Motor and socio-cognitive mechanisms explaining peers' synchronization of joint action across development in autistic and non-autistic children.

When partners coordinate their movement in time and space to reach a goal, they perform joint action, an important part of every interaction. Joint action involves motor abilities and socio-cognitive skills like theory of mind. Autistic children's lower joint motor coordination (joint action) abilities as well as their motor functioning and theory of mind difficulties may interfere with efficient peer interaction. However, the shared contribution of motor and theory of mind to partners' joint action was not yet explored. This study investigated those contributors (motor and theory of mind) along with group and age differences in 84 autistic children ages 6-16 years and 64 non-autistic children matched by age, sex, and IQ across three age-groups: early-childhood, preadolescence, and adolescence. Basic and advanced theory of mind skills and most motor tasks were higher among adolescents versus early-childhood. However, the autistic group consistently underperformed the non-autistic group in basic and advanced theory of mind levels and in all gross- and fine-motor tasks across all age-groups, revealing unique motor development characteristics in autism. A significant joint full mediation effect emerged for motor and theory of mind skills on joint action performance in both study groups. Understanding that motor and theory of mind skills together underlie joint action opens up a new channel of intervention to facilitate peer interaction.Lay abstractWhen two or more people move together in a coordinated way at the same time and in the same place, they perform "joint action," which is an important part of everyday social interaction. Joint action involves the activation of both motor skills and the social-cognitive understanding of others' thoughts, feelings, and desires-their ability to hold "Theory of Mind." Motor functioning and Theory of Mind may be challenging for autistic individuals. We wanted to investigate how motor skills and the ability to understand others' minds develop in autistic and non-autistic children and adolescents and to explore how these skills contribute to joint action performance. We compared 84 autistic children with 64 non-autistic children matched by age, sex, and IQ. Among these 6- to 16-year-olds, we examined three age-groups: early-childhood, preadolescence, and adolescence. We found that older participants, both in the autistic and non-autistic groups, showed better abilities than younger participants in basic and advanced Theory of Mind skills and in most motor tasks. However, non-autistic children outperformed autistic children in Theory of Mind (at basic and advanced levels) and also in all gross-motor and fine-motor tasks, across all age-groups. The autistic group's motor patterns were characterized by greater variability in tasks' rated difficulty levels compared to their non-autistic peers, who showed more intact, uniform patterns. Both motor and Theory of Mind skills were found to significantly impact joint action performance in both study groups. These findings are important for understanding joint action's underlying mechanisms and for refining social intervention programs for autistic individuals.