LAUSR

Down syndrome (DS) is the most frequent genetic cause of intellectual disability (ID). Individuals with DS exhibit an elevated risk of other neurodevelopmental disorders, including autism spectrum disorder (ASD). The primary objective of this study was to explore the clinical characteristics of co‐occurring ASD in children and adolescents with DS using a case–control approach. We compared the adaptive and behavioral profiles of a group of participants with both DS and ASD (ASD group) with a group of participants with DS‐only matched by age, sex, and IQ (CON group). Participants in the ASD group exhibited significantly lower adaptive skills than the CON group, despite the IQ‐matching in the moderate/severe ID range. No group differences emerged on internalizing or externalizing behavioral symptoms, but participants in the ASD group exhibited significantly higher social withdrawal, stereotyped behavior, and restricted interests. These findings indicate that, although children with DS—with or without ASD—who have similar low cognitive functioning often exhibit common behavioral traits, paying close attention to the elevation of stereotyped behaviors or restricted interests can improve the detection of co‐occurring ASD in this population, enabling more personalized interventions. Conversely, the presence of ritualistic behaviors or behaviors related to insistence on sameness may not be a strong indicator of underlying ASD in children with DS. Additionally, acknowledging that the presence of ASD contributes to adaptive behavior deficits beyond the impact of moderate‐to‐severe ID alone highlights the critical need for early interventions to enhance daily living skills in this population.