In 1980, the third edition of the manual—the DSM-3—said children needed to show severe language deficits, bizarre reactions to their environment and a lack of interest in people, starting before 30 months old, to get a diagnosis. By 1994, the DSM-4 expanded the definition to more social and behavioral traits including repetitive behaviors or intense interests and created categories including Asperger’s syndrome, for milder cases.
Starting in 2013, the DSM-5 merged those categories into a single spectrum with three levels, added sensory issues and loosened the age limit on when symptoms needed to appear. At the time, some suspected that the shift could lower rates by leaving out kids who previously qualified, but that didn’t materialize, researchers said.
“Clinicians are assigning this diagnosis in a way they absolutely weren’t when autism first entered the DSM,” said David Mandell, an autism researcher and director of the Penn Center for Mental Health at the University of Pennsylvania.
Clinicians might also be less frequently assigning other labels such as intellectual disability, formally referred to as mental retardation, some data suggest. Substituting one diagnosis for another accounted for an estimated 64% of the increased autism cases from 2000 to 2010 in U.S. special-education enrollment, according to one 2015 analysis.