Intellectual Ability and Autism

Background: While many children with Autism Spectrum Disorders (ASD) undergo intelligence testing, little is known about the distribution of intellectual ability of this population. Previous studies estimated the rate of intellectual disability (ID) in children with ASD to be between 30% and 70%. It is important to reassess the expression of intellectual ability in ASD children.   

Objectives: The purpose of this study was to investigate the distribution of intelligence among ASD children. Case-specific data were analyzed to determine differences between ASD children on different intellectual levels.  

Methods: Data were collected as part of an ASD surveillance investigation carried out in four New Jersey counties.  Using an active case-finding method established by the Centers for Disease Control and Prevention (CDC), ASD surveillance data were collected for 8-year-old children living in the region in 2006.  Data were based on review and analysis of information contained in health and education records.  Demographic information referencing SES, ASD diagnosis, case-specific data and intelligence quotient (IQ) results were analyzed. ASD diagnosis included Autistic Disorder and ASD-NOS (representing all other ASD diagnoses). Statistical analysis was performed using t-tests and Chi-square tests.   

Results: We identified 232 ASD children having both an IQ test and an ASD diagnosis from a community provider. Of these children, 37.1% met the ID criteria, scoring 70 or below on an IQ test. There were no significant differences between IQ and sex. Associations between race and IQ as well as SES and IQ were highly significant (p<0.001). However, when both factors were combined and analyzed in relation to IQ, no statistical difference was observed. The rate of ID differed by race, where 22.7% of white children, 57.8% of black children and 41.3% of Hispanic children met the criteria for ID (p<0.001). Compared to children with AD, children with ASD-NOS were more likely to have an average IQ score of 85 or above (44.9% v. 17.0%, p<0.001). The rate of ID in children with AD was significantly higher than those with ASD-NOS (63.8% v. 25.4%, p<0.001).  

Conclusions: Consistent with other recent studies, 37% of ASD children in our sample have ID. However, children with AD were more likely to present with ID. However, one out of four children with ASD-NOS still met the criteria for ID. Contrary to other studies, we did not identify higher rates of ID among girls with ASD. It is important to note that almost 40% of all children in our sample had an IQ of 85 or above, demonstrating the strong intellectual ability of children with ASD. The high prevalence of ID in children with Autistic Disorder could allow clinicians to more accurately diagnose cases of autism. While socio-economic and race factors showed significant differences with regards to IQ and rate of ID, further research must be done to determine what factors contribute to these variations.  There may be race and/or SES-based differences in the intellectual ability of children with ASD. If these differences are confirmed, we would expect there to be parallel differences in educational placement and functional outcomes.