Characteristics of Boys and Girls with and without Autism Spectrum Disorder in the Study to Explore Early Development

Background: In population-based studies of autism spectrum disorder (ASD), there is a 4:1 sex ratio of boys to girls. Girls with ASD may be under-recognized by healthcare professionals, which could increase the likelihood for delayed or absent ASD diagnosis. Consequently, some studies have examined whether sex differences exist among behavioral and developmental characteristics of children with ASD and if these differences contribute to differential performance on ASD screening and diagnostic instruments. These studies have produced mixed results, especially among young children. Moreover, no studies have included a population-comparison group (POP) to examine how sex differences in ASD compare to those in the general population.

Objectives: (1) Compare preschool boys and girls with ASD, other (non-ASD) developmental delay (DD), and POP on behavioral and developmental characteristics; and (2) Compare preschool boys and girls with ASD on results of ASD screening and diagnostic instruments, as well as the presence of a previous ASD diagnosis.

Methods: Children 2-5 years of age were enrolled in the Study to Explore Early Development Phases1&2 (SEED1+2). The Social Communication Questionnaire (SCQ) screened all children for ASD symptoms upon study enrollment, and the Mullen Scales of Early Learning (MSEL) assessed developmental characteristics in all children. Those with an SCQ score of 11 or higher or a previous ASD diagnosis received additional evaluation that consisted of the Autism Diagnostic Observation Schedule (ADOS) and Autism Diagnostic Interview – Revised (ADI-R). Results of the ADOS and ADI-R determined ASD status. Parents reported whether their child had a previous ASD diagnosis and completed the Child Behavior Checklist (CBCL) to assess child behavioral characteristics.

Results: The sample included 4,936 children: 1,480 ASD, 1,805 DD, and 1,651 POP. The sex ratio varied by case classification with more boys in the ASD sample (81.7%) versus DD (66.9%) and POP (52.5%), and more boys in DD than POP (p<.01). There were no sex differences in child age, family income, maternal age, maternal ethnicity, or maternal race in any study group. Girls in the POP group had better MSEL expressive language, receptive language, and fine motor abilities than boys (p<.001). These sex differences were not present in the ASD or DD samples. There were no sex differences in MSEL non-verbal cognitive abilities or CBCL externalizing or internalizing behavior problems in any study group. Among children with ASD, there were no sex differences in SCQ total score, ADOS domain scores, ADI-R domain scores, or presence of a previous ASD diagnosis.

Conclusions: Children in the ASD and DD groups did not show similar sex differences in developmental or behavioral characteristics than those in the POP group, where girls scored higher in language and motor development than boys. There were no sex differences in the behavioral or developmental characteristics of children with ASD, or with their performance on ASD screening and diagnostic instruments. Future research of sex differences in other samples of preschool children could consider whether sex differences are unique to particular ASD phenotypes and/or emerge as children with ASD age.