The increased prevalence of autism in people born after 1980 is mostly the result of changes in reporting and diagnostic practices, according to Forbes. In the early 1990s, the diagnostic criteria for autism changed to encompass a wider range of symptoms and behaviors. A Danish study of 670,000 children born between 1980 and 1991 concludes that 60 percent of the apparent increase in autism's prevalence since 1980 is attributable to these changes in diagnostic criteria.
Autism first received an official medical definition in 1980 with the release of the third edition of the Diagnostic and Statistic Manual of Mental Disorders, explains Unstrange Minds. Infantile Autism describes a set of six criteria related to social responsiveness and language development, but a diagnosis could only be made if all criteria were met and if all symptoms manifested during early childhood, according to the DSM III.
The 1987 revision to this edition changed the name to Autistic Disorder and removed the age limit, notes Unstrange Minds. This revision also expanded the set of diagnostic criteria from six symptoms to 16, encompassing social behaviors, imaginative activity, language disorders and physical activity. A diagnosis required that a patient only exhibit eight of these symptoms, which expanded the number of potential diagnoses. The DSM IV further refined the definition, describing a range of 14 criteria and requiring that the patient exhibit six of them.
The 2013 release of the DSM V changed the name to Autism Spectrum Disorder, reflecting the wider range of behaviors that the diagnosis covers, as noted by the Centers for Disease Control and Prevention. A diagnosis now accounts for the severity of symptoms and no longer requires a patient to meet a specific number of criteria, according to the DSM V's definition. Moreover, while previous editions of the DSM describe complete sets of behaviors, the DSM V notes that its criteria are illustrative, not exhaustive.