Renee Bergeron and Randy Floyd examined the performance of children with
intellectual developmental disorder (IDD) compared with typically functioning
children on the Woodcock-Johnson test. They wanted to determine how much weight
should be given to broad cognitive abilities, as opposed to general
intellectual ability, in understanding the functioning of children with IDD.
This question arose against the background of the fact that, since 2002, the
U.S. Social Security Administration has allowed a child to be diagnosed as
having IDD on the basis of index scores (for example, broad abilities) rather
than a general IQ score when there is reason to question the validity of the
general IQ score.
Thirty children with IDD, aged 8–18, including 17 girls, were selected
for the study according to the following criteria:
• A psychological evaluation conducted within the previous five years in
which the child met the criteria for IDD.
• An IQ between 40 and 70.
• Deficits in adaptive behavior skills and cognitive difficulties that could
not be attributed to a medical or neurological condition such as Down syndrome.
Each child with IDD was matched with a child of average achievement from
the WJ-III standardization sample. The children were matched for age, sex,
socioeconomic status, and ethnic background, whenever possible.
The children with IDD completed the first 14 tests of the American version of the WJ III. Their mean general intellectual ability score was 54.8, with a standard deviation of 12.45. Data on their mean scores in the various abilities are presented in the table below (click on the slide to enlarge).
Performance of 30 children with IDD on the WJ III
Here are the most interesting findings from this study:
The profile of the
children with IDD was not flat. Sixty percent of the children with IDD (18
children) obtained a score of 85 or higher in at least one cognitive ability.
In other words, 60% of them had at least one cognitive ability within the
average range. Note the range of scores in the table. Some children with
IDD obtained scores of 93, 97, and 100 in auditory processing, visual
processing, and processing speed, respectively.
According to Detterman, one of the leading researchers in the field of
intelligence, IDD is caused by impairment in one or more cognitive abilities
that are central to intelligence and that lower the functioning of the system
as a whole. The more central a cognitive ability is, the higher its correlation
with general intellectual ability. This relationship is referred to as the
ability's g-loading. In this study, the correlation between the
performance of the children with IDD and the g-loadings of the broad abilities
was −0.75. In other words,
the more central the ability was to general intelligence, the lower the score
of a child with IDD tended to be on that ability.
The children with IDD showed a significant intraindividual strength in
visual processing and auditory processing, and a significant intraindividual
weakness in crystallized knowledge and long-term storage and retrieval. The
highest mean score of the IDD group was in visual processing (79.6), an ability
with a relatively low loading on general intellectual ability in the American
version of the Woodcock-Johnson test. The lowest mean score of the IDD group
was in long-term storage and retrieval (59.10). Long-term storage and retrieval
has a moderate-to-high loading on general intellectual ability.
It should be noted
that very few children with IDD showed a profile similar to that of the group
as a whole. In other words, there is no single profile of ability scores
that characterizes children with IDD.
The individual cognitive profiles of the children with IDD in this study
also support Detterman's hypothesis. All of the children with IDD showed impairment in at least one of the
two abilities with the highest loadings on general intellectual ability:
crystallized knowledge and fluid reasoning. Many of the children with IDD
showed impairment in both abilities.
The findings of the study indicate that when general intellectual
ability is low, not all cognitive abilities are necessarily low. The reverse is
also true: when a child obtains a very, very low score in a particular
cognitive ability, one should not expect all of the child's other abilities to
be low as well.
The impaired cognitive abilities are not the same in all children with
IDD. Each child with IDD
has his or her own pattern of strengths and weaknesses.
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