Designing a Robot’s Proactivity: Adjusting the Robot’s Communication Modality to the Participant’s Cognitive Load

Social robots are increasingly expected to initiate interactions, yet proactive behavior often produces negative experiences if presented when the users are not available. We argue that availability is not binary and depends on the user's cognitive load. Moreover, proactivity can take different forms, suggesting that proactive cues can be adjusted to the user’s current load. We tested whether matching proactivity modality (verbal and nonverbal cues) to cognitive-load level improves robot perception. In a 2×2×2 between-participants study, 87 participants completed a high-load writing task or a low-load screw-sorting task while an ElliQ robot initiated interaction using verbal cues, nonverbal cues, both, or neither. Perceptions varied across conditions. Under low load, verbal proactivity was preferred regardless of nonverbal cues. Under high load, nonverbal-only proactivity produced the best experience. These findings suggest that proactivity design is not trivial and should focus on aligning the robot's proactive behavior with the user's cognitive load.