The Level and Nature of Autistic Intelligence II: What about Asperger Syndrome?

Background: Autistic children and adults score on average 30 percentile points higher on the pre-eminent fluid intelligence test, Raven’s Progressive Matrices (RPM), than on widely used Wechsler subtest batteries (WAIS-III, WISC-III), while typical individuals do not display this discrepancy (Dawson et al., 2007). Unlike individuals diagnosed with autism, individuals diagnosed with Asperger syndrome do not, by diagnostic definition, present with delays in speech production. Also, individuals diagnosed with Asperger syndrome are, by diagnostic definition, all characterized by intelligence test scores in the normal range, whereas individuals diagnosed with autism may score in any range. Although testing with RPM does not require extensive speech production and comprehension, typical individuals are proposed to rely on language processes to perform many RPM items (Lohman, 2005). How the performance of Asperger adults and children on RPM compares with their Wechlser performance remains uncertain. Preliminary findings have shown that Asperger children (ages 6-12yrs) obtain significantly higher raw scores on RPM than typical children matched on age and Wechsler performance (Hayashi et al., 2008).

Objectives: This study aimed to replicate and extend findings concerning the performance of autistic individuals on RPM in a population of strictly (rather than clinically) defined Asperger children and adults.

Methods: The child sample included 23 Asperger and 24 typically developing children, matched on age (6 to 16 years old). The adult sample included 29 Asperger and 19 control adults, matched on age (16 to 49 years old). An Asperger diagnosis was given for participants eligible for an autism diagnosis on ADI-R or ADOS (module 3 or 4), but who did not have delayed speech, echolalia, pronoun reversal, or stereotyped speech, all as defined in the ADI-R. All participants completed the standard version of RPM. In addition, children completed the Wechsler Intelligence Scale for Children (WISC-III), whereas adults completed the Wechsler Adult Intelligence Scale (WAIS-III). The three instruments were administered by clinicians unaware of the hypotheses of this study.

Results: Asperger children obtained on average a Wechsler Full-Scale IQ at the 48^th percentile and a RPM score at the 60^th percentile. Planned one-way analyses of variance revealed a significantly higher RPM performance than Wechsler FSIQ performance by the Asperger children, but no difference between performance on the two tests by the control children (Wechsler FSIQ 70^th percentile and RPM at 72^nd percentile). Similarly, Asperger adults scored significantly higher on RPM (75^th percentile) than on the Wechsler FSIQ (49^th percentile), whereas there was no significant difference between performance on RPM (82^nd percentile) and Wechsler FSIQ (75^th percentile) for the control adults.

Conclusions: These findings demonstrate that, while differing from autistics in history of speech delay and in individually having Wechsler scores in the normal range (here with group averages near the 50^th percentile for children and adults), Asperger children and adults are similar to autistic children and adults in presenting with significantly higher RPM than Wechsler scores. These findings suggest that strengths in RPM performance and therefore in fluid reasoning are pervasive across the autistic spectrum.