No sex differences in core autism features, social functioning, cognition or co-occurring conditions in young autistic children: A systematic review and meta-analysis

Autism Spectrum Disorder (ASD) is a neurodevelopmental condition with more males than females diagnosed, and researchers have considered whether the existence of a female-specific ASD phenotype may contribute to differences in rates of diagnosis. We sought to inform this issue through a systematic review and meta-analysis of potential sex differences specifically in young autistic children across a range of domains including core ASD features, social functioning, cognition, and co-occurring internalising and/or externalising conditions. The systematic review identified 35 studies examining sex differences in young autistic children. Conflicting results were evident across studies, with some reporting small sex differences and others reporting no sex differences. Meta-analysis revealed no overarching significant sex differences in the domains investigated. However, the meta-analytic effect for the RRB domain approached significance, with females demonstrating fewer RRBs than males. Many of the primary studies included here utilised data from standardised diagnostic instruments to measure autism features, so while this study suggests non-significant sex differences in early childhood ASD, it remains possible that current tools are insufficiently sensitive to detect differences in ASD presentation by sex at this age. It is also possible that the diagnostic criteria may reflect a predominately 'male phenotype' and this may obscure the detection of genuine sex differences in young autistic children.