Lay Abstract Children with Autism Spectrum Disorder (ASD) have numerous impairments in social interaction that can severely impede mental and physical development, learning, and behavioral functioning at home and in the community and also make treatment difficult. In addition, many individuals have difficulty performing movements of the body that involve both large and small actions. We explored the role of an overlooked dimension of social interaction, social motor synchrony, in ASD, and evaluated its relationship with body movement. In particular we examined the ability of children with and without ASD to perform body movements alone or with another individual. We found that children with ASD were less able to synchronize their body with an experimenter and children with ASD performed single-person motor movements that were slower and more variable in both spacing and timing. Children with ASD had trouble performing movements that were of a consistent size and tempo over the course of the interaction. Such lack of consistency in movement likely makes coordination with another person more difficult. This raises the possibility that these types of body movements could provide new insights into understanding the social problems in ASD. Scientific Abstract Impairments in social interaction and communicating with others are core features of autism spectrum disorder (ASD) but the specific processes underlying such social competence impairments are not well understood. An important key for increasing our understanding of ASD-specific social deficits may lie with the social motor synchronization that takes place when we implicitly coordinate our bodies with others. Here, we tested whether dynamical measures of synchronization differentiate children with ASD from controls and further explored the relationships between synchronization ability and motor control problems. We found (a) that children with ASD exhibited different and less stable patterns of social synchronization ability than controls; (b) children with ASD performed motor movements that were slower and more variable in both spacing and timing; and (c) some social synchronization that involved motor timing was related to motor ability but less rhythmic synchronization was not. These findings raise the possibility that objective dynamical measures of synchronization ability and motor skill could provide new insights into understanding the social deficits in ASD that could ultimately aid clinical diagnosis and prognosis.

Impairments in social interaction and communication are critical features of ASD but the underlying processes are poorly understood. An under-explored area is the social motor synchronization that happens when we coordinate our bodies with others. Here, we explored the relationships between dynamical measures of social motor synchronization and assessments of ASD traits. We found (a) spontaneous social motor synchronization was associated with responding to joint attention, cooperation, and theory of mind while intentional social motor synchronization was associated with initiating joint attention and theory of mind; and (b) social motor synchronization was associated with ASD severity but not fully explained by motor problems. Findings suggest that objective measures of social motor synchronization may provide insights into understanding ASD traits.