• Some types of autistic disorder are inherited, either alone or as part of a genetic syndrome, while other types of autistic disorder seem to be caused by environmental factors. Autistic disorder that runs in families is due in large part to genetic factors. In some instances, autistic disorder is a feature of a known genetic syndrome. However, much more frequently no specific cause is found (this is called idiopathic autistic disorder, meaning autistic disorder of unknown cause). There is a great deal of evidence that autistic disorder with an unknown cause actually is caused by changes or "mutations" in genes. Twin studies show much higher concordance rates for autism among identical twins than among non-identical (fraternal) twins. Concordance refers to the likelihood that if one twin has a diagnosis of autism, the second twin will also have a diagnosis of autism. Family studies have found that relatives of a person with autistic disorder have a higher chance of having autistic disorder than members of the general population. These factors, together with other research, suggest a strong genetic component in autistic disorder.

Asperger Disorder
Individuals with Asperger disorder do not have a delay in spoken language development, but can have serious deficits in social and communication skills.

Pervasive Developmental Disorder Not Otherwise Specified (PDD/NOS)
Is a diagnosis often considered for children who show some signs of autistic disorder, but who do not meet the specific diagnostic criteria for the other PDDs.

Rett Disorder
Rett disorder is among the most common genetic cause of profound intellectual and physical disability in girls, occurring more commonly than 1 in 10,000 female births.

Childhood Disintegrative Disorder
Children with childhood disintegrative disorder develop normally for a relatively prolonged period before developing a condition that resembles autistic disorder.