That was the finding reported by Mélina Rivard and colleagues  looking at the co-occurrence of intellectual disability (also called learning disability here in Blighty) and autism spectrum disorder (ASD) "in young children". Based on an analysis of over 200 children diagnosed with an ASD, researchers recorded various details before participants entry into an "early behavioral intervention program" including those relating to IQ and adaptive behaviours. Most of those children falling into the category of ID were in the 'mild' range (IQ 50-69) as opposed to the moderate range (IQ 35-49).
|Roses @ Wikipedia|
This is an important paper insofar as presenting some further details about how widespread ID is in cases of autism. Indeed, a figure hovering somewhere around 30-40% of all cases of autism (accepting the issue of variations in subtypes on the autism spectrum) presenting with ID is pretty much what many people have discussed on the basis of other research in this area . That being said, the recent CDC prevalence estimates (yes, estimates) of autism (see here) kinda hinted that the rates of ID in autism might be on the move as per their reporting: "31% of children with ASD were classified as having IQ scores in the range of intellectual disability (IQ ≤70)" . We'll have to wait and see how this goes alongside the introduction of the DSM-5 diagnostic criteria (see here).
Having reliable data on the rate of ID in autism also goes beyond just knowing how many people fall into that category. When present alongside each other, one might assume that there may be shared factors at work in terms of aetiology as for example, discussed by Srivastava & Schwartz . I've talked a few times on this blog about another triad of symptoms (autism, epilepsy & learning disability) appearing in some quite rare genetic conditions (see here) which provides some interesting details about possible onset and indeed intervention.
That there may also be a greater risk of autism and ID in specific populations is another detail to bear in mind. Take for example the 2013 report on autism in the Somali population in Minneapolis (see here) and the finding: "Somali children with ASD were more likely to also have an intellectual disability than children with ASD in all other racial and ethnic groups in Minneapolis". I say this acknowledging that inferring causation from such epidemiology is not necessarily going to be straight forward.
Perhaps also important is mention of the impact that comorbid ID can have on various other outcomes with autism in mind. Yes, we know that this probably means a greater requirement for service provision (see here) and inevitably this will have an economic cost attached to it (see here). But then there are issues like self-injurious behaviour (see here) and the topic no-one really likes to talk about, early mortality (see here) which might also be differentially affected by the presence of ID, or at least some behaviours which might place someone at greater risk of danger (see here). I personally see these as some of the more important effects of ID on autism.
Eliza Doolittle to finish, and Pack Up.
 Rivard M. et al. Indicators of Intellectual Disabilities in Young Children with Autism Spectrum Disorders. J Autism Dev Disord. 2014 Jul 29.
 Chakrabarti S. & Fombonne E. Pervasive developmental disorders in preschool children: confirmation of high prevalence. Am J Psychiatry. 2005 Jun;162(6):1133-41.
 Developmental Disabilities Monitoring Network Surveillance Year 2010 Principal Investigators. Prevalence of autism spectrum disorder among children aged 8 years - autism and developmental disabilities monitoring network, 11 sites, United States, 2010. MMWR Surveill Summ. 2014 Mar 28;63(2):1-21.
 Srivastava AK. & Schwartz CE. Intellectual disability and autism spectrum disorders: Causal genes and molecular mechanisms. Neurosci Biobehav Rev. 2014 Apr 4. pii: S0149-7634(14)00077-3.
Rivard M, Terroux A, Mercier C, & Parent-Boursier C (2014). Indicators of Intellectual Disabilities in Young Children with Autism Spectrum Disorders. Journal of autism and developmental disorders PMID: 25070470
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