It is a disorder in its own right, though, characterized by intellectual functioning that is well below average.
Often the patient has an IQ of around 70 or less.
Patients of autistic disorder are not necessarily retarded; they just may appear so because of the 'withdrawal symptoms' that they depict with respect to communication and socializing.
Down syndrome has patients suffering from serious language difficulties and a few cognitive ones.
However, they do not often have the social withdrawal symptoms that are so heavily characteristic of classic autism.
Quite on the contrary, sufferers of Down-syndrome tend to be rather affectionate and very responsive to others.
Tourette's syndrome is a neurological disorder that is characterized by involuntary body movements and the tics, vocal outburst that the patient cannot control.
In between the sudden occurrences, though, the patients have normal social and emotional development that is uncharacteristic of autism.
Added to this, their intelligence is unimpaired in any way.
And then there are a number of other diseases and disorders that share a few common symptoms with autism but do not have anything to do with the autism-spectrum.
Symptoms such as scratching or biting oneself, head banging, and using objects in the immediate environment to hurt oneself happen from time to time in autism sufferers, but they are also characteristic of a number of other genetic disorders like Cornelia de Lange syndrome, Lesch-Nylan, and the fragile X syndrome.
You want to know these conditions that share various similarities to classic autism but as not PDDs -- pervasive developmental disorders - or part of the autism-spectrum.
One day, perhaps, you may save a life.
even when you never thought possible.
So - yes, no knowledge is ever wasted.