The median rate of Autistic Disorder in epidemiological studies is 5 cases per 10,000 individuals, iwth reported rates ranging from 2 to 20 cases per 10,000 individuals.   It remains unclear whether the higher reported rates reflect differences in methodology or an increased frequency of the condition.

      By definition, the onset of Autistic Disorder is prior to age 3 years.   In some instances, parents will report that they have been worried about the child since birth or shortly afterward because of the child's lack of interest in social interaction.   Manifestations of the disorder in infancy are more subtle and difficult to define than those seen after age 2 years.   In a minority of cases, the child may be reported to have developed normally for the first year (or even 2 years) of life.   Autistic Disorder follows a continous course.   In school-age children and adolescents, developmental gains in some areas are common (e.g.,increased interest in social functioning as the child reaches school age).   Some individuals deteriorate behaviorally during adolescence, whereas others improve.   Language skills (e.g.,presence of communicative speech) and overall intellectual level are the strongest factors related to ultimate prognosis.   Available following-up studies suggest that only a small percentage of individuals with the disorder go on as adults to live and work independently.   In about one-third of cases, some degree of partial independence is possible.   The highest functioning adults with Autistic Disorder typically continue to exhibit problems in social interaction and communication along with markedly restricted interests and activities.

      There is an increased risk of Autistic Disorder among siblings of individuals with the disorder, with approximately 5% of siblings also exhibiting the condition.   There also appears to be risk for various developmental difficulties in affected siblings.

      Periods of developmental regression may be observed in normal development, but these are neither as severe or as prolonged as in Autistic Disorder.   Autistic Disorder must be differentiated from other Pervasive Developmental Disorder.   Rett's Disorder differs from Autistic Disorder in its characteristic sex ratio and pattern of deficits.   Rett's Disorder has been diagnosed only in females, whereas Autistic Disorder occurs much more frequently in males.   In Rett's Disorder, there is a characteristic pattern of head growth deceleration, loss of previously acquired purposeful hand skills and the appearance of poorly coordinated gait or trunk movements.   Particularly during the preschool years, individuals with Rett's Disorder may exhibit difficulties in social interaction similar to those observed in Autistic Disorder, but these tend to be transient.   Autistic Disorder differs from Childhood Disintegrative Disorder, which has a distinctive pattern of severe developmental regression in multiple areas of functioning following at least 2 years of normal development.   In Autistic Disorder, developmental abnormalities are usually noted within the first year of life.   When information on early development is unavailable or when it is not possible to document the required period of normal development, the diagnosis of Autistic Disorder should be made.   Asperger's Disorder can be distinguished from Autistic Disorder by the lack of delay or deviance in early language development.   Asperger's Disorder is not diagnosed if criteria are met for Autistic Disorder.

      Schizophrenia with childhood onset usually develops after years of normal, or near normal, development.   An additional diagnosis of Schizophrenia can be made if an individual with Autistic Disorder develops the characteristic features of Schizophrenia with active-phase symptoms of prominent delusions or hallucinations that last for at least 1 month.   In Selective Mutism, the child usually exhibits appropriate communication skills in certain contexts and does not have the severe impairment in social interaction and the restricted patterns of behavior associated with Autistic Disorder.   In Expressive Language Disorder and Mixed Receptive-Expressive Language Disorder, there is a language impairment, but it is not associated with the presence of a qualitative impairment in social interaction and restricted, repetitive, and stereotyped patterns of behavior.   It is sometimes difficult to determine whether an additional diagnosis of Autistic Disorder is warranted in an individual with Mental Retardation, especially if the Mental Retardation is Severe or Profound.   An additional diagnosis of Autistic Disorder is reserved for those situation in which there are qualitative deficits in social and communicative skills and the specific behaviors characteristic of Autistic Disorder are present.  Motor stereotypies are characteristic of Autistic Disorder; and additional diagnosis of Stereotypic Movement Disorder is not given when these are better accounted for as part of the presentation of Autistic Disorder.   Symptoms of overactivity and inattention are frequent in Autistic Disorder, but a diagnosis of Attention-Deficit/Hyperactivity Disorder is not made if Autistic Disorder is present.