Although not so stated in the AAIDD manual, the elevation of general intelligence to such high status, combined with statements that current intelligence measures are not available to reliably and validly assess multiple cognitive abilities (a statement that is simply wrong--see PPT show link in last post in series), implicitly has the potential to convey this stereotype. I argued that individuals with ID/MR show just as much heterogeneity in profiles of cognitive abilities as individuals without ID/MR.
This past week a colleague reminded me of one article that makes my point clear. Within the field of ID/MR, there are a number of rare genetic-based disorders. Such genetically-based disorders typically result in a greater degree of similarity (homogeneity) among individuals with the condition. Williams syndrome (WS) is one such ID/MR disorder. Of course, individuals with WS are not those being evaluated in typical Atkins death penalty cases, but the common assumption and lore is that WS individuals show a "syndrome-specific pattern of cognitive strengths/weaknesses"----high verbal abilities and much lower visual-spatial abilities.
I would argue, as have others, that this WS syndrome-specific cognitive stereotype is largely due to the fact that historically MR/ID researchers only had the V/P organized Wechsler batteries as their primary IQ battery...and that the "profile" may be due to this research being constrained by batteries that did not validly measure a greater breadth of cognitive functioning. This is not a criticism of the past research, as researchers had limited theories of intelligence and measures of constructs from which to work. However, now that CHC theory has emerged as the consensus psychometric model of cognitive abilities and, more importantly, there are a significant number of well-standardized and psychometrically sound IQ batteries of multiple cognitive abilities, I'm not surprised that a syndrome with a strong genetic core, which typically results in more within-group similarity, when measured by more contemporary CHC-based IQ batteries display considerable variability/heterogeneity in patterns of cognitive abilities.
Below is the abstract for 2005 study that reported that WS individuals do NOT display the classic and historical syndrome-specific pattern of cognitive strengths and weaknesses when measured with a more contemporary CHC-based cognitive battery (WJ-R: conflict of interest note--I am a coauthor of the next edition..the WJ III).
This study clearly suggests that even a population of individuals with a shared genetic causal mechanism display significant individual differences in patterns of cognitive abilities. If this is found in ID/MR populations with a strong shared genetic causal mechanism, one would be hard-pressed to argue that such variability does not exist for more milder forms of ID/MR and the general population.
My point (again)---I'm very concerned that the AAIDD 11th Edition ID manual's "stuck on g" position is out of synch with contemporary intelligence theory and measurement and has the potential to cause serious harm when potentially life-altering decisions are made on the basis of a single g-based composite IQ scores that ignores the heterogeneity of human cognitive abilities across the ability spectrum and different disorders.
Porter, M. A. & Coltheart, M. Cognitive Heterogeneity in Williams Syndrome. Developmental Neuropsychology, 27 (2), 275-306. (click here to view article)
This study used the Woodcock-Johnson Tests of Cognitive Ability-Revised to investigate a wide range of cognitive abilities in people with Williams syndrome (WS). It involved a comparatively large sample of 31 people with WS, but took a case-series approach. The study addressed the widespread claims of a characteristic "WS cognitive profile" by looking for heterogeneity rather than homogeneity. People with WS showed a variety of preserved (significantly above mental age [MA]), expected (at MA), and significantly impaired (significantly below MA) levels of functioning. Such results provide clear evidence for heterogeneity in cognitive functions within WS. We found the most homogeneity on a test of phonological processing and a test of phonological short-term memory, with half of the WS sample performing at MA levels on these tests. Interestingly, no WS individual showed a weakness on a test of nonverbal reasoning, and only one WS individual showed a weakness on a test of verbal comprehension. In addition, we found that strengths on analysis-synthesis and verbal analogies occurred only for WS individuals with an MA less than 5.5 years (our sample median MA); people with an MA greater than 5.5 years performed at MA level on these 2 tests. Results also provided preliminary evidence for distinct subgroups of WS people based on their cognitive strengths and weaknesses on a broad range of cognitive functions. On the basis of the findings, caution should be made in declaring a single cognitive profile that is characteristic of all individuals with WS. Just as there is heterogeneity in genetic and physical anomalies within WS, not all WS individuals share the same cognitive strengths and weaknesses. Also, not all WS individuals show the profile of a strength in verbal abilities and a weakness in spatial functions.
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