One of the many unanswered questions when it comes to autism is just how many adults have autism. While this may seen to be an academic question at first, the answer to the question is quite important. The reason is that if we knew how many adults have autism it would give us a clue to whether autism is more common now than it was in the past. If there are just as many adults out there are children, that would mean that autism is not becoming more common and it is likely that we are just better at recognizing it. If there are not as many then that would strongly suggest that something else is going on.
Unfortunately, there have been very few (if any) high quality studies that have looked for autism in adults. For example, the most recent study that looked for autism in adults in the UK came up with a figure of 1% but the study had major problems with how it was done. As a result, we can only guess at how many adults have autism.
With that in mind, consider a recent study published that was recent published in the Journal of Intellectual Disability Research. I wish I could say that it directly looked at the autism rates in adults but it didn't. What it did do is look at how common autism is in adults with intellectual disability.
Intellectual disability (ID) is roughly defined as having an IQ under 70 along with potential problems with motor, language, and self help skills. ID is thought to be fairly common in people with autism - estimates range from 20% all the way up to 80% when you include borderline ID.
In this study, the researchers looked at all of the adults (ages 18 to 67) with severe intellectual disability (IQ less than 50) who were registered for government services in in Reykjavik, Iceland in 2000. The city had a population of 70,007 and there were a total of 256 adults were registered for services for severe ID.
The researchers sent out letters to all 256 adults and attempted to recruit them for this study. However, only 119 of the 256 people elected to participant. Those who elected to participant were significantly younger and more likely to live in group homes than those who didn't. As you will see, this fact become important later on.
These 119 people were then screened (by psychology students working on their thesis) using a slightly modified Bryson scale followed by a the Childhood Autism Rating Scale (CARS) with modified cut-off point (26 or above instead of the usual 30 or above). Afterwards, an experienced clinician administered an ADI-R to confirm that the person did in fact have a form of autism.
The overall results were not that surprising. The study identified a total of 25 cases of autism, 14 of which had did not previous diagnosis of autism. Seventeen of these cases were autism while the other 8 where atypical autism (think PDD-NOS). I could not determine from reading the study what the exact breakout of autism vs atypical autism was in the initial 11 cases compared to the new 14 cases. It appears that the 11 people who already had a diagnosis had a diagnosis of autism which means 6 of the new cases were autism while 8 where atypical autism, but I could easily be wrong.
The breakout of autism vs atypical autism in the groups may sound like a minor point, but it actually can help us determine why the cases were "missed" in the past. One of the arguments against the rate of autism actually increasing is that the diagnosistic criteria have undergone major revisions over the years. The earlier criteria were much more limited in what they would call autism while the most recent crieria have expanded their scope, especially in the area of atypical autism or PDD-NOS. Therefore, if the newly diagnosed group were mostly in the atypical group, it could be argued that they were "missed" earlier because they would not have met the criteria for any form of autism.
But from what I can gather, it is a mixed bag with only about half of the newly diagnosed people falling under an atypical label which means that the change in the diagnostic criteria only played a partial role in the new cases.
Regardless, one of the other interesting points here is that most of the people diagnosed were relatively young. The ages ranged from 19 to 63 with a mean of 32.56 and a standard deviation of 10.92. The study does not give more of a break out of how these cases are distributed by age, but from looking at the statitics, you can tell that there are far more younger cases than older ones. I don't know whether this age bias is a function of the subset of the population that responded (which we know were younger), or whether it is a function of autism becoming more common each year (younger people have a higher autism rate), or both. I suspect it is both.
Going back to the population of adults with severe ID, the rate of autism in this group was about 21%. This number is not surprising and, if anything, is on the lower side. Other similar studies have suggested that the rate is higher, possibly around 30%.
But in general, the results of this study agree with earlier results and doesn't give us much new information.
Saemundsen E, Juliusson H, Hjaltested S, Gunnarsdottir T, Halldorsdottir T, Hreidarsson S, Magnusson P. Prevalence of autism in an urban population of adults with severe intellectual disabilities - a preliminary study. J Intellect Disabil Res. 2010 Aug;54(8):727-35. PubMed PMID: 20633201.