Several structural brain abnormalities have been associated with autism, but none have been consistently replicated and no single abnormality is seen in all persons with autism. A potential factor contributing to these inconsistencies is the inherent difficulty in controlling for genetic and environmental variables that influence brain development. To overcome this difficulty, Dr. Barnea-Goraly will compare brain structure in siblings discordant for autism (sibling pairs in which one has autism and the other does not). This design minimizes genetic and environmental differences that could influence brain morphology. The study will use an innovative analysis of brain-surface complexity in individuals with autism, their non-autistic siblings, and in control subjects to examine the relationship between brain structure and autism. The study will also assess brain-surface complexity in very young subjects with autism (23-43 months of age) as compared to age- and gender-matched control subjects. This very young population will allow investigation of brain structure at the earliest ages in which autism diagnosis is possible. In addition, measures of brain-surface complexity will be compared to behavioral features of subjects with autism. What this means for people with autism: This study will help specify the brain regions most highly associated with autism, and how these regions develop throughout childhood. A better understanding of the neurobiological underpinning of autism will allow for the identification of meaningful subgroups of individuals with this condition and the development of more targeted treatments for cognitive and behavioral symptoms.