Attenuated adaptive growth of interpersonal synchrony in autism

During social interactions, people continuously align their movements and rhythms, a process known as interpersonal synchrony that supports rapport, mutual understanding, and smooth communication. In autism spectrum disorder (ASD), previous studies have often reported atypical or reduced synchrony, but most have relied on aggregate or session-averaged measures that may miss how coordination develops over time. It therefore remains unclear whether interactional differences in autism reflect a general reduction in synchrony or altered temporal dynamics of interpersonal coordination. We examined the temporal dynamics of head-movement synchrony during a structured face-to-face communication task, comparing non-autistic dyads (two typically developing [TD] partners) with mixed-neurotype dyads (one TD speaker paired with one autistic listener), using gyroscope-based tracking and time-resolved trajectory modelling. Phase-based synchrony, indexed by the phase-locking value (PLV), was lower overall in mixed-neurotype dyads. Critically, time-resolved analyses revealed a marked group difference in synchrony trajectories: non-autistic dyads showed progressive, adaptive growth in synchrony over the interaction, whereas mixed-neurotype dyads showed a significantly attenuated, flatter pattern. These findings suggest that autism may involve altered temporal organization of social coordination rather than simply reduced synchrony overall. Lay AbstractWhen we talk with someone, we often naturally match their body language and rhythms without even realizing it. This physical "syncing up" helps us feel connected, builds trust and shared understanding, and makes communication flow easily. Research shows that autistic people might sync their movements differently during conversations compared to non-autistic people. However, past studies usually just measured an overall average of this syncing across a whole interaction. This approach misses how human interactions actually unfold over time. We wanted to know: do autistic people just sync less overall, or does their syncing change differently as the conversation goes on? To find out, we used small motion sensors to track the head movements of adults having structured face-to-face conversations and compared two types of pairs: non-autistic pairs, where both people were non-autistic, and mixed-neurotype pairs, where one non-autistic speaker talked to one autistic listener. We found a notable difference in how the two groups interacted over time. For the non-autistic pairs, the physical syncing grew progressively stronger as the conversation progressed; they progressively "tuned in" to each other. In contrast, mixed-neurotype pairs showed a flatter pattern--their level of syncing stayed relatively constant from start to finish without that same gradual build-up. These findings are important because they suggest that differences in autistic communication are not simply a "lack" or "deficit" in social coordination. Instead, autistic individuals have a distinct style of interacting--one that maintains social engagement without relying on the progressive build-up of physical syncing that non-autistic people use. Taken together, our results highlight the importance of examining how interactions evolve over time to better understand the different ways autistic and non-autistic people communicate.