Examining Autism Symptom Severity and Structure in IQ Groups

The purpose of this study was to compare the severity and structure of autism symptoms in four cognitive-level groups: (1) those without an intellectual disability (No ID; IQ: 85;), (2) those with borderline cognitive ability (Borderline; IQ: 70-84), (3) those with an intellectual disability with verbal language (ID Verbal; IQ < 70), and (4) those with an intellectual disability without verbal language (ID Non-Verbal; IQ < 70). The data for this study were based on a sample of 2,819 children with autism who participated in the Simons Simplex Collection (SSC; Fischbach & Lord 2010). In Objective 1, the correlations between IQ and autism symptom severity were examined, as well as comparing symptom severity in each of the four IQ groups using the Autism Diagnostic Interview-Revised (ADI-R). IQ was negatively correlated with ASD symptom severity, ranging from weak to moderate correlations. The No ID group had significantly lower scores on the Social, Communication, and Restricted Repetitive Behaviour domains in comparison to the ID Verbal and ID Non-Verbal groups. Also, the No ID group had significantly lower scores on the Social and Communication domains in comparison to the Borderline group. In Objective 2, confirmatory factor analyses were used to compare different models of autism symptom structure in the four IQ groups using the ADI-R and then the Autism Diagnostic Observation Schedule (ADOS). Most of the common models in the literature were acceptable representations of the structure of autism, especially for children with low IQ. The DSM-IV and DSM-5 models may better capture those with lower cognitive ability and more severe autism symptoms. For children with higher cognitive ability, it may be beneficial to consider elements of a 4-factor model consisting of Social, Communication, Play/Peer, and RRBI factors. This model had good fit in all four IQ groups using the ADI-R, and in three of the groups using the ADOS. It was also the only model that had good fit for the groups with higher IQ based on the analyses from the ADI-R. This study has implications for how we conceptualize the construct of autism, specifically in terms of diagnostic criteria and assessment measures. Further replication of these findings is warranted with different measures