"The prevalence of ID [intellectual disability] in WA [Western Australia] has increased over the past 10 years compared with previous estimates... This increase is associated in a large part with an increased prevalence of ASDs [autism spectrum disorder] for whom 70% had comorbid ID or an unknown level of ID."
Those were some of the findings reported by Jenny Bourke and colleagues  (open-access available here). Drawing on data derived from the Intellectual Disability Exploring Answers (IDEA) database, a resource designed to 'provide high-quality complete and population-based information on Western Australians with an intellectual disability', authors set about looking at cases of ID for those born between 1983 and 2010. Intellectual disability (ID) - sometimes called learning disability here in Blighty - is typically diagnosed when IQ is assessed as being below 70 where a score of between 55-69 denotes mild ID, a score of between 40-54 denotes moderate ID and a score below 40 denotes severe ID. Authors were also able to cross-reference cases with information in other databases in terms of race, gender and location of birth.
Results: covering a total of nearly 750,000 live births during the period of inspection, some 10,000 infants "were identified with an ID by 2010." This equated to a total prevalence of ID of 17 per 1,000 live births. Most cases of ID were defined as being mild or moderate in terms of IQ scores (where available) and when compared to previous data from this authorship group  authors reported "an overall increase in prevalence of ID of 19% from 1999 to 2010."
Insofar as the possible causes of ID, various factors are reported to be potentially contributory including ID accompanying Down's syndrome, ID linked to various other genetic/chromosomal issues, birth defects and infection(s). Increasing preterm birth and survival rates are also suggested to be another contributory factor. The authors also add: "It is also possible that a proportion of the observed increase in mild or moderate ID may be attributable to un-diagnosed Fetal Alcohol Syndrome."
The link between autism and ID is also discussed by Bourke et al. On the basis of other research (see here) suggesting that approximately 30-40% of cases of autism will also include a degree of ID, I was pretty interested to see the authors of this latest research suggesting something a little bit different. To quote: "Of the 2307 [diagnosed with an ASD], 675 (29.3%) definitely did not have an ID." I've underlined the word 'not' because the implication is that up to 70% of those with autism did have some level of ID. I say this bearing in mind that Bourke did include children diagnosed with an ASD where "children with an autism diagnosis but an unknown level of ID were classified within the comorbid ASD and ID group." But how far will the true figure of autism and ID combined fall by excluding such unknowns?
There is just one more detail of the results that I want to draw your attention to with regards to ID: "The prevalence for Aboriginal children was 39.0/1000 compared with 15.7/1000 for non-Aboriginal children." The idea of disparities in rates of ID among Indigenous Australians and other groups is not necessarily a new one as per other research  and strengthen calls for a lot more research focus on this and other groups  from a variety of different clinical perspectives.
In terms of what to make of these combined findings, I'd like to think there are some important issues requiring further study. The idea that autism and ID can and do frequently co-exist is paramount to discussions. We can talk and discuss about the hows and whys until the cows come home but the link remains strong and indeed, among different populations, might be more variable  than previously suggested. Also, for many years in autism research circles, there have been discussions upon discussions about how the quite spectacular rise in autism cases (including that in Australia) might have been at the expense of diagnostic switching from categories such as ID. The Bourke data seem to suggest that not everywhere in the world is necessarily experiencing a corresponding 'drop' in cases of ID supportive of this diagnostic switching argument. Indeed, I'm minded to suggest that one has to be quite careful about explaining away any 'real increase' in autism cases solely using the ID switching argument (see here)...
 Bourke J. et al. Population-Based Prevalence of Intellectual Disability and Autism Spectrum Disorders in Western Australia: A Comparison With Previous Estimates. Medicine (Baltimore). 2016 May;95(21):e3737.
 Leonard H. et al. Prevalence of intellectual disability in Western Australia. Paediatr Perinat Epidemiol. 2003 Jan;17(1):58-67.
 Leonard H. et al. Autism and intellectual disability are differentially related to sociodemographic background at birth. PLoS One. 2011 Mar 30;6(3):e17875.
 Bennett M. & Hodgson V. The missing voices of Indigenous Australians with autism in research. Autism. 2016 May 25. pii: 1362361316643696.
 Postorino V. et al. Intellectual disability in Autism Spectrum Disorder: Investigation of prevalence in an Italian sample of children and adolescents. Research in Developmental Disabilities. 2016; 48: 193-201.
Bourke J, de Klerk N, Smith T, & Leonard H (2016). Population-Based Prevalence of Intellectual Disability and Autism Spectrum Disorders in Western Australia: A Comparison With Previous Estimates. Medicine, 95 (21) PMID: 27227936