Autism: What We Know

by National Institute of Health

Even though autism was first described in the 1940s, little was really known about the disorder until the 1990s. Even today, there is a great deal that researchers, scientists, and health care providers don't know about autism.

But there are things that we do know about autism. This fact sheet offers broad information about autism and answers some of the more common questions that parents and families often have about the disorder. Keep in mind that the articles listed are geared toward scientists and researchers, so the information is more technical than what is presented here. Knowledge of autism is always growing as research examines more and different sides of the disorder. The NICHD joins other federal agencies, organizations, and support groups in helping those with autism achieve their full potential to live healthy, productive lives.

What is autism?

Autism is a complex neurobiological disorder of development that lasts throughout a person's life. It is sometimes called a developmental disability because it usually starts before age three, in the developmental period, and because it causes delays or problems in many different skills that arise from infancy to adulthood.

The main signs and symptoms of autism involve language, social behavior, and behaviors concerning objects and routines:

Communication - both verbal (spoken) and non-verbal (unspoken, such as pointing, eye contact, or smiling).

Social interactions - such as sharing emotions, understanding how others think and feel (sometimes called empathy), and holding a conversation, as well as the amount of time a person spends interacting with others.

Routines or repetitive behaviors - often called stereotyped behaviors, such as repeating words or actions, obsessively following routines or schedules, playing with toys or objects in repetitive and sometimes inappropriate ways, or having very specific and inflexible ways of arranging items.

People with autism might have problems talking with you, or they might not look you in the eye when you talk to them. They may have to line up their pencils before they can pay attention, or they may say the same sentence again and again to calm themselves down. They may flap their arms to tell you they are happy, or they might hurt themselves to tell you they are not. Some people with autism never learn how to talk. These behaviors not only make life challenging for people who have autism, but also take a toll on their families, their health care providers, their teachers, and anyone who comes in contact with them.

Because different people with autism can have very different features or symptoms, health care providers think of autism as a "spectrum" disorder - a group of disorders with a range of similar features. Based on their specific strengths and weaknesses, people with autism spectrum disorders (ASDs) may have mild symptoms or more serious symptoms, but they all have an ASD. This fact sheet uses the terms "ASD" and "autism" to mean the same thing.

What conditions are in the ASD category?

Currently, the ASD category includes: Autistic disorder (also called "classic" autism), Asperger syndrome, Pervasive Developmental Disorder Not Otherwise Specified (or atypical autism).

In some cases, health care providers use a broader term - pervasive developmental disorders (PDD) - to describe autism. The PDD category includes the ASDs mentioned above and: Childhood disintegrative disorder, and Rett syndrome.

Depending on specific symptoms, a person with autism may fall into the ASD or the PDD category. Sometimes, the terms "ASD" and "PDD" are used to mean the same thing because autism is in both categories.

How many people have autism?

Currently, researchers don't know the exact number of people with an ASD in the United States.

Researchers use different ways to determine prevalence that often give different results. Some estimates of prevalence rely on previously published studies. Researchers review all the published data on a topic and take the averages of these calculations to determine prevalence. Independent researchers3 recently conducted two such reviews. Based on these studies, the best conservative estimate4 of the prevalence of ASDs in the United States is that one child in 1,000 children has an ASD.

Is autism more common now than it was in the past?

Researchers are not certain whether autism is more prevalent now than in the past for a number of reasons. Although more cases of autism are being identified, it is not clear why. Some of the increase may result from better education about the symptoms of autism or from more accurate diagnoses of autism.

The new definition of autism as a spectrum disorder means that even people with mild symptoms can be classified as having an ASD, which could also account for the increase in identified cases. As research moves forward using the current definition of ASDs, more definite numbers may be available to answer this question.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.