Autism is classified as a neurodevelopmental disorder that manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior.
Children with autism appear to lack "theory of mind", the ability to see things from another person's perspective, a behavior cited as exclusive to human beings above the age of five and, possibly, other higher primates such as adult gorillas, chimpanzees and bonobos. Typical 5-year-olds can develop insights into other people's different knowledge, feelings, and intentions, interpretations based upon social cues (e.g., gestures, facial expressions). An individual with autism seems to lack these interpretation skills, an inability that leaves them unable to predict or understand other people's actions. The social alienation of autistic and Asperger's people is so intense from childhood that many of them have imaginary friends as companionship. However, having an imaginary friend is not necessarily a sign of autism and also occurs in non-autistic children.
Asperger's and Kanner's syndrome
Asperger described his patients as "little professors". 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]elays 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 autistics 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.