Mario Costantino¹, Erez Freud¹; ¹Department of Psychology, York University

Daily activities, such as eating with a peer or walking in a crowd, inherently unfold within a social context. One mechanism thought to promote synchrony between individuals is the formation of shared task representations, in which people implicitly encode elements of a partner’s actions alongside their own. While previous research has extensively examined deliberate coordination driven by competition or collaboration, far less is known about the spontaneous, uninstructed synchronization that emerges when actors pursue independent goals side-by-side. Here, we examined how social context modulates visuomotor control and whether co-action promotes alignment even in the absence of an explicit shared goal. Pairs of young adults completed a reach-to-grasp task under three conditions: acting individually, acting concurrently with identical objects (congruent), and with different-sized objects (incongruent). In paired conditions, partners sat face-to-face and could observe each other’s movements. Motion-tracking technology captured hand kinematics. To assess the fine-grained spatiotemporal structure of the movement trajectories and quantify interpersonal synchrony, we applied Dynamic Time Warping (DTW). DTW preserved temporal structure across partners, allowing us to quantify trajectory similarity independent of speed differences, overcoming limitations of time-averaged metrics. Results revealed two effects of co-action. First, movement time and time-to-maximum grip aperture were faster during co-action than during individual performance, regardless of object size. Second, co-action altered the structure of participants’ movements. During the paired condition, lateral hand trajectories became more similar than in the individual condition, indicating that co-action induces spontaneous alignment even when partners pursue independent goals. Additionally, synchrony in grip-aperture trajectories emerged when partners grasped identical objects. Grip-aperture profiles converged more in the congruent than in the individual condition, reflecting enhanced alignment when task parameters were matched. These findings indicate that co-action generates a baseline tendency toward synchronous movement, and that shared task parameters amplify this coordination beyond the effects of co-presence alone.