Autism is a developmental disorder that appears in the first 3 years of life, and affects the brain's normal development of social and communication skills.

Autism is a physical condition linked to abnormal biology and chemistry in the brain. The exact causes of these abnormalities remain unknown, but this is a very active area of research. There are probably a combination of factors that lead to autism.

Genetic factors seem to be important. For example, identical twins are much more likely than fraternal twins or siblings to both have autism. Similarly, language abnormalities are more common in relatives of autistic children. Chromosomal abnormalities and other nervous system (neurological) problems are also more common in families with autism.

A number of other possible causes have been suspected, but not proven. They involve:

• Diet
• Digestive tract changes
• Mercury poisoning
• The body's inability to properly use vitamins and minerals
• Vaccine sensitivity

The exact number of children with autism is not known. A report released by the U.S. Centers for Disease Control and Prevention (CDC) suggests that autism and related disorders are more common than previously thought. It is unclear whether this is due to an increasing rate of the illness or an increased ability to diagnose the illness.

Autism affects boys 3 - 4 times more often than girls. Family income, education, and lifestyle do not seem to affect the risk of autism.

Some parents have heard that the MMR vaccine children receive may cause autism. This theory was based, in part, on two facts. First, the incidence of autism has increased steadily since around the same time the MMR vaccine was introduced. Second, children with the regressive form of autism (a type of autism that develops after a period of normal development) tend to start to show symptoms around the time the MMR vaccine is given. This is likely a coincidence due to the age of children at the time they receive this vaccine.

Several major studies have found NO connection between the vaccine and autism. The American Academy of Pediatrics and the Center for Disease Control and Prevention report that there is no proven link between autism and the MMR vaccine, or any other vaccine.

Some doctors believe the increased incidence in autism is due to newer definitions of autism. The term "autism" now includes a wider spectrum of children. For example, a child who is diagnosed with high-functioning autism today may have been thought to simply be odd or strange 30 years ago.

Other pervasive developmental disorders include:

• Asperger syndrome (like autism, but with normal language development)
• Rett syndrome (very different from autism, and only occurs in females)
• Childhood disintegrative disorder (rare condition where a child learns skills, then loses them by age 10)
• Pervasive developmental disorder - not otherwise specified (PDD-NOS), also called atypical autism.