Structural and functional brain studies of autism, Down syndrome, and idiopathic mental retardation are reviewed to help identify brain areas and parameters that may be related to intelligence. Six brain factors appear common among people with autism, Down syndrome, and idiopathic mental retardation: cerebellum size, brain stem size, hippocampus size, abnormal dendritic development, whole brain size, and whole brain metabolism. How these areas may relate to intelligence is discussed.