Autism prevalence gets a lot of discussion here and elsewhere on blogs. But what about other prevalence numbers? Say, for intellectual disability? This is important on its own in this discussion. Intellectual disability (ID) is more common amongst autistics than in the general population. In addition, it is worth noting that even with something like ID, prevalence values vary.
In general, the prevalence of ID is about 1%. Similar to autism prevalence. The prevalence of ID varies by country and by income, and by age.
Prevalence of intellectual disability: A meta-analysis of population-based studies
Abstract
Intellectual disability is an extremely stigmatizing condition and involves utilization of large public health resources, but most data about its burden is based on studies conducted in developed countries. The aim of this meta-analysis was to collate data from published literature and estimate the prevalence of intellectual disability across all such studies. The review includes studies published between 1980 and 2009, and includes data from populations that provided an overall estimate of the prevalence of intellectual disability. Meta-analysis was done using random effects to account for heterogeneity. Sub-group analyses were also done. The prevalence of intellectual disability across all 52 studies included in the meta-analysis was 10.37/1000 population. The estimates varied according to income group of the country of origin, the age-group of the study population, and study design. The highest rates were seen in countries from low- and middle income countries. Studies based on identification of cases by using psychological assessments or scales showed higher prevalence compared to those using standard diagnostic systems and disability instruments. Prevalence was higher among studies based on children/adolescents, compared to those on adults. Higher prevalence in low and middle income group countries is of concern given the limitations in available resources in such countries to manage intellectual disability. The importance of using standardized diagnostic systems to correctly estimate the burden is underlined. The public health and research implications of this meta-analysis have been discussed.
Research highlightsThe prevalence of intellectual disability across the world is around 1%. The prevalence is almost two times more in low and middle income countries compared to high income countries. Highest prevalence was seen in child and adolescent population. Using standardized diagnostic instruments and disability measurements leads to lower estimates compared to simple psychological assessment tools even if confirmed by clinicians.
emphasis added.
As with autism prevalence, we need to look at these numbers in context: the culture of the country, the methods used, diagnostic criteria.
Whenever you hear someone say something like “One in 110 children born today will have autism” you are listening to someone who doesn’t really understand the “estimate” part of prevalence estimate.
Left Brain Right Brain 
“Whenever you hear someone say something like “One in 110 children born today will have autism” you are listening to someone who doesn’t really understand the “estimate” part of prevalence estimate.”
What do you mean by this comment? I thought the 1/110 was a pretty close estimate. Is it not that accurate? Count me in as one of those who do not understand the estimate” part of prevalence estimate. 🙂
Shannon,
thanks for the comment–sorry for the terse wording of that.
Autism prevalence estimates are estimates. Most studies don’t go out and really test a group of people and see how many are autistic. They don’t do something like say, let’s test everyone in Kansas with some standard tests and count how many autistics we find. The CDC numbers, for example, are found by looking at school records and medical records and working out from those who would be considered autistic. These people are doing a great job, but they can’t get all the information. What about the kid who is in regular class who has autism but no IEP, no medical record? What about the kid with an IEP but with incomplete records so they can’t determine if he/she is autistic? The CDC reports on 8 year olds. We’d like to think that all kids are identified by that age, but it isn’t true.
I don’t think that, say, Alabama has about 1/2 the “rate” of autism as New Jersey. But the CDC estimates report that is the case. We don’t say that 1 in 200 kids in Alabama will be autistic.
There are “rates” already out there up to about 1.5%. I seem to recall even 2%.
We have estimates. Some better than others. But they are just estimates.
I think this estimate issue is a really important point. As explained above, prevalence statistics get thrown around but people rarely seem to consider where those stats come from. The question that always comes to my mind when people make claims about a certain country having the highest ASD rate is what the social awareness of ASD is, what early intervention and data collection programs that country has, what public assessment services do parents have access to, what does the assessment process involve? And so on. It may be possible to argue those countries with the highest reported rates of ASD simply have the best health care, assessment and recording procedures for ASD, rather than pure prevalence.
Didn’t Jim Coplan say at one point that we won’t move from estimates to actuality until we do a very detailed, almost door-to-door survey using accurate diagnostic instruments? (I seem to have loaned my book to someone….)
I can’t find the comment at
http://www.psychologytoday.com/blog/making-sense-autistic-spectrum-disorders
but I think it was in the comments, anyway.
Sullivan- Thank you for taking the time to explain that! I really appreciate it.
“What about the kid who is in regular class who has autism but no IEP, no medical record? What about the kid with an IEP but with incomplete records so they can’t determine if he/she is autistic?”
Also, what about the kid who meets all criteria for autism, but the IEP team simply decides on another category?
What about the IEP’s where a child meets all criteria for autism but a parent or professional objects for personal, proffessional, or philosophical reasons?
What about the the kids under age 9 (age 8 in some States) who will be almost certainly be categorized under “developmental delay” even if it is tremendously obvious that they are autistic?
The IEP executive data bases only report on a single service category for each child.