Date of Completion


Embargo Period



autism spectrum disorder; developmental delay; low mental age

Major Advisor

Deborah Fein, PhD

Associate Advisor

Jeffrey Burke, PhD

Associate Advisor

Rhiannon Smith, PhD

Field of Study



Doctor of Philosophy

Open Access

Open Access


Diagnosis of Autism Spectrum Disorder (ASD) is complicated in children with low mental age (low MA; cognitive functioning below a 12-month level) due to the way in which diagnostic measures function in this group, and due to limited understanding of symptom presentation in children with low MA. Indeed, no commercially available ASD diagnostic tools are designed or recommended for use in children below 12 months. Utility of the Autism Diagnostic Observation Schedule (ADOS) and the Childhood Autism Rating Scale (CARS) in discriminating ASD from Global Developmental Delay (GDD), as well as symptom profiles measured by individual ADOS item scores, were examined in two-year-old children with ASD-low MA (n = 53), GDD (n = 175), and ASD with MA over 12 months (n = 425). Both the ADOS and CARS were similar in their agreement with clinical best estimate (i.e., 79.2% and 83.3%, respectively). Yet, in cases of disagreement, the ADOS over-diagnosed ASD in children with low MA, whereas the CARS both over-classified (though less than the ADOS) and under-classified these children. Reciprocal social interaction (e.g., eye contact, social interest), but not more advanced social behaviors (e.g., pointing or play), best distinguished children with ASD (with and without low MA) from those with GDD. ASD-low MA, a more severe autism subtype, may benefit from a modified ADOS algorithm or alternative direct measure of symptoms to facilitate accurate, timely diagnosis.