Autism is classified as a neurodevelopmental disorder that manifests itself in markedly abnormal social interaction, communication ability, patterns of interests, and patterns of behavior.
Although the specific etiology of autism is unknown, many researchers suspect that autism results from genetically mediated vulnerabilities to environmental triggers.
And while there is disagreement about the magnitude, nature, and mechanisms for such environmental factors, researchers have found at least seven major genes prevalent among individuals diagnosed as autistic. Some estimate that autism occurs in as many as one United States child in 166, however the National Institute of Mental Health gives a more conservative estimate of one in 1000.
For families that already have one autistic child, the odds of a second autistic child may be as high as one in twenty. Diagnosis is based on a list of psychiatric criteria, and a series of standardized clinical tests may also be used."