Autism presents in a wide degree, from those who are nearly dysfunctional and apparently mentally handicapped to those whose symptoms are mild or remedied enough to appear unexceptional ("normal") to the general public. In terms of both classification and therapy, autistic individuals are often divided into those with an IQ<80 referred to as having "low-functioning autism" (LFA), while those with IQ>80 are referred to as having "high-functioning autism" (HFA). Low and high functioning are more generally applied to how well an individual can accomplish activities of daily living, rather than to IQ. The terms low and high functioning are controversial and not all autistic's accept these labels.

This discrepancy can lead to confusion among service providers who equate IQ with functioning and may refuse to serve high-IQ autistic people who are severely compromised in their ability to perform daily living tasks, or may fail to recognize the intellectual potential of many autistic people who are considered LFA. For example, some professionals refuse to recognize autistic's who can speak or write as being autistic at all, because they still think of autism as a communication disorder so severe that no speech or writing is possible.

As a consequence, many "high-functioning" autistic persons, and autistic people with a relatively high IQ, are under diagnosed, thus making the claim that "autism implies retardation" self-fulfilling. The number of people diagnosed with LFA is not rising quite as sharply as HFA, indicating that at least part of the explanation for the apparent rise is probably better diagnostics.

In the current Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR), the most significant difference between Autistic Disorder (Kanner's) and Asperger's syndrome is that a diagnosis of the former includes the observation of "[d]belays or abnormal functioning in at least one of the following areas, with onset prior to age 3 years: (1) social interaction, (2) language as used in social communication, or (3) symbolic or imaginative play, while a diagnosis of Asperger's syndrome observes "no clinically significant delay" in these areas.

The DSM makes no mention of level of intellectual functioning, but the fact that Asperger's autistic's as a group tend to perform better than those with Kanner's autism has produced a popular conception that Asperger's syndrome is synonymous with "higher-functioning autism," or that it is a lesser disorder than autism. There is also a popular but not necessarily true conception that all autistic individuals with a high level of intellectual functioning have Asperger's autism or that both types are merely geek's with a medical label attached. Also, autism has evolved in the public understanding, but the popular identification of autism with relatively severe cases as accurately depicted in Rain Man has encouraged relatives of family members diagnosed in the autistic spectrum to speak of their loved ones as having Asperger's syndrome rather than autism.

Another view of these disorders is that they are on a continuum known as autistic spectrum disorders. A related continuum is Sensory Integration Dysfunction, which is about how well we integrate the information we receive from our senses. Autism, Asperger's syndrome, and Sensory Integration Dysfunction are all closely related and overlap.

There are two main manifestations of classical autism, regressive autism and early infantile autism. Early infantile autism is present at birth while regressive autism begins before the age of 3 and often around 18 months. Although this causes some controversy over when the neurological differences involved in autism truly begin, some believe that it is only a matter of when an environmental toxin triggers the disorder. This triggering could occur during gestation due to a toxin that enters the mother's body and is transferred to the fetus. The triggering could also occur after birth during the crucial early nervous system development of the child due to a toxin directly entering the child's body.

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