Probably the most devastating factor in the field of autism is the belief that these people function at a mentally retarded level. This belief is so strong that it is even stated in the DSM IV, which means that preschool children are often given a diagnosis of mental retardation at the same time as that of autism. All decisions made for them after this point are based on this belief....and the incredible intelligence level they have is lost to the world forever.
The problem we have measuring intelligence is that the majority of our tests are based directly on a person's ability to communicate clearly. Since autism is an impairment in communication, we are not actually measuring the intelligence level with these tests. The impairment to communicate in autism makes all intelligence testing invalid.
However we are beginning to clear the debris that has built up through the years. A research study from the University of Montreal (2006) has been released which indicates the error of our ways. Read this story in Le Devoir.