Research on the neural underpinnings of Autism Spectrum Disorder (ASD) has focussed primarily on impairments of social interaction and communication. Less is known though about the second diagnostic criterion of restricted behaviors and interests. Uniquely in this domain, alongside impairments stands an 'ASD advantage' characterised by superior performance on many visual tasks. We recently found that 2-year-olds with ASD dramatically outperform age-matched, typically developing controls on visual search. Here we use task-evoked, phasic pupil responses--a sensitive, involuntary measure of effort and a biomarker of the locus coeruleus-norepinephrine (LC-NE) system's modulation of attention--to isolate a causal factor: a 'hyperphasic' LC-NE system compels (here, advantageously) focussed attention. However, this focussed attention in other contexts may contribute to restricted behaviors and interests.