Mental retardation is NOT a mental illness.
Psychology only looks at the outward behavior and understand VERY LITTLE ELSE!
That is very useless when it comes to modern medicine.
I'll say it again; psychology is little more than glorified philosophy in behavioral studies.
There are various genetic diseases that cause various types of retardation, and were it up to psychology alone, it would all be lumped into one category; you've got Fragile X, Downs, Autism, and a few others that I can't remember at the moment. They all have different causes, and therefore have different treatments and different ways of preventing (not always possible)