can someone thats mentally retarded endorse the back of a paycheck, or sign there name at all?

Answer 1

Think of what a child can do. A first-grader can write his or her name in some way, even younger children can. So adults with that same level of intellectual development can write their name as well, as long as they've been taught something about letters. Of course some people have a developmental delay and/or neurological deficits that are too profound even for them to sign their name.

Answer 2

A mentally retarded person has the same legal rights as anyone else unless they have been declared Legally Incompetent. In that case, the guardian may need to sign. However, even if they are found legally incompent they may have the right to handle their own money depending on the specific provisions of the guardianship.

Answer 3

Of course, both my x-wifes, do it all the time.