Why do mentally disabled people sometimes hit themselves?

Gene > brain >: behavior is studied by translational research. OMIM, a genetic database, lists 1,077 molecular genetic defects that cause Intellectual Disability, often moderate-severe, +/- Autistic Spectrum Disorder with Self-Injurious Behavior in their Neurobehavioral profiles. SIB used by a non- verbal person to avoid situations or attain desires often responds to behavioral therapy +/- medications.