Do some researchers exclude autistics with intellectual disability because they are trying to distort the research information or because they are controlling confounders? Some folks would have you think that researchers deliberately exclude those with intellectual challenges in order to distort information.
Some folks give the example of the latest research concerning brain scans, which exclcuded those with ID. In order to get a picture of whether there are differences in the architecture of the brain that would allow diagnosis of autism, comparisons need to be made, and this is done incrementally. Notice this study also didn't have females or children included, or left-handed individuals and that comorbid conditions like epilepsy were excluded.
Based on the nature of the study, a baseline needed to be found. Add those other variables in and you have no idea if the differences in the scans are because the person was a female, left-handed child with an intellectual disability or because of the autism. There are plenty of reasons to discuss this study. There's even room to criticize the media coverage and blogging coverage. What there shouldn't be room for is ignorance of the scientific method, the specific science behind a particular study, and the constraints in preliminary (or basic) research.
So, how much research is out there that involves these twin questions of intellectual disability and autism? The EBSCO database at my local university, using the following databases, Academic Search Complete, Psychological and Behavioral Sciences Collection, Health Source: Nursing/Academic Edition, PsycINFO, and PsycARTICLES, found 2,458 full-text peer reviewed journal articles with both intellectual disability and autism as search terms. In nearly 2500 full-text articles, both ID and autism were examined! That's not a dearth of research involving both autism and ID. Remove the full-text limit and 3,210 results are available. Remove the peer review limiter, and 3,504 articles written between 1977 and 2010 come up in the search results. When you replace intellectual disability with intelligence, the full-text, peer reviewed articles available for the same time span rises to 10,327 articles. Remove the limiters except for time, and it rises to 14,330 articles.
Expand out from the narrow parameters of autism and ID and focus in only on intellectual disability, and there are 18, 475 full-text peer reviewed articles available in my university's database. Add therapy into the mix, and you find that there are over a thousand articles that have looked at therapy for ID.
It should be noted that this was a limited database search (there are many more possible databases to look at, and I only looked for what I could immediately pull and read). There is a phenomenal amount of scientific literature out there, both on the combined disabilities of autism and intellectual disability and intellectual disability alone.
It seems to me that the charge that researchers are ignoring intellectual disability when researching autism is at best an absurd allegation based on a woefully inadequate understanding of the available science as well as ignorance on how studies are designed and carried out.
I understand that people want their particular issue looked at and an answer found, that there is a peculiar need to rage against anyone who doesn't see the world in the exact same way, but charges, specifically in regards to the recent study by Ecker et al. and their decision to exclude ID, is nonsensical when their research is viewed in its appropriate context.