Overlap and Differences of Autism and ADHD: Digital Phenotyping of Movement and Communication During Development.

Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD) often co-occur, which complicates diagnosis across development. Here, we test whether automatic analysis of naturalistic video can disentangle shared from distinct behavioral signatures of the two disorders. We analyzed videos of 2,341 youths (ages 5-22) from a community sample while they described an emotive short film. Multivariate models revealed that language deficits previously attributed to ADHD were largely explained by age. Difficulties in understanding and recalling a social narrative, sometimes attributed to ADHD, were uniquely predicted by ASD. Increased motor activity was a specific marker of the hyperactive-impulsive domain of ADHD. Conversely, ASD showed structurally intact language but significant impairments in narrative ability and perspective-taking, coupled with a unique vocal profile of higher pitch, intensity, and altered voice quality. These findings suggest that despite substantial comorbidity, ADHD and ASD exhibit separable behavioral profiles that can be measured objectively at scale.