Date of Completion


Embargo Period



autism, low mental age, diagnostic stability

Major Advisor

Dr. Deborah Fein

Associate Advisor

Dr. Chi-Ming Chen

Associate Advisor

Dr. Kimberly Cuevas

Field of Study



Doctor of Philosophy

Open Access

Open Access


Autism Spectrum Disorders (ASDs) involve deficits in social and communication abilities and the presence of repetitive and restricted behaviors, and affect approximately 1 in 68 children. Early identification and intervention significantly improve the prognosis of children with ASDs. Research has demonstrated that screening and diagnosis of ASDs is reliable in children as young as 18 months. Cognitive impairment affects up to 70% of individuals with ASDs, and there is diagnostic overlap between ASDs and Intellectual Disability (ID). A subset of children with ASDs present with low mental ages (Low MA), defined as an age equivalent of below 12 months in all developmental domains, at the time of their first evaluation. While research has demonstrated that ASDs can be distinguished from ID diagnostically, the validity of utilizing ASD-specific screening and diagnostic tools in the presence of Low MA has not been investigated. In this study we investigated the diagnostic stability, developmental outcomes, and ASD symptomatology in a sample of children that were initially diagnosed with ASD Low-MA (n = 25), Autistic Disorder (AD; n = 111), and PDD-NOS (n = 82). The majority of children with ASD Low-MA (96%) remained on the autism spectrum at follow-up, compared to AD (86.5%) and PDD-NOS (73.2%). The ASD Low-MA group had the lowest mean ratio IQ scores and made the least developmental progress in all subdomains of the Mullen Early Scales of Development; the PDD-NOS had higher scores than the AD group in all domains. This trend was observed for the Vineland Adaptive Behavior Scales as well, with exceptions in the Domestic and Community domains; each group had comparable ratio IQ’s initially, but experienced a decrease in ratio IQ across time. The ASD Low-MA group had the highest total scores on the CARS, ADOS, and DSM-IV at both time points, indicating high and stable ASD symptom severity. The AD group demonstrated a higher rate of symptom improvement than the PDD-NOS and ASD Low-MA groups. These results suggest the validity of diagnosing ASDs in children that present with Low MA, as well as the high stability of diagnosis and symptoms across all ASDs.