Reasons for the Sex Difference in the Prevalence and Age of Autism Spectrum Disorder Diagnosis

Author: Rachel Hiller

Hiller, Rachel, 2014 Reasons for the Sex Difference in the Prevalence and Age of Autism Spectrum Disorder Diagnosis, Flinders University, School of Psychology

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In the absence of intellectual impairment, girls are diagnosed with Autism Spectrum Disorder (ASD) both substantially less and later than boys. In this thesis I explored potential reasons for the sex difference in the prevalence and age of the diagnosis of ASD. Two theories were explored. First, girls may genuinely develop ASD less than boys, due to an advantage in the typical development of cognitive and environmental factors associated with social development. Alternatively, girls may be better able to hide their underlying impairments and present with different overt behaviours, resulting in the under-detection of the disorder. While little evidence was found for the first theory, across two studies I found evidence to support the theory that ASD presents differently girls, and is thus potentially more difficult to detect. Findings indicate that the current estimate of the rate of ASD in girls likely underestimates the true prevalence of the disorder in this population. To investigate whether girls may genuinely develop ASD at a lesser rate to boys, I explored sex differences in the cognitive and social profiles of typically-developing pre-schoolers. In particular, I focussed on cognitive factors (theory of mind and executive function), communication style with parents (mental state talk), and play style, all of which are linked to social development. Results from 68 pre-school aged children (27 girls) failed to show robust evidence of girls being protected by better developed cognitive or social skills. While girls were more readily exposed to complex social environments, through parent-interaction and through a preference for pretend-play, this was not related to more advanced social competence. However, robust evidence was found for the second theory, that girls may be diagnosed less with ASD due to the under-detection of the disorder. This theory was investigated over two studies. In the first, I explored the pre-diagnosis concerns of 152 caregivers (60 of girls) whose cognitively able children were late-diagnosed with ASD. In the second, I explored sex differences in a sample of 69 boys and 69 girls all diagnosed with ASD, based on clinician and teacher ratings. Evidence across both studies showed girls were reportedly better able to imitate, and use this in a social environment in an attempt to copy social interactions. Further, while girls were equally impaired as boys in some key underlying social impairments, this manifested in quite different overt behaviours. It is likely many of these overt behaviours (e.g., better use of nonverbal gestures) further camouflages girls' underlying impairments. This ability to camouflage seemed most notable when in school, with teachers reporting far fewer concerns for girls than for boys, including the majority of girls being rated by teachers as having quite typical social skills. Outside of the social domain, girls were also found to present with different types of restricted interests to boys, which were potentially more difficult to detect as atypical, or indeed as a sign of ASD. Results provided insight into why the disorder may be more difficult to detect in girls, particularly in the younger years and by professionals not specifically trained in the diagnosis of ASD. Further, results provide a framework for how we can better identify the disorder in girls.

Keywords: autism spectrum disorder,sex differences,gender
Subject: Psychology thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of Psychology
Supervisor: Associate Professor Robyn Young