Children who are on the severe end of the autism spectrum are increasingly being left out of studies on treating the developmental disorder, new research suggests.

A review of 367 studies on children with autism conducted between 1991 and 2013 finds that the proportion of research that included those with severe autism “decreased significantly.”

“We found a marked decline in the inclusion of people severely affected by autism in clinical research, even using a very liberal definition of severity,” said Matthew Siegel of Tufts University School of Medicine and Maine Behavioral Healthcare. Siegel is a senior author of the literature review, which was published recently in the Journal of Autism and Developmental Disorders and is believed to be the first to assess whether research is overlooking those with severe autism.

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Overall, about half of the studies that were reviewed included those who are severely affected. But, the proportion declined substantially from more than 90 percent in the early 1990s to just 30 percent in the 2010s, the study found.

For each year, the odds that individuals with severe autism were included in studies dropped by 16 percent over the course of the time examined.

That lack of representation has consequences, researchers said.

“The picture of autism that emerges from the recent literature appears to be increasingly skewed toward the high functioning population, and risks leaving behind those who arguably have the greatest morbidity and need,” Siegel said.

Beyond representation alone, the study also found that research frequently left out information on participants’ communication abilities and used varied measures for intelligence and adaptive functioning, making it difficult to draw conclusions across studies.

Those behind the review said that greater awareness is needed about including people who are severely affected in studies and they recommended establishing consistent measures for reporting how those participating in studies are affected by autism.