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Effect sizes of studies comparing individuals with, without autism decrease over time

Differences between individuals with autism and those without have lessened over time, which may be tied to changes in diagnostic practices, according to a meta-analysis published in JAMA Psychiatry.

“The diagnostic criteria for autism have been revised several times, and our understanding of autism has evolved from a narrowly defined clinical picture to a spectrum of conditions of uncertain similarity,” Laurent Mottron, MD, PhD, Hôpital Rivière-des-Prairies in Montreal, and colleagues wrote. “Possible changes in diagnostic practices may have resulted in empirical studies assessing an increasingly heterogeneous population, including individuals with less profound deviations from normal that would not have previously been classified as autistic.”

In their meta-analysis, the researchers examined the link between publication year and effect size of autism-control group comparisons across several domains of published autism neurocognitive research in 11 meta-analyses.

Mottron and colleagues included analyses that had data within a 15-year or longer span, compared group-level differences between individuals with autism and controls and reported effect size, sample size and relevant task or method for each study. They also examined the temporal evolution of similar variables in schizophrenia to distinguish temporal trends specific to autism from possible confounders.

The 11 meta-analyses included data on 27,723 individuals. The researchers reported that the effect sizes for seven distinct differences in psychological and neurological constructs within social, executive and neurologic domains between groups with autism and control groups decreased over time.

Although Mottron and colleagues found downward temporal trends for effect size for all constructs, the trends were significant in five of the seven constructs:

• emotion recognition (slope: –0.028; 95% CI, –0.048 to –0.007);
• theory of mind (slope: –0.045; 95% CI, –0.066 to –0.024);
• planning (slope: –0.067; 95% CI, –0.125 to –0.009);
• P3b amplitude (slope: –0.048; 95% CI, –0.093 to –0.004); and
• brain size (slope: –0.047; 95% CI, –0.077 to –0.016).

Contrastingly, the temporal trends were not significant for any of the three analogous constructs in schizophrenia.

“Pioneers of a spectrum view of autism argued that research on narrowly defined subtypes is of limited value because findings can only be generalized to a small group of individuals. However, gradual changes to a diagnostic category, such as autism, and blurring of the distinction between autistic traits and autism could potentially affect our ability to advance mechanistic models of the condition,” the investigators wrote. “The belief within autism research that large heterogeneous populations are preferable compared with small narrowly defined ones in the search for scientific truth may be open to question.” – by Savannah Demko

Disclosure: The authors report no relevant financial disclosures.