Autism is one of the five pervasive developmental disorders (PDD), which are characterized by widespread abnormalities of social interactions and communication, and severely restricted interests and highly repetitive behavior. Unlike with autism, people with Asperger syndrome have no substantial delay in language development. whereas autism itself is often called ''autistic disorder'', ''childhood autism'', or ''infantile autism''. In this article, ''autism'' refers to the classic autistic disorder; in clinical practice, though, ''autism'', ''ASD'', and ''PDD'' are often used interchangeably.
The manifestations of autism cover a wide spectrum, ranging from individuals with severe impairments—who may be silent, mentally disabled, and locked into hand flapping and rocking—to high functioning individuals who may have active but distinctly odd social approaches, narrowly focused interests, and verbose, pedantic communication. Because the behavior spectrum is continuous, boundaries between diagnostic categories are necessarily somewhat arbitrary. or on how much support the individual requires in daily life; these subdivisions are not standardized and are controversial. Autism can also be divided into syndromal and non-syndromal autism; the syndromal autism is associated with severe or profound mental retardation or a congenital syndrome with physical symptoms, such as tuberous sclerosis. Although individuals with Asperger syndrome tend to perform better cognitively than those with autism, the extent of the overlap between Asperger syndrome, HFA, and non-syndromal autism is unclear.
Some studies have reported diagnoses of autism in children due to a loss of language or social skills, as opposed to a failure to make progress, typically from 15 to 30 months of age. The validity of this distinction remains controversial; it is possible that regressive autism is a specific subtype,
Research into causes has been hampered by the inability to identify biologically meaningful subpopulations and by the traditional boundaries between the disciplines of psychiatry, psychology, neurology and pediatrics. Newer technologies such as fMRI can help identify biologically relevant phenotypes (observable traits) that can be viewed on brain scans, to help further neurogenetic studies of autism. It has been proposed to classify autism using genetics as well as behavior, with the name ''Type 1 autism'' denoting rare autism cases that test positive for a mutation in the CNTNAP2 gene.
