Mental Retardation is basically having an IQ below 70, autism is a mental disorder that causes social difficulties etc, but many people with autism have an IQ well above 100. Mental retardation is a catch-all term that refers to people who fall outside the norm in skills that we all take for granted, and who usually need some sort of supervision or assistance in coping with daily life. People with autism have a neurological disorder that is not yet completely understood.

It affects the way they relate to the world, but not necessarily their ability to learn to function in it. In fact, the insights that come from their unique viewpoints have proven in some cases to be extremely valuable mental retardation is still a co-morbid of autism, so they often come together, but they're also many autistics are the opposite of mentally retarded.