Although it is well established that the prevalence of autism spectrum disorder (ASD) is higher inmales than females, there is relatively little understanding of the underlying mechanisms and their developmentaltime course. Sex-specific protective or risk factors have often been invoked to explain these differences, but suchfactors are yet to be identified.Methods:We take a developmental approach, using a prospective sample of 104 infants at high and low familialrisk for ASD, to characterise sex differences in infant markers known to predict emerging autism symptoms. Weexamine three markers previously shown to be associated with later autistic social-communication symptoms: theAutism Observation Scale for Infants (AOSI) total score, attention disengagement speed and gaze followingbehaviour. Our aim was to test whether sex differences were already present in these markers at 1 year of age,which would suggest sex-specific mechanisms of risk or protection.Results:While no sex differences were found in any of the three markers investigated, we found sex differences intheir relationship to 3-year autism traits; all three markers significantly predicted later autism traitsonly in the boys.Conclusions:Previously identified‘early autism markers’were associated with later autism symptoms only in boys.This suggests that there may be additional moderating risk or protective factors which remain to be identified. Ourfindings have important implications for prospective studies in terms of directly testing for the moderating effect ofsex on emerging autistic traits.