Can a human have autism and mental retardation?

i think my cousn does.

Some children with autism also are MR, however... Many children with autism are given IQ tests at a stage when it would be impossible to get an accurate measure of IQ and receive scored as low as 9! This of course is in the severely MR range. When these kids are tested a year or two later (after intensive thearpy) they may test in the normal range... they were never really MR. See Interactive Autism Network for more information on this

The people I've met with autism,
seem to have areas that they excell in,
and other areas where growth is stunted.

Then, too, their social skills are the autistic part

Some of the symtoms of mental retardation are similar to autism. He may not actually have both. Leave it up to a psychiatrist to make a diagnosis.

Yes. I worked for years with a guy. Very difficult person to get through too.

Yes.

I work in mental health and some of our clients have both.

I worked with profoundly mentally retarded adults/teenagers as a Social Worker in an institution. I found that a large amount of them preferred to be by themselves and seemed to be in their own little world--oblivious to others. The higher functioning clients (who were still mentally retarded) often became more attached to objects then people.
This is not to say that all individuals who are mentally retarded exhibit signs of autism. In fact, there are times when autistic kids are misdiagnosed with mental retardation if the autism is extreme.