[10][11][12] Autism is a highly variable neurodevelopmental disorder[13][14] whose symptoms first appear during infancy or childhood, and generally follows a steady course without remission.

[20] Individual symptoms of autism occur in the general population and appear not to associate highly, without a sharp line separating pathologically severe from common traits.

Autistic toddlers differ more strikingly from social norms; for example, they have less eye contact and turn-taking, and do not have the ability to use simple movements to express themselves, such as pointing at things.

[22] Three- to five-year-old autistic children are less likely to exhibit social understanding, approach others spontaneously, imitate and respond to emotions, communicate nonverbally, and take turns with others.

[25] Differences in communication may be present from the first year of life, and may include delayed onset of babbling, unusual gestures, diminished responsiveness, and vocal patterns that are not synchronized with the caregiver.

[20] An estimated 0.5% to 10% of individuals with classic autism show unusual abilities, ranging from splinter skills such as the memorization of trivia to the extraordinarily rare talents of prodigious autistic savants.

[20] It was presumed initially that there was a common cause at the genetic, cognitive, and neural levels for classic autism's characteristic triad of symptoms.

[34] However, over time, there was increasing evidence that autism was instead a complex and highly heritable disorder whose core aspects have distinct causes which often co-occur.

One study done by Elia et al. (2000) used magnetic resonance imaging (MRI) on the midsagittal area of the cerebrum, midbrain, cerebellar vermis, corpus callosum, and vermal lobules VI and VII to measure brain atypicalities in children with low-functioning autism.

The results suggested that the midbrain structures correlate with certain developmental behavioral aspects such as motivation, mnemonic, and learning processes, though there is more research needed to confirm this.

[38] Although theories regarding vaccines lack convincing scientific evidence, are biologically implausible,[40] and originated from a fraudulent study,[41] parental concern about a potential vaccine link with autism (and subsequent concern about ASD) has led to lower rates of childhood immunizations, outbreaks of previously controlled childhood diseases in some countries, and the preventable deaths of several children.

The children would also be required to exhibit six other symptoms from three macro-categories pertaining to qualitative impairment in social interactions, quantitative abnormalities in communication, and restricted/repetitive/stereotyped patterns of behavior, interests, and activities.

[48] Classic autism was said to be characterized by widespread abnormalities of social interactions and communication, severely restricted interests, and highly repetitive behavior.

[15] Of the PDDs, Asperger syndrome was closest to classic autism in signs and likely causes; Rett syndrome and childhood disintegrative disorder share several signs with it, but were understood to potentially have unrelated causes; PDD not otherwise specified (PDD-NOS; also called atypical autism) was diagnosed when the criteria were not met for one of the other four PDDs.

[52][53] Those who display symptoms for LFA usually have "impairments in all the three areas of psychopathology: reciprocal social interaction, communication, and restricted, stereotyped, repetitive behaviour".

Specific behavioral impairments that may be exhibited by a person with LFA include adherence to nonfunctional rituals or routines, repetitive motor functions such as hand flapping or complex whole body movements, and restrictive or obsessive patterns of interest that are abnormal.

[25] The main goals when treating autistic children are to lessen associated deficits and family distress, and to increase quality of life and functional independence.

Applied behavioral analysis (ABA) is considered the most effective therapy for autism spectrum disorders by the American Academy of Pediatrics.

[74] Early, intensive ABA therapy has demonstrated effectiveness in enhancing communication and adaptive functioning in preschool children;[75] it is also well-established for improving the intellectual performance of that age group.

[77] Treatment approaches have little empirical support in quality-of-life contexts, and many programs focus on success measures that lack predictive validity and real-world relevance.

[5] After it was recognised as a distinct disorder, reports of autism cases substantially increased, which was largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness[82][83] (particularly among women).

They include: The Neo-Latin word autismus (English translation autism) was coined by the Swiss psychiatrist Eugen Bleuler in 1910 as he was defining symptoms of schizophrenia.

[91] The word autism first took its modern sense in 1938 when Hans Asperger of the Vienna University Hospital adopted Bleuler's terminology autistic psychopathy in a lecture in German about child psychology.