Autism, also called autism spectrum disorder[a] (ASD) or autism spectrum condition (ASC),[8] is a neurodevelopmental disorder marked by restricted and repetitive patterns of behavior and deficits in reciprocal social communication. Other common signs include perseverative interests, stereotypic body movements (stimming), rigid routines, hyper- or hyporeactivity to sensory input, and difficulty with social interaction and verbal and nonverbal communication. Autism is clinically regarded as a spectrum disorder, meaning that it can manifest very differently in each person. For example, some are nonspeaking, while others have proficient spoken language. Because of this, there is wide variation in the support needs of people across the autism spectrum.

Psychiatry has traditionally classified autism as a mental disorder, but the autism rights movement (and an increasing number of researchers) see autistic people with low support needs as a part of humanity's natural neurodiversity.[9] From this point of view, autistic people may also be diagnosed with a disability of some sort, but that disability may be rooted in the systemic structures of a society rather than in the person;[10] thus, proponents argue that autistic people should be accommodated rather than cured.[11] The neurodiversity perspective has led to significant controversy among those who are autistic and advocates, practitioners, and charities.[12][13]

There are many theories about the causes of autism; it is highly heritable and mainly genetic, but many genes are involved, and environmental factors may also be relevant.[14] The syndrome frequently co-occurs with attention deficit hyperactivity disorder, epilepsy, and intellectual disability. Disagreements persist about what should be included as part of the diagnosis, whether there are meaningful subtypes or stages of autism,[15] and the significance of autism-associated traits in the wider population.[16][17] The combination of broader criteria, increased awareness, and the potential increase of actual prevalence, has led to a trend of steadily increasing estimates of autism prevalence,[18] unintentionally aiding the disproven myth perpetuated by anti-vaccine activists that it is caused by vaccines.[19]

Although early intervention services based on applied behavior analysis (ABA) can help children with autism gain self-care, social, and language skills,[20][21][22][23] independent living is unlikely in more severe cases. Speech and occupational therapy, as well as augmentative and alternative modes of communication, are effective adjunctive therapies, but some in the autism rights movement consider ABA therapy unethical and unhelpful.[24] Pharmacological treatments may also be useful; the atypical antipsychotics risperidone and aripiprazole are empirically validated for alleviating co-morbid irritability, though these drugs tend to be associated with sedation and weight gain.[25]

Classification
Spectrum model
Before the DSM-5 (2013) and ICD-11 (2022) diagnostic manuals 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 the syndrome formerly known as 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.[26]

The DSM-5 and ICD-11 use different categorization tools to define this spectrum. DSM-5 uses a "level" system, which ranks how in need of support the patient is,[27] while the ICD-11 system has two axes, intellectual impairment and language impairment,[28] as these are seen as the most crucial factors.

Autism is currently defined as a highly variable neurodevelopmental disorder[29] 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.[30] 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 person.[31] How it presents in a person can depend on context, and may vary over time.[32]

While the DSM and ICD greatly influence 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.[33][34][35][36]

As of 2023, empirical and theoretical research is leading to a growing consensus among researchers that the established ASD criteria are ineffective descriptors of autism as a unitary biological entity, and that alternative research approaches must be encouraged, such as going back to autism prototypes, exploring new causal models of autism, or developing transdiagnostic endophenotypes.[37] Proposed alternatives to the current disorder-focused spectrum model deconstruct autism into at least two separate phenomena: (1) a non-pathological spectrum of behavioral traits in the population,[38][39] and (2) the neuropathological burden of rare genetic mutations and environmental risk factors potentially leading to neurodevelopmental and psychological disorders,[38][39] (3) governed by an individual's cognitive ability to compensate.[38]