Summary: A new meta-analysis of autism data reveals the difference between people diagnosed with autism and the general population is shrinking. Measurable differences between people with ASD and people not on the spectrum has decreased over the past 50 years. Researchers say the definition of autism may be getting too blurry to be meaningful, and the condition may be at risk of being trivialized.
Source: University of Montreal
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.
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 in JAMA Psychiatry, the most prestigious journal in the field of psychiatry. Given the importance of its findings, the study is also the subject of the journal’s editorial.
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.
University of Montreal
Jeff Heinrich – University of Montreal
The image is credited to University of Montreal.
Original Research: Open access
“Temporal Changes in Effect Sizes of Studies Comparing Individuals With and Without Autism: A Meta-analysis”. Mottron et al.
JAMA Psychiatry. doi:10.1001/jamapsychiatry.2019.1956
Temporal Changes in Effect Sizes of Studies Comparing Individuals With and Without Autism: A Meta-analysis
The definition and nature of autism have been highly debated, as exemplified by several revisions of the DSM (DSM-III, DSM-IIIR, DSM-IV, and DSM-5) criteria. There has recently been a move from a categorical view toward a spectrum-based view. These changes have been accompanied by a steady increase in the prevalence of the condition. Changes in the definition of autism that may increase heterogeneity could affect the results of autism research; specifically, a broadening of the population with autism could result in decreasing effect sizes of group comparison studies.
To examine the correlation between publication year and effect size of autism-control group comparisons across several domains of published autism neurocognitive research.
This meta-analysis investigated 11 meta-analyses obtained through a systematic search of PubMed for meta-analyses published from January 1, 1966, through January 27, 2019, using the search string autism AND (meta-analysis OR meta-analytic). The last search was conducted on January 27, 2019.
Meta-analyses were included if they tested the significance of group differences between individuals with autism and control individuals on a neurocognitive construct. Meta-analyses were only included if the tested group difference was significant and included data with a span of at least 15 years.
Data Extraction and Synthesis
Data were extracted and analyzed according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline using fixed-effects models.
Main Outcomes and Measures
Estimated slope of the correlation between publication year and effect size, controlling for differences in methods, sample size, and study quality.
The 11 meta-analyses included data from a total of 27 723 individuals. Demographic data such as sex and age were not available for the entire data set. Seven different psychological and neurologic constructs were analyzed based on data from these meta-analyses. Downward temporal trends for effect size were found for all constructs (slopes: –0.067 to –0.003), with the trend being significant in 5 of 7 cases: emotion recognition (slope: –0.028 [95% CI, –0.048 to –0.007]), theory of mind (–0.045 [95% CI, –0.066 to –0.024]), planning (–0.067 [95% CI, –0.125 to –0.009]), P3b amplitude (–0.048 [95% CI, –0.093 to –0.004]), and brain size (–0.047 [95% CI, –0.077 to –0.016]). In contrast, 3 analogous constructs in schizophrenia, a condition that is also heterogeneous but with no reported increase in prevalence, did not show a similar trend.
Conclusions and Relevance
The findings suggest that differences between individuals with autism and those without the diagnosis have decreased over time and that possible changes in the definition of autism from a narrowly defined and homogenous population toward an inclusive and heterogeneous population may reduce our capacity to build mechanistic models of the condition.