Coleman (2005) writes "over 2000 different disease entities have been described in which patients have mental retardation. Mental retardation is a series of neurodevelopmental syndromes due to chromosomal, genetic, infectious, endocrine, and toxic etiologies; in almost all cases, the disease process is underway prior to birth. Regarding genetic disease, when mutation of a single gene is both necessary and sufficient to cause disease, it is called a monogenetic trait; it is now known that over 200 of the mental retardation diseases are classified as monogenetic diseases (Zechner et al. 2001)" (page 3).
Anyone who tries to argue that all cases of intellectual disability and autism are the same condition, are one condition, is not relying on scientific evidence. In fact, anyone who does so has turned his back to science completely and is asserting something on faith and ideology.
What the most recent genetic studies show is that some of the multiple genes involved in complex, previously unidentified causations of both idiopathic (no known cause) autism and ID is an overlap of genes in the two. No one who follows the scientific evidence denies that there is a large overlap of ID and autism; studies show varying degrees of overlap/coexistence. There is not a hard and fast number because different studies show different levels of overlap. In fact, that is the point. Anyone who uses ONE number and insists it's the right number isn't following the scientific evidence that shows a wide range in the percentage of individuals with both ID and an ASD. Scientists know there's a large overlap. Scientists also know that historically, many studies simply carried forth the figures from other studies rather than verifying in their own sample the degree of overlap. Anyone nonverbal is going to be unable to comply with an IQ that depends on being verbal. The CDC's attempts to measure the degree of overlap is important and necessary. These, too, though, should not and are not taken by scientists as the final numbers. Nor would scientists ever do something like take the CDC number and decide that there are equal numbers of Asperger's and autistic disorder, so the figure must be 80% in autistics, while ignoring the three other PDDs, nor would they decide that PDD-NOS is milder, so there can't be ID there (there can in fact be -- it isn't that PDD-NOS is milder, but that it does not meet the criteria for autistic disorder), so the new, artificially constructed number still works. That's, again, not relying on science.
Scientists measure. And they keep measuring. They keep refining their questions. They don't speak in certainties and absolutes, and if they are really scientifically-minded, they don't make numbers up. Or conditions, like Wakefield did.
It's one of the ways those trained in science distinguish between those committed to the scientific method and those who follow pseudoscientific principles. Those who rely on scientific evidence accept a certain level of uncertainty and are constantly looking at what the data shows, not what they want it to show.
Autism spectrum disorders are complex, with many causal factors. Intellectual disability has literally thousands of causes. Will autism spectrum disorders ultimately have thousands of causes identified? It's too early to tell. What is evident is that there are a number of genes, both separately and combined, that appear to work together with the environment to create the behaviors we now call autism.
In its own way, this kind of narrow-minded, obsessive need to focus on one thing and conflate it with the other and insist it's one thing and one thing only is just as bad as those who put forth the AoA rhetoric. And it serves none of the individuals who have an intellectual disability or an ASD or both.
Genetic research is important and necessary. So, too, is environmental research. But even more importantly, is research that focuses on helping those living with an ID or an ASD, or both, to live the fullest lives possible, to make as much progress as possible towards independent life skills. In tandem should be the focus on funding safe living and working environments for individuals who will always need assistance.
Coleman, M. (2005). The Neurology of Autism.