The severely incapacitating symptoms are caused by physical disorders of the brain. In some families there appears to be a pattern of autism or related disabilities which suggests there may be a genetic basis to the disorder, although at this time no one gene has been directly linked to autism. No known factors in the psychological environment of a child have been shown to cause autism. Its expression ranges from the very severely involved to those who are only mildly affected and may appear very close to normal. Autism is a developmental diagnosis, i.e., expression of the syndrome varies with age and the developmental level of the person affected.
Autism is a retrospective diagnosis.
The diagnosis cannot be made without taking a careful developmental history from parents and persons involved in a child or adult's life. Autism can co-exist with any other condition that any person may have. The most commonly co-existing condition is mental retardation. Just because a person with autism does not mean that he cannot have other diagnosable conditions.