Medical researchers are exploring different explanations for the various forms of autism. Although no one cause of autism is known, current research links autism to biological or neurological differences in the brain. Scans such as the MRI (Magnetic Resonance Imaging) and PET (Positron Emission Tomography) shows abnormalities in the structure of the brain, with significant differences within the cerebellum, including the size and number of Purkinje cells. In some families there appears to be a pattern of autism or related disabilities. This suggests there is a genetic basis to the disorder, although no one gene has been directly linked to autism. Several previous theories about the cause of autism have been proven false. Autism is not a mental illness. Children with autism are not unruly kids who choose not to behave. Autism is not caused by bad parenting. Furthermore, no known psychological factors in the development of the child have been shown to cause autism.