This does not equate to an 80% figure of ID for those with autistic disorder, no matter how one parses the numbers. In fact Yeargin-Allsop's 2003 study disconfirms the idea that most individuals with the AD diagnosis have ID. Some individuals are fond of the 80% number for ID and autism comorbidity (like others are so fond of the 80% divorce rate, also untrue).
The CDC's press briefing on the latest autism findings offer this:
"Catherine Rice: So in terms of mental retardation, it's now more commonly referred to as intellectual disability. We know there's quite an overlap in intellectual disability and autism spectrum disorders. For many years, the best statistics and disorders told us 75% or three-quarters of children with autism also had an intellectual disability. Now the numbers that we're identifying in our study shows us it's more about 40% or more specifically 41% of children with autism having an intellectual disability. So overall, this is a population with less intellectual impairment, sometimes referring to as more a higher functional population. So there are many theories out there in terms of this diagnostic shifting and how we look at things or are we really seeing a more high functioning population? That is challenging to sort out."
In an earlier post, I covered the latest findings regarding ID and autism:
Yeargin-Allsopp et al. (2003) write: "Children with autism were identified as part of the Centers for Disease Control and Prevention's (CDC) Metropolitan Atlanta Developmental Disabilities Surveillance Program (MADDSP), an ongoing, active population-based surveillance program to monitor the occurrence of 5 DDs (autism, cerebral palsy, hearing loss, mental retardation [MR], and vision impairment) among 3- to 10-year-old children in the 5-county metropolitan Atlanta area.25-26 The total number of 3- to 10-year-old children residing in metropolitan Atlanta in 1996 was 289 456 (51% male; 58% white, 38% black, and 4% other racial group)."
According to Yeargin-Allsopp et al. (2003), "Psychometric data were available for 880 (89%) of the 987 children with autism. Of these, 676 (77%) had been administered a standardized intelligence test, and the others had received a developmental test (a list of psychometric tests is available from the authors). Children with a full-scale IQ of 70 or less or a score of 2 or more standard deviations below the mean on the cognitive domain of a developmental test were classified as having a cognitive impairment."
So what did Yeargin-Allsopp et al.find?: "Among the children with autism (N = 987), 62% had at least 1 coexisting MADDSP-defined disability or epilepsy. Of the children with an IQ or developmental test result (N = 880), 68% had cognitive impairment (64% based on IQ data alone). Among children with psychometric test data (N = 880), 20% had mild MR, 11% moderate MR, 7% severe MR, 3% profound MR, and 28% with an unspecified level of cognitive impairment that included 9% classified as MR-NOS using IQ data and 19% classified using developmental scores (Table 2). In addition, of the children with autism, 8% had epilepsy, 5% had cerebral palsy, 1% had vision impairment, and 1% had hearing loss. We found that as the severity of MR increased the sex ratio decreased (4.4 to 1.3), indicating a greater proportion of females in the severe and profound levels of impairment (Table 2)."
In other words, about two-thirds had a cognitive impairment, but of those most were mild cases, not severe. Standardized IQ tests for autistic individuals have potential problems; it can be tremendously difficult to ascertain capability from a noncompliant individual. In addition, since language deficits are part of the autism triad, the verbal portion can be expected to be lower than nonautistic peers, and it does not appear that the nonverbal IQ test is always (or often administered) in place of the traditionally used IQ test. Where noncompliance, lack of inherent interest, difficulty with communication, and discomfort with strangers can all get in the way of a reliable test result, IQ tests must be taken with a grain of salt, especially where performance outside the testing arena demonstrates more competency than the IQ score would have predicted.
Individuals with autism are likely to have additional diagnoses, as well: "Many children (70%) we identified with autism had more than 1 diagnostic evaluation, and 61% (data not shown) were seen at more than 1 educational or medical program in the community, thus providing independent information on the behaviors used to determine case status."
Scientists aren't in agreement with an 80% rate of ID in autistic individuals. In fact, the findings show that only 7% of those with autism are severely intellectually disabled. The overwhelming majority who have ID have mild to moderate ID. Many individuals with mild ID are able to live independent lives. Having autism comorbid with ID makes full independence more challenging, but it is the autism that creates this impediment, not the ID.
The portrait of autism has changed over the last forty years. More mildly affected individuals are being diagnosed with autism spectrum disorders as our awareness and understanding of autism has grown.
Most of those diagnosed with autism are not severely disabled. That doesn't mean that severely impaired individuals who require 24/7 care shouldn't be acknowledged and recognized. But restricting autism to this minority isn't going to happen.
Like it or not, our understanding of autism has broadened to include those on the broader autism phenotype who have less significant barriers. Personally, as the mother of three children who have tremendous commonalities with each other despite the variation in severity of the autism and the variation in intellectual impairment (my son has an intellectual disability but my daughters give every indication of being gifted), I respect and appreciate the increased awareness. Individuals who are extremely bright may still face significant challenges and have impairments that impede their ability to live independently.
Trying to manipulate numbers to fit one's notion of autism as also being intellectual disability is not scientifically accurate, and the historical numbers of 70% or so were based not on sound science and replication, but on taking a few original studies and their interpretation of 70% and carrying it forward in time rather than measuring the IQ of the study participants. In other words, it was a readily accepted and rarely questioned axiom that ID and autism went hand-in-hand. Assessing the IQ in noncompliant individuals (as many autistic individuals can be) is a difficult enterprise, so previous studies' assessments were used.
Times have changed, though, as our awareness and willingness to question have increased. Scientists now assess IQ. Recognition that traditional IQ testing may not best assess capability has also grown. Remember, IQ as assessed by the WAIS-III and Stanford-Binet assess the likelihood of success at academic performance. If a person hasn't had the kind of instruction or exposure to those kinds of material, he will not score well. It doesn't tell us about functional life skills. It doesn't tell us about the ability to hold a job, live semi-independently, socialize appropriately.
These changes in how we view and assess autism are good things. Whether the APA gets their act together and manages to put together a severity scale that is useful without pejorativeness is another matter entirely, as is the willingness of interested members of the autism community to accept communally drawn definitions of autism based on the scientific research.
Yeargin-Allsopp, M., Rice, C., Karapurkar, T., Doernberg, N., Boyle, C., & Murphy, C. (2003). Prevalence of Autism in a US Metropolitan Area. JAMA: Journal of the American Medical Association, 289(1), 49. Retrieved from Academic Search Complete database.