What is autism?

Autism is one of a group of disorders known as autism spectrum disorders (ASDs). ASDs are developmental disabilities that cause substantial impairments in social interaction and communication and the presence of unusual behaviors and interests. Many people with ASDs also have unusual ways of learning, paying attention, and reacting to different sensations. The thinking and learning abilities of people with ASDs can vary—from gifted to severely challenged. An ASD begins before the age of 3 and lasts throughout a person's life.

ASDs include autistic disorder, pervasive developmental disorder - not otherwise specified (PDD-NOS, including atypical autism), and Asperger syndrome. These conditions all have some of the same symptoms, but they differ in terms of when the symptoms start, how severe they are, and the exact nature of the symptoms. The three conditions, along with Rett syndrome and childhood disintegrative disorder, make up the broad diagnosis category of pervasive developmental disorders.

Who is affected?

ASDs occur in all racial, ethnic, and socioeconomic groups and are four times more likely to occur in boys than in girls. CDC’s Autism and Developmental Disabilities Monitoring (ADDM) Network released data in 2007 that found about 1 in 150 8-year-old children in multiple areas of the United States had an ASD.

When can autism spectrum disorders be detected?

ASDs can often be detected as early as 18 months. While all children should be watched to make sure they are reaching developmental milestones on time, children in high-risk groups—such as children who have a parent or  brother or sister with an ASD—should be watched extra closely. A child with any of the warning signs of ASDs  should be checked by a health care professional.

Research shows that early intervention can greatly improve a child’s development.[1],[2]  CDC is working with national partners on a public awareness campaign to educate parents about how important it is to track their child’s development in the first few years of life. The campaign, “Learn the Signs. Act Early,” teaches parents, health care professionals, and child care providers about early childhood development, including early warning signs of autism and other developmental disabilities.

Is autism a new disorder?

Autism may seem like a modern disorder, but it’s not. People have probably lived with what we know today as autism spectrum disorders throughout history. Some of the earliest published descriptions of behavior that sounds like autism date back to the 18th century. But the disorder did not have a name until the middle of the 20th century.

Autism was first identified as a specific disorder in 1943 by child psychiatrist Dr. Leo Kanner. Based on a study of 11 children, Dr. Kanner published the first description of what he called “autistic disturbances of affective contact.”[3] At about the same time, German scientist Dr. Hans Asperger, based on his study of 400 children, described another form of autism that became known as Asperger syndrome.[4]

The criteria used to diagnose ASDs have changed many times since Kanner’s original description.

What causes autism?

We have learned a lot about the symptoms of ASDs and have improved efforts to track the disorders, but we still don’t know a lot about the causes of ASDs. Scientists think that both genes and the environment play a role, and there might be many causes that lead to ASDs. 

Family studies have been most helpful in understanding how genes contribute to autism. Studies have shown that among identical twins, if one child has autism, then the other will be affected about 75% of the time. In non-identical twins, if one child has autism, then the other has it about 3% of the time. Also, parents who have a child with an ASD have a 2%–8% chance of having a second child who is also affected.[5],[6]

For most people with ASDs, the cause is not known. But ASDs tend to occur more often than expected among people who have certain other medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful drugs taken during pregnancy also have been linked with a higher risk of autism, specifically, the prescription drug thalidomide.

CDC’s Centers for Autism and Developmental Disabilities Surveillance and Epidemiology (CADDRE) are working together on a large, population-based study to better understand the possible risk factors for and causes of autism. Called the Study to Explore Early Development (SEED), this project will help answer the many questions needed to find the causes of autism and—if possible—come up with strategies to prevent this complex disorder.

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[1] Handleman, J.S., Harris, S., eds. Preschool Education Programs for Children with Autism (2nd ed). Austin, TX: Pro-Ed. 2000. 

[2] National Research Council. Educating Children with Autism.  Washington, DC: National Academy Press, 2001.

[3] Kanner, L.  Autistic disturbances of affective contact. Nervous Child 1943; 2:217-250.

[4] Asperger, H. Die “Autistichen Psychopathen” Kindesalter. Arch Psychiatr Nervenkr 1944; 117:76-136.

[5] Boyle C, Van Naarden Braun K, Yeargin-Allsopp M. The Prevalence and the Genetic Epidemiology of Developmental Disabilities.  In: Genetics of Developmental Disabilities. Merlin Butler and John Meany eds. 2004 (Table 3, p. 716-717).

[6] Muhle R, Trentacoste V, Rapin I. The Genetics of Autism. Pediatrics 2004;113;472-486