The purpose of this research is to better understand the genetic, biochemical, cognitive and behavioral symptom abnormalities that contribute to autism spectrum disorders. The investigators anticipate recruiting at least 100 participants with autism spectrum disorder and large head size, at least 100 participants with autism spectrum disorder without large head size and at least 40 healthy siblings. Biological parents are expected to be recruited only as genetic changes are identified in individuals with autism spectrum disorders to better understand the nature of these genetic changes. Participants are asked to complete cognitive testing, a blood draw, urine collection and measurement of his/her height, weight and head circumference. Parents or caregivers may be asked to complete a diagnostic evaluation and will complete questionnaires that examine the participant's medical and family history as well as his/her current symptoms, functioning, and quality of life. A brief report simply listing and giving a basic description of any behavioral diagnostic information, autism symptoms, adaptive functioning, and a listing of results from cognitive testing will be provided as part of this study.