Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Psychological Science, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.Approximately four times more males are diagnosed with Autism Spectrum Disorder (ASD) than females and relatively few studies to date have investigated gender differences in the early signs and later manifestations of autism and cognitive development in infants and

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... oung children. Of these, few studies have combined observational measures with clinical reports to understand any gender differences in these children, and there are few follow-up studies that have explored development of gender differences over time. Three complementary follow-up studies were conducted to investigate gender differences in children later diagnosed with ASD with the aims of identifying gender differences in a) the early indicators of autism (Study 1), b) early cognitive development and autism manifestations (Study 2) and c) the development of social attention and restricted and repetitive behaviours (RRBs) from 2- to 4-years of age of age (Study 3) in the same cohort. The current research utilised data from the Social Attention and Communication Study (SACS; Barbaro and Dissanayake, 2010). The combined results from all three studies indicated a similar developmental trajectory in boys and girls in the early indicators during the first two years of life, and from toddlerhood to pre-school age on cognitive ability, autism manifestations and observed social attention. However, boys showed slightly more RRBs than girls from toddlerhood to pre-school age. The results are discussed in relation to the Extreme Male Brain Theory (EMB; Baron-Cohen, 2002) as well as providing an explanation to why boys may be more likely to receive an ASD diagnosis than girls. Further research is needed to understand similarities and differences in the progression of ASDs in girls and boys, and to understand the female profile [...]