Three of the criteria
for an autism diagnosis are related to sociability," he said. "Fifty
years ago, one sign of autism was a lack of apparent interest in others.
Nowadays, it's simply having fewer friends than others. Interest in others can
be measured in various ways, such as making eye contact. But shyness, not
autism, can prevent some people from looking at others."
To complicate matters, the
term "autism" has fallen out of favour, replaced by "autism
spectrum disorder," a sign that there's a new belief that there various
different forms of the condition exist. This has prompted some people to
question whether autism exists at all.
"And yet, autism is a
distinct condition," says Dr. Mottron. "Our study shows that changes
in diagnostic practices, which have led to a false increase in prevalence, are
what's fuelling theories that autism doesn't really exist."
Even though Dr. Mottron
recognizes that there is a continuum between people with autism and those
without it, he believes that such a continuum could result from the
juxtaposition of natural categories. "Autism is a natural category at one
end of the socialization continuum. And we need to focus on this extreme if
want to make progress," he said.
In his opinion, autism studies
include too many participants who aren't sufficiently different from people
without autism. In contrast to the generally prevailing scientific belief, Dr.
Mottron thinks that including more subjects in studies on autism, as it is
currently defined, reduces the likelihood of discovering new things about the
mechanisms of the disorder. No major discoveries have been made in this field
in the last 10 years.
Story Source:
Materials provided by Université de Montréal.
Original written by Daniel Baril. Note: Content may be edited for style
and length.
Journal Reference:
Eya-Mist Rødgaard, Kristian
Jensen, Jean-Noël Vergnes, Isabelle Soulières, Laurent Mottron. Temporal
Changes in Effect Sizes of Studies Comparing Individuals With and Without
Autism. JAMA Psychiatry, 2019; DOI: 10.1001/jamapsychiatry.2019.1956
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