1. Introduction
Robots can be used to perform a series of complex actions [
1]. A service robot performs service tasks for humans or devices [
2]. It is an autonomous robot capable of interacting with people and completing specific service tasks [
1]. The development of artificial-intelligence technology has popularized service robots, such as educational robots, therapeutic robots, and entertainment robots [
3]. However, human acceptance of service robots is the main obstacle to popularizing service robots [
4]. Since service robots have certain social attributes [
5] and human-like characteristics that can encourage humans to treat service robots as social participants, human-like characteristics can influence the service effectiveness of robots [
5,
6]. The human-like characteristics of robots can effectively influence human attitudes toward robots [
6]. Human acceptance of the human-like characteristics of robots promotes human acceptance of service robots [
7], whereas human non-acceptance of the human-like characteristics of robots inhibits human acceptance of service robots [
6].
However, scholars have different views on human acceptance of human-like robots. Some scholars believe that humans have positive emotions toward human-like robots and are more willing to deal with a robot that has more human-like features [
7,
8,
9], while some other scholars believe that more human-like robots can cause fear and anxiety in people, decreasing their willingness to interact with the robot [
6,
10]. This paper focused on how the human likeness of a service robot affects human acceptance of it.
Human likeness refers to the degree to which a robot looks and behaves like a human [
11]. Human likeness includes two categories: human-like appearance and human-like behavior [
12,
13]. Appearance describes the static aspects of the robot (look, sound, sense of touch, etc.) [
14,
15,
16], while behavior describes the dynamic aspects of the robot (actions, expressions, emotions, etc.) [
11,
12]. To enhance the human likeness of the service robot, the designer would endow the service robot with more human characteristics. For example, the designer would make a robot’s face look like a human’s or add more human characteristics to its actions [
13]. A few previous empirical studies have explored the human-like behavior of service robots (HLBR) [
8]. However, this factor also has an important effect on human–robot interaction [
8]. Therefore, this paper focused on the effects of HLBR on human acceptance of a service robot.
In previous studies, scholars used two types of constructs to measure human acceptance of a service robot: (i) psychological constructs, such as trust [
16,
17,
18], likes [
11], use intention [
19], and satisfaction [
20]; and (ii) sociological constructs, such as social distance [
21]. Most studies have employed psychological constructs, while few have employed sociological constructs. However, it is important to examine the human acceptance of a service robot from a sociological perspective. The previous literature has shown that the social rules in people-to-people interactions apply to human–robot interaction [
22], and robots can be viewed as social actors with specific behavioral patterns [
23]. This paper focused on the sociological aspect of human beings’ acceptance of service robots (i.e., social distance). Social distance refers to the closeness of the relationship between the two individuals in people-to-people interactions [
24]. The social distance between humans and service robots (SDHR) can measure the closeness of the relationship between humans and service robots [
25]. Thus, SDHR can indicate human acceptance of a robot [
21].
This paper examined how HLBR affects SDHR. This paper has two contributions. First, this paper extends the outcomes of HLBR. Concerning the effect of HLBR on human acceptance of a robot, previous studies examined only the attitudes [
8] and likes [
26] but not any other outcomes. This paper examined the effect of HLBR on SDHR. Second, this paper introduced perceived competence and perceived warmth as mediators. The mediating effect of HLBR and SDHR has not been studied in the previous literature. This paper employed the social-identity theory to explain the mediating effect of HLBR and SDHR.
In addition, previous studies have shown that cultural background can affect human responses to robots [
27,
28,
29]. In the US and China, robots are widely used in various fields, including the service industry [
30], such as Sony’s entertainment robot AIBO and Takara’s home-care robot TERA [
27]. However, the two countries differ in their views on robots. Americans regard robots as assistants, while Chinese tend to regard robots as friends [
31]. It is generally believed that the US is an individualist country, and China is a collectivist country [
32,
33]. Compared to individualism, interpersonal relationships are more intimate in the context of collectivism [
34]. Social rules in interpersonal communication can also apply to human–robot interaction [
22]. A cross-cultural study of human–robot interaction found that Chinese people have a higher sense of intimacy with robots than Americans [
27]. Therefore, we validated our theoretical model with participants from two different cultures (China and the US).
5. Discussion
Our mediation-effect model was supported in both groups (the Chinese and American participants), indicating that the mediating effects of perceived competence and perceived warmth are applicable across cultures (China and the US), to a certain extent. However, the two groups differed in some aspects, stemming from cultural differences.
First, the Chinese participants’ score on anthropomorphism was higher than that of the US participants. Cross-cultural studies have found that compared to Americans, Chinese people more strongly advocate animism, so Chinese people are more inclined to anthropomorphize robots [
31]. Therefore, when faced with the same HLBR, the Chinese people more strongly anthropomorphized the service robot in the experiment.
Second, the Chinese participants’ score on perceived warmth was higher than that of the American participants. Since the external characteristics of service robots (such as small size and slow movement speed) are more in line with Eastern cultural preferences [
27], people from Eastern cultural backgrounds might have a more positive attitude toward human-like robots and a higher evaluation of the robot’s cuteness and friendliness [
27,
63]. These characteristics are linked to the human perception of the warmth of the human-like robot [
45]. As a result, the Chinese people had a stronger perception of the warmth of the human-like robot.
Finally, the Chinese participants’ score on SDHR was lower than that of the American participants. Previous studies have shown that Americans are high on individualism, while Chinese are high on collectivism [
32,
33,
34]. Compared to individualists, collectivists have a more pronounced in-group preference and maintain a smaller social distance from in-group members [
39]. Since the Chinese people tend to view robots as in-group members [
64], they keep a smaller social distance from human-like service robots.
5.1. Theoretical Contributions
First, this paper extends the study on the outcomes of HLBR. Few previous empirical studies have examined HLBR, and the existing literature has mainly discussed HLBR related to customer attitude [
8] and user preference [
29]. This paper linked HLBR with SDHR and examined the outcome of HLBR from a sociological perspective.
Second, this paper extended the study on the antecedents of SDHR. Social distance is an important indicator of human acceptance of robots [
21], but only a few empirical studies have explored the antecedents of social distance. The existing literature has studied only the effect of the robot’s language form on social distance [
25]. This paper extended the antecedents of social distance to HLBR (including the conversation contents, voice intonation, and actions of the robots when they interact with humans) to investigate the effect of the robot’s behavioral characteristics on SDHR.
Third, this paper demonstrated that perceived competence and warmth mediate the relationship between HLBR and its outcome. Previous studies on the robot’s human likeness and human acceptance of the robot have found that mediators include social-interaction needs [
43] and the sense of social presence [
65]. The mediators (i.e., perceived competence and perceived warmth) identified in this paper are fundamental dimensions of social cognition [
45], and they help us understand the mechanisms by which HLBR produces outcomes, from the perspective of human perception.
Fourth, this cross-cultural study tested the model’s universality, with participants from China and the US. This paper also found differences between the Chinese and American participants in their perception of service robots. These results complement cross-cultural studies on human attitudes toward robots [
66,
67].
5.2. Practical Implications
First, it is necessary to consider human-like behavior when designing service robots. Robots’ expressions, attitudes, and actions can enhance human acceptance of service robots. This paper responded to the debate on the necessity of the human-like design of robots [
7,
9,
10,
19]. The findings of our study suggest that investing resources in the design of HLBR is beneficial.
Second, designing the HLBR is conducive to humans’ acceptance of service robots. The findings of our study showed that the design of HLBR can enhance human in-group identification with service robots (represented by a smaller social distance), making service robots not only accepted by humans but also better integrated with human groups in a sociological sense [
25,
68].
Finally, the detailed design of HLBR, such as the conversation contents, specific voice intonation, and specific actions, should aim to promote individuals’ positive perceptions of the competence and warmth of service robots as the mediators of the relationship between a robot’s human-like behavior and service satisfaction [
43,
49,
50].
5.3. Limitations and Future Research
First, the model in this paper did not include human characteristics. However, the existing studies showed that human characteristics are also important factors affecting SDHR. Future research may incorporate human characteristics into the model. Second, this paper used video as the stimuli, rather than stimuli generated by real contact between humans and robots, which may have led to different results than would be expected. Future research may conduct field studies to examine real human—robot contact. Third, the Chinese and American participants showed some differences in the perception of service robots, for multiple reasons that we discussed in the previous section. Future research may conduct a more nuanced cross-cultural analysis. Fourth, we selected participants based on their country of origin; therefore, selection bias may have affected the experimental results. Future research could improve the selection method.