"Autism spectrum disorder (ASD) is a strongly genetic, highly prevalent neurodevelopmental disorder characterized by striking social deficits (1, 2). Among the most scientifically challenging features of ASD are its phenotypic heterogeneity and genetic variability, which constrain successful identification of genes underlying the clinical syndrome. Despite these challenges, we hypothesize that the various factors contributing to the expression of ASD might exert their effects through a circumscribed set of neuroanatomical structures (3); that is, it is possible that the simplest and potentially most powerful signature of ASD will be found at the level of brain systems. Such “neural signatures” of ASD may serve as critical endophenotypes to facilitate the study of the pathophysiological mechanisms underlying this devastating and highly prevalent neurodevelopmental disorder."
"As illustrated in Table 1, the three groups of participants were matched on chronological age and were of similar cognitive ability, all within the average range. Cognitive ability was assessed using either the Differential Ability Scale (DAS-II) (18) or the Wechsler Abbreviated Scale of Intelligence (WASI) (19)."
Undoubtedly there will be more and more studies of Autism Spectrum Disorders which will exclude subjects with autism and intellectual disability. On a common sense basis I do not believe you can draw conclusions concerning this entire spectrum of disorders by excluding a major group, a group whose participants are severely affected. If you don't study them you can't draw conclusions about them.
Of course the professionals involved with the DSM and ICD revisions may have a smple solution - simply define the problem away. By expanding autism in the DSM-IV to include those with Aspergers who are, by definition, not cognitively challenged, the APA reduced the numbers of those with autism and intellectual disability from the 75-80% range to approximately 40%. The DSM-5 may reduce the figure further by including sub-clinical, almost autistic groups, in the new Autism Spectrum Disorder and may simply define Autism Spectrum Disorder so as to exclude persons with autism and allegedly "co-morbid" conditions like Intellectual Disability.
In the meantime researchers like the Yale group are forging ahead studying the New ASD and drawing conclusions about the entire spectrum without studying those with ASD and Intellectual Disabilty:
In this study, we have characterized neural signatures of the state of having ASD, the underlying trait of vulnerability to develop ASD, and regions of compensatory activity that distinguish US from children with those with ASD and TD. Measurement of activity in the pSTS region allows us to subdivide the autism spectrum by severity. Our results identifying trait activity provide a possible neuroendophenotype ofASDand hold promise for future genetic research.