Although considerable work has documented higher prevalence rates of autism spectrum disorder (ASD) in boys, fewer studies have focused on sex differences within samples of young children at‐risk for ASD. This study examined sex differences in ASD symptom domains and ASD screening outcomes among toddlers (18–35 months) and preschoolers (36–72 months) with ASD‐related concerns. Participants included 480 children between 18 and 72 months evaluated by university‐based ASD specialty clinics. Results revealed significant sex differences in severity of social communication (SC) deficits across age groups. Within the toddler group, girls diagnosed with ASD displayed greater SC deficits according to standardized observation and clinician severity ratings. Within the preschool group, girls diagnosed with ASD were rated by parents as having more severe SC deficits, but these differences were not corroborated by standardized observations or clinician ratings. No sex differences emerged for severity of restricted repetitive behaviors (RRBs) for either age group. Across the entire referred sample, boys and girls did not differ in terms of scores on commonly used screening instruments. Importantly, results suggest that two of the most commonly used ASD screeners (i.e., Modified‐Checklist for Autism in Toddlers‐Revised with Follow‐up and Social Communication Questionnaire ) may underidentify RRBs in toddler and preschool‐aged girls as screening scores were only influenced by severity of SC deficits. Greater SC deficits in young girls with ASD along with its impact on screening status suggests greater attention be placed on the under‐identification of ASD in girls as well as current screening measures’ ability to tap into the topography of ASD symptoms across genders. Autism Res 2020, 13: 157–166. © 2019 International Society for Autism Research, Wiley Periodicals, Inc.
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