# Vocal Synchrony of Robots Boosts Positive Affective Empathy
## 1 Introduction  
Robots that can converse with humans play increasingly important roles in society [1] by providing automated services, assisting with communication, and offering psychological support. For example, as the number of single-person households increases, feelings of social isolation have increased owing to a decrease in the number of partners available for daily conversation. This can contribute to decreases in happiness and physical activity as well as an increased risk of dementia, particularly among the elderly. To enhance the role of social welfare workers in providing psychological support, dialog robots have recently begun to attract attention. In particular, numerous studies have examined robots that interact with people in daily life [2, 3]. However, robots have not yet been designed to interact with people as conversation partners over a long periods. One issue that arises in human-robot conversations is the robot's inability to elicit affective empathy [4, 5] from humans. To address this problem, improvement in human-like countenance in robots should be based on the strategy of imitating human-human communication [6, 7, 8, 9]. Here, we focus on vocal synchrony as one feature of such communication, which is defined as the dynamic and reciprocal adaptation of the temporal structure of behaviors between interactive partners [10]. Vocal synchrony is categorized as a nonverbal type of synchrony. Ample evidence in human psychological studies has shown that various types of behavioral synchrony (e.g., gesture mimicking) underlies human empathic interactions [11, 12]. Studies have shown that behavioral synchrony heightens empathic feelings including harmoniousrelationships [13] and partner likeability [14] as well as empathic behaviors such as helping [15]. Several researchers have developed behaviorally synchronized robots and have proposed that such synchrony can produce empathic impressions in human users and might produce empathic understanding in robots [16; 17; 18; 19; 20; 21; 22]. Following this concept, several previous studies on robotics have reported that the effects of behavior synchrony (e.g., physical gestures) in robots can facilitate empathic reactions in human users [23; 24]. Previous psychological studies have reported that vocal synchrony is associated with empathic understanding in humans [25; 26; 27] and that the intentional use of vocal synchrony improves the speaker's affective empathy [25; 28; 29]. Vocal synchrony in robots is expected to have similar effects [30; 31], although no research has verified such results. We hypothesize that robots with the ability to vocally synchronize with humans during conversation would boost empathic feelings in humans compared with robots not programmed with such an ability. To test this hypothesis, we developed in this work a robot that can synchronize with the user's voice in terms of vocal prosody, particularly the speech rate, pitch, and volume. To verify the effect of the robot's vocal synchrony, we conducted an experiment in which the robot and human participants conducted predefined conversations with vocal synchrony. The conversations were prepared to induce positive impressions in readers based on preliminary experiments. As the control condition, the robot was desynchronized from the prosody of participants such that the robot uttered no synchronized voice with comparable prosodic variance. We evaluated the subjective and objective responses regarding affective empathy during the conversation. Specifically, we assessed seven subjective responses related to affective empathy (e.g., emotional connection) using a questionnaire. To complement the subjective rating, which could result in bias such as demand characteristics [32; 33], we measured the physiological responses including facial electromyography (EMG) from the corrugator supercili (i.e., brow lowering) and zygomatic major (i.e., lip corner pulling) muscles as well as the skin conductance level (SCL).