Social Touch in Human–Robot Interaction: Robot-Initiated Touches can Induce Positive Responses without Extensive Prior Bonding

Social touch interactions can, depending on the type and strength of the dyadic social relationship, elicit a plethora of physiological, emotional, and behavioral responses; both beneficial and disadvantageous. With the intention to expand the communicative capabilities of humanoid social robots, we investigated whether robot-initiated touches could elicit beneficial responses in the human user that are comparable to responses to human touch. In addition, we investigated whether having a pre-existing positive social bond with the robot modulates these responses. To this end, we conducted a {\$}{\$}2 {\backslash}times 2{\$}{\$}2{\texttimes}2between subjects experiment (N = 67) in which participants either did or did not establish a bond with the robot prior to interacting with it during stressful circumstances. This interaction either did or did not comprise robot-initiated touches. We hypothesized that robotic touches would attenuate the subjective and physiological stress responses during the stressful event (H1a), enhance the perceived relation with the robot (H1b), and increase one's pro-social behavior (H1c), as contrasted with interactions without touch. Based on findings from human touch, we also expected that the effects of H1a and H1b would be more outspoken when a bond with the robot was established (H2). Our findings imply that robotic touches attenuated physiological stress responses and increased the perceived intimacy of the human--robot bond. No effects were found on pro-social behavior and all effects were independent of whether a bond was formed or not. Although no full support for our hypotheses was found, the findings suggest that robot-initiated touch can, under specific circumstances, be a valuable extension of a social robot's nonverbal communication repertoire.