Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

autism (autism spectrum disorder)

What Is It?

Autism is a developmental disorder of the brain. People with autism have problems communicating and interacting socially. They also may have unusual patterns of behavior, interests and activities.

Doctors use the term autism spectrum disorder (ASD). This term includes the five kinds of autism.

The three main types of autism are:

• Classic autism

• Asperger's syndrome — A milder form

• Nonspecific pervasive developmental disorder (PDD-NOS) — Describes children who don't quite fit the criteria for the other types

There are also two rare autism disorders:

• Rett syndrome — Affects mostly girls. It includes problems with movement and speech, along with autistic features.

• Childhood disintegrative disorder — A severe type of autism. The child loses more physical, language and social skills than in classic autism.

Signs of autism spectrum disorder are typically first seen before a child's third birthday. However, only half of children with autism are diagnosed before kindergarten.

Rett syndrome appears to be caused by a genetic mutation.

The causes of the other types of autism remain unknown. Various studies suggest that autism may:

• Be inherited

• Be caused by infection or the effects of an environmental toxin

• Result from a brain injury or abnormality that occurs in the womb or in early infancy

• Result from abnormal levels of chemicals messengers in the brain

Studies have found no link between vaccines and autism.

All types of autism except Rett syndrome are more common in boys than in girls.