Supplemental Material for Children’s reciprocity and relationship formation with a robot across age.

Reciprocity is central to the formation and maintenance of relationships. Reciprocity and relationship formation change with children's development and are key aspects in human–robot interaction. So far, it is unclear how children reciprocate and build a relationship with a social robot and how this reciprocity develops with age. In the present study, we collected data from 147 children aged 5–12 years to investigate age differences in reciprocity and relationship formation toward a social robot. To test reciprocity, children completed an Alternated Repeated Ultimatum Game with a social robot and another child. Children also completed a survey on relationship formation to assess robot-related closeness, trust, and social support. Results from a linear-mixed effects Bayesian analysis indicated that children reciprocated similarly to a robot as to another child. While reciprocity differed across age with lower values for 8–10-year-olds compared to younger and older children, these age differences in reciprocity were also observed when children interacted with the robot. Children's relationship formation with a social robot also changed with age. Our findings suggest that established theories from human–human literature (e.g., age differences in reciprocity) are also relevant for human–robot interaction. Children's age is an important determinant for how children interact with and perceive robots.