model Before the diagnostic manuals the DSM-5 (2013) and ICD-11 (2022) were adopted, what is now called ASD was found under the diagnostic category pervasive developmental disorder. The previous system relied on a set of closely related and overlapping diagnoses such as Asperger syndrome and Kanner syndrome. This created unclear boundaries between the terms, so for the DSM-5 and ICD-11, a spectrum approach was taken. The new system is also more restrictive, meaning fewer people now qualify for diagnosis. The DSM-5 and ICD-11 use different categorisation tools to define this spectrum. DSM-5 uses a "level" system, which ranks how in need of support the patient is, while the ICD-11 system has two axes: intellectual impairment and language impairment, as these are seen as the most crucial factors. It is now known that autism is a highly variable neurodevelopmental disorder that is generally thought to cover a broad and deep spectrum, manifesting very differently from one person to another. Some have high support needs, may be non-speaking, and experience developmental delays; this is more likely with other co-existing diagnoses. Others have relatively low support needs; they may have more typical speech-language and intellectual skills but atypical social/conversation skills, narrowly focused interests, and wordy, pedantic communication. They may still require significant support in some areas of their lives. The spectrum model should not be understood as a continuum running from mild to severe, but instead means that autism can present very differently in each individual. How a person presents can depend on context, and may vary over time. While the DSM and ICD are greatly influenced by each other, there are also differences. For example, Rett syndrome was included in ASD in the DSM-5, but in the ICD-11 it was excluded and placed in the chapter on Developmental Anomalies. The ICD and the DSM change over time, and there has been collaborative work toward a convergence of the two since 1980 (when DSM-III was published and ICD-9 was current), including more rigorous biological assessment—in place of historical experience—and a simplification of the classification system. ICD The World Health Organization's International Classification of Diseases (11th Revision), ICD-11, was released in June 2018 and came into full effect as of January 2022. It describes ASD as follows: ICD-11 was produced by professionals from 55 countries out of the 90 involved and is the most widely used reference worldwide. DSM The American Psychiatric Association's Diagnostic