Autism also called autistic disorder is a neurodevelopmental disorder that manifests itself before the age of three years Children with autism are marked by impairments in social interaction, impairments in communication, and restricted and repetitive behaviour These three characteristics reflect Leo Kanner's first reports of autism emphasizing autistic aloneness and insistence on sameness Autism is one of the five pervasive developmental disorders PDD or autism spectrum disorders ASD which have no outward physical characteristics Diagnosis is typically based on a complete patient history and physical and neurological evaluation Estimates of prevalence vary widely