Autism was first described by Leo Kanner in 1943. He described 11 children with “extreme autistic aloneness” p. 242, failure to use language in a communicative fashion, and an obsessive desire for the maintenance of sameness. In the 60 years since this classic paper, there have been numerous studies on every aspect of autism, with the pace of research accelerating greatly in the last 15 years. Kanner's original description has held up remarkably well, and forms the basis for the three domains of diagnostic criteria found in DSM-IV-TR (American Psychiatric Association, 2000).

Impairment in social relationships, the first domain, has four behavioral markers: (a) impaired nonverbal communication, including eye contact and gesture, (b) poor peer relationships, including lack of interest in peers when young, and odd, one-sided relationships later on, (c) lack of joint attention (pointing to indicate interest, bringing items to show others, following a point), and (d) lack of emotional reciprocity, such as failure to notice or share another's distress.

The second domain is impairment in language and symbolic capacity, and includes: (a) language delay, (b) impaired ability to carry on a two-way conversation (when there is sufficient language), (c) perseverative and repetitive language, such as repeating what others say or what the child has heard in commercials or videos, or repeating favorite phrases over and over, and (d) absent, delayed, or repetitive pretend play.