In a recent post, I examined data for cognitive impairment in autistic individuals. Another study examines data collected in 1996 from the same collection program. 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."
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.