1 Introduction
Over the years, a plethora of computer mediated communication (CMC) devices and applications has been developed to address humans’ innate need for social connectedness and intimacy [
8]. However, CMC is often criticized for impoverishing and devaluing human communication [
91], and may even have contributed to the prevalence of individualism and feelings of loneliness [
15]. That is to say, brief textual or audio-visual messages supposedly cannot match the intimacy as experienced during face-to-face contact. To achieve more intimate CMC, interfaces that modulate one’s emotions via the sense of touch [
32] could be particularly suitable. These
‘Affective Haptic Interfaces’ [
90] could be utilized in a non-social way, by merely drawing upon their physical qualities. Different materials and textures (e.g., [
35,
81,
84,
96,
106]), or haptic actuators such as vibrotactile [
79,
80], air jet based [
89], friction-based [
77], or force feedback [
5] can be employed to elicit affective responses in the receiver. Another approach to use affective haptics is to directly attribute the physical stimuli to a
social source, as is for instance the case with touching social robots [
88,
101] or in
Mediated Social Touch [
26,
36,
40]. The latter approach may alter the perceptions of the physical stimuli and the associated affective responses due to the social connotations the message entails. To better understand how affective haptic CMC that is aimed at increasing social connectedness and intimacy should be designed, it is important to discern between the non-social and social qualities of physical stimuli.
Using non-noxious physical warmth is often proposed for affective CMC [
9,
33,
47,
87,
98], as it provides a pleasant, comfortable feeling, is semantically associated with positive emotional valence [
9], and activates brain regions associated with positive emotions [
82,
104]. Moreover, perceptions of physical warmth can activate perceptions of
‘social warmth’ (i.e., the experience of being connected to another) [
47,
102]: a
physical-social warmth link. In practice this means that holding warm objects, or residing in relatively warm environments, may activate pro-social behaviors and cognitions [
44] and decrease feelings of loneliness [
6]. While warmth may elicit beneficial responses in every user, it may be particularly suitable for people that suffer from loneliness, stress, or psychological pathologies [
6,
72]. In this work, we set out to test the hypothesis that the physical-social warmth link is also applicable in CMC, and that physical warmth thus can increase feelings of connectedness and comfort between remotely located interactants. Moreover, we investigate whether attributing different meanings to the warmth results in different affective responses from the receiver. That is, the warmth is described to the participant either as a mere physical, non-social quality of the interface, or as (social) warmth that represents the sender’s body heat.
In the remainder of this paper we will substantiate our hypotheses by discussing why physical warmth actually may be valuable for social connectedness and comfort in CMC. Moreover, we describe related work, and elaborate on the role of attribution in CMC, after which we describe and discuss two studies. With the work as presented here we intend to further inform the design of haptic CMC interfaces that can effectively provide feelings of social connectedness and comfort.
1.1 The link between physical and social warmth
The physical-social warmth link supposedly stems from our period as newborns, during which we are often held closely by our parents and perceive their bodily warmth [
56,
103], and during which the foundations of psychological mechanisms such as affect and social bonding are formed [
11,
44]. Moreover, the perceptions of physical and social warmth seem to share neurobiological mechanisms, which appear to be unrelated to the hedonic qualities of warmth [
48]. Both the perception of physical and social warmth can activate the same areas in the insular cortex (responsible for social cognitions and emotions) [
19,
47], and neurochemical substrates [
48].
Perceiving physical warmth thus seems to imply physical closeness to a source of affiliation and therefore a sense of social connectedness. This suggestion was supported by a study in which people perceived the physical distance to a warm drink as smaller than to a cold drink, after holding it [
28]. Related studies demonstrated that people, when residing in a warmer environment or when being primed with a warm object, use more language that is associated with feeling socially connected to others. Moreover, people focus more on relational characteristics of abstract figures rather than on their individual attributes, and they feel more social proximity towards another [
45,
78]. Also, the desire to bond was higher when primed with physical warmth [
28]. This increased social connectedness resulted for instance in increased altruistic behavior such as sharing small gifts [
44,
102], whereas coldness made people invest less with an anonymous partner and decreased trust [
53]. Moreover, holding a warm drink, contrary to a cold one, led people to perceive others as socially warmer [
102]. They attributed more
‘socially good’ personality traits such as friendliness [
4,
31] to an ambiguously described other person, whereas traits that are unrelated to this socially warm-cold dimension remained unaffected by the warmth.
The physical-social warmth link appears to be bidirectional. When people think about relationships and communal feelings [
83], or when they are physically close to others [
46], they tend to perceive the ambient temperature as higher. When people experience social stress such as exclusion or guilt, they perceive the temperatures as lower [
6,
108] and they actually become physically colder as their skin temperature decreases [
41,
49]. Socially stressed people show an increased need for compensatory or restorative physical warmth. They for instance have more warm drinks or take more warm baths, and this warmth supposedly reduces the stress [
6]. Since warmth may be beneficial for social connectedness, physical comfort,
and psychological comfort, it seems suitable for affective CMC.
The exact underlying mechanisms of the physical-social warmth link are not fully understood though, as the research is still in its infancy. Moreover, the research may suffer from publication bias or even false positives, for instance due to underpowered studies [
43]. This is reflected in failed attempts, with significantly larger sample sizes, to replicate the effects of warmth on perceived personality [
63], the relation between communal feelings and the ambient temperature [
25], and the need for compensatory warmth during social stress [
24]. Since the majority of the effects of warmth has been found in direct contrast with coldness, and because coldness decreased altruistic behavior [
53], it could be that physical coldness has a negative impact on people, rather than warmth having a positive effect [
6,
99]. Moreover, rather than being a specific response to social stress, the decrease in skin temperature may also be a common sympathetic response to negative, stressful events [
54,
75], which is reflected by vasoconstriction in for instance the fingertips or the tip of the nose. The inconclusiveness regarding earlier research on the physical-social warmth link and its underlying mechanisms warrants an empirical validation of the suggested beneficial effects concerning social connectedness and comfort of warmth in CMC [
9,
33,
47,
72,
87,
98].
On the premise that social touch is one of the most intimate forms of face-to-face communication and is essential for wellbeing, social bonding, and behavior [
29,
30,
32,
66], affective haptics have been employed to simulate social touches with the intention to induce similar beneficial effects [
26,
36,
40]. Since body warmth is an important physical quality of a human touch [
1,
38], it has also been included in several mediated touch devices. Examples include artificial representations of hugging (e.g., [
22,
86]) and holding hands [
34], and a more abstract interaction in which squeezing a “White Stone” results in the heating of another augmented stone [
87]. Although such devices supposedly can enhance the social connectedness while being separated, and may be particularly suitable for the conveyance of affective messages [
60,
61], empirical evidence for this claim is sparse and inconclusive. Perceiving a warm “thermal hug” around the waist, that was sent by the partner during a mediated collaborative task, increased social presence [
33]. Also, physical interactions with warm non-human social entities, robots with ‘body heat’, resulted in a more positive attitude towards these robots and a stronger sense of trust and friendship [
67,
69]. With regard to one’s emotional state, research has demonstrated that stimulation with different thermal patterns elicits various emotional responses [
76]. When haptic patterns consisting of warmth, vibration, and a tickle sensation were attributed to another person and presented while watching an emotional movie, these mediated touches reduced physiological stress responses with a similar rate as real human touch [
13]. On the contrary, warmth did not affect the social perceptions of a fictitious peer after mediated interaction [
99]. Applying the physical-social warmth link as means of affective CMC thus is not trivial. Moreover, it remains unclear whether the effects found were mainly caused by the attribution of the warm message to “the sender” and the associated social meaning, or merely by the physical qualities of the interface. In the former explanation, the warm messages could be considered a form of disclosing affect and personal emotional information; mechanisms that are known to contribute to intimacy during communication [
16,
58]. In the latter case, the effects of the warm messages would mainly draw upon the inherent physical comfort and/or the physical-social warmth link.
In the work as presented here, we intend to shed additional light on the roles that these aspects may have in affective haptic CMC, by actively manipulating both the physical temperature and the source, and thus the social meaning, of the warmth. This is done in two studies in which participants were invited to communicate with another person via an affective haptic communication interface, and to evaluate this haptic message afterwards. The interface contained heat-pads that were either switched off (i.e., the “Room Temperature” condition), or switched on. When the interface was warm, we manipulated the attribution of the warmth to either a non-social or social source. The warmth was either presented as a physical attribute of the interface that was switched on beforehand, and thus did not have social meaning (the “Artificial Heat” condition), or as a real-time display of the communication partner’s “Body Heat”, with inherent social meaning. During these two three-conditions between-subjects experiments, we set out to find support for the following hypotheses:
-
H1: Direct physical contact with warmth provided by a haptic communication medium will, contrary to neutral temperature, result in increased social connectedness, socially warmer perceptions of the other, and comfort.
-
H2: The beneficial effects of physical warmth are larger when the warmth is attributed to the other person (as mediated body heat), rather than to a non-social inanimate attribute of the haptic communication device.
With this work, we intend to further inform the design of affective haptic CMC interfaces by investigating the applicability of the physical-social warmth link. Moreover, we intend to further inform the approach to take in the design of effective affective haptic interfaces; for instance with regard to increasing social connectedness. That is, this work could show whether an approach that mainly relies on the perception of the
physical qualities of the interface may already suffice in sorting beneficial psychological and physiological effects, or that more elaborate processing, such as interpreting the actual
meaning or
intentions of the haptic signal, is required.
4 Meta analyses and post-hoc power tests
Although none of the between-subjects effects reached statistical significance, the majority of the scores did emerge in the hypothesized direction. As one of the criticisms regarding the supposed effects of warmth concerns the power of the studies [
43], we decided to conduct random-effect model meta-analyses on the recurring measures of Study 1 and 2 (i.e., IOS, Warm/Cold Personality Traits, and Arousal and Comfort (TPT)), in order to increase the power. The effects of warmth (i.e., Room Temperature vs warm conditions combined;
\(\textit{n} = 120\)) and attribution (i.e., Artificial Heat vs Body Heat;
\(\textit{n} = 81\)) were investigated. The results of the meta-analyses corroborate the findings of the individual studies, as no significant effects were found for any of the measures (see Table
5).
Moreover, post-hoc power analyses were carried out to provide a better understanding of the practical significance of the effects. The effect sizes of all statistical tests of Study 1 and 2, computed with G*Power [
27], can be considered
small at best, according to the conventions by Cohen [
18]. Given the small effect sizes, the power (
\(1-\upbeta \)) to actually detect effects varied between 0.08 and 0.34, with the exception of the Communication Experience test in Study 2, of which the effect size was
medium and the power 0.88. With the power set at the recommended 0.80 [
18], the required sample sizes for the tests would have to increase to at least
\(\textit{n} = 123\), and even to
\(\textit{n} = 1188\) for the least powered test, for the group differences to reach statistical significance. This means that even if the effect would reach significance for large samples, its practical relevance is still limited.
5 General discussion
With two studies and meta-analyses, we set out to find empirical support for the suggestion that interaction via a physically warm medium may increase comfort and pro-social cognitions about one’s communication partner, as compared to interaction via an unheated medium [
9,
33,
47,
72,
87]. We intended to verify whether the physical-social warmth link is also applicable in CMC. To further inform the design of haptic communication devices, we also discerned between warmth that is attributed to non-social, inanimate technology, and warmth that has inherent social connotations [i.e., the body heat of a stranger (Study 1) or one’s own partner (Study 2)]. Although the participants thought that a device that mediates body temperature provided more opportunities for emotional expression, this was not reflected in other effects of physical warmth. Despite the longer exposure to the warmth, a richer situational context, and the consideration of individual personality characteristics in Study 2, neither the temperature nor the attribution manipulation did alter perceptions of closeness to the interaction partner or provided additional emotional or physiological comfort during stressful circumstances. Our results are in line with earlier failed attempts to find support for the existence of the physical-social warmth link [
24,
25,
63,
99] and the supposed modulating role of attachment style [
59,
99], but contradict a growing amount of studies that suggest the existence of this link [
43]. Here, we discuss our methodology and findings and relate these to earlier research on the physical-social warmth link. We will argue that contrary to what is often suggested, this link may not necessarily be suitable for affective CMC.
The absence of the hypothesized effects of physical warmth may be explained by the medium that was used. First of all, Nakama may have been perceived as childish which may have reduced the overall impressions of the messages. Moreover, due to its design, the effects of warmth on comfort and social connectedness could not be investigated in isolation. That is, holding a soft object during communication may in itself already alleviate stress responses. A decrease in cortisol level (i.e., the “stress-hormone”) was for instance found while hugging a pillow during a phone call [
81,
106], and interactions with soft robotic plush animals increased the “love-hormone” oxytocin [
84] and had beneficial effects on coping with stress on the longer term as well [
96]. There is no added value of warmth, over simply holding a bear at room temperature, with regard to stress reduction. It is however unlikely that the childishness and potential physical comfort provided by Nakama also account for the absence of physical-social warmth effects. One may of course suggest that (a combination of) the mediated touches, heartbeats, and perhaps the other channels have occluded the effects of physical warmth on social connectedness, as these signals may have increased the social connectedness by themselves [
26,
36,
40,
50]. This suggested ceiling effect seems however unlikely considering the ‘mildly warm’ perceptions of the interaction partner (according to the scores in both studies); the influence of the other communication channels seems negligible. We can conclude that
if warmth has positive effects on comfort and/or social warmth, it does not prevail over the other physical qualities and communication channels of Nakama. An additional caveat that has to be placed is the fact that sensitivity to thermal changes differs from person to person (e.g., [
51,
52], see also Sect.
2.1.2). While it is unlikely that these interpersonal differences affected the outcomes of our studies (as earlier research on the physical-social warmth link did also present generic thermal stimuli), future research could benefit from a paradigm in which thermal changes are applied in reference to the participant’s current body temperature (e.g., [
76]).
After Study 1, stricter boundary conditions within which physical warmth in CMC supposedly could induce effects were proposed. In line with earlier research (e.g., [
28,
99,
102]) the results of our studies suggest that the duration of the exposure to the warmth is not an essential boundary condition. It may however be the case that the
contrast between the warmth and room temperature was not large enough to detect effects. That is, the majority of studies in favor of the physical-social warmth link discern between warm and
cold conditions (e.g., [
28,
44,
102]), and coldness supposedly has a negative impact on pro-social cognitions, rather than warmth having a positive impact [
6,
53,
99]. People may have a default orientation towards others that can be described as ‘mildly warm’ [
6], and likely a warmer attitude towards their own partners; a claim that can also be supported by the scores in both studies. Additional physical warmth may not change this default as much as coldness can [
6]; in particular because avoidance motivations towards the social world (which in this case would be represented by coldness) are usually stronger than approach motivations (or warmth) [
6,
14]. This would not necessarily mean that warmth has no effects, but that the added value of it can mainly be found in its restorative or compensatory effects in cases of social stress, such as feelings of loneliness, guilt, or exclusion, or with psychological pathologies [
6,
39,
43,
55,
72,
93]. Despite the introduction of a clear context for the haptic messages in Study 2, namely comforting the partner in a situation of stress, no
social stress was induced. Moreover, the participants knew that their partner was literally close by and directly available after the scary movie. Since we did not actively induce social stress, or at least control for it, one may say that our methodology was not suited to actually detect the supposed effects of warmth in CMC. This argument does not seem sound however, as the majority of studies that report positive effects of physical warmth do not include a social stress context either (e.g., [
28,
33,
45,
78,
102]). If the physical-social warmth link actually would be applicable in CMC, we would at least have found some indications that warmth positively influenced social cognitions.
Although there is a growing body of evidence in support of the physical-social warmth link, neither the underlying mechanisms nor the boundary conditions are fully understood yet; the research is still in its infancy [
42,
43]. Moreover, some of the studies may have had a participant sample that was too small to actually support the tested hypotheses, which could have resulted in possible publication bias or even false positives [
43]. Although the sample sizes in our studies and meta-analyses (
\(\textit{n} = 65\), 62, 120, and 81, respectively) can be considered substantial, they appeared, according to our post-hoc power analyses, too small to actually demonstrate effects of warmth and the attribution thereof. This may simply be because the effects, if these actually exist, do not seem to be as outspoken as suggested. Moreover, as mentioned, there have been several attempts, often with significantly larger sample sizes, to exactly replicate results (e.g., [
24,
25,
59,
63] or to find similar effects (e.g., [
99]) that were unsuccessful. Abovementioned considerations raise the question whether the supposed effect actually exists, and if so, to what extent these significant findings are of actual practical use. Our results suggest, contrary to earlier research [
9,
33,
47,
72,
87], that the effects on comfort and social warmth, if any, are not substantial enough to be of practical significance during CMC. This is however not to say that we should disregard warmth as a communication medium altogether. Valuable insights in the conveyance of emotions through different thermal patterns have been gained in earlier research (e.g., [
60,
61,
76]). However, the elicitation of beneficial responses in CMC through the use of warmth should be based on other principles (e.g., abstract thermal messages) than on the physical-social warmth link.
Although the majority of the results suggest that physical warmth does not induce outspoken social responses in CMC, participants did indicate that mediated body heat increased the perceived opportunity for emotional expression. These findings may seem contradicting. It could however be the case that the participants considered the option to switch the non-social, inanimate temperature on or off merely as an attribute of the device, whereas the conveyance of body heat may have been considered an actual social communication channel. Having this additional communication channel could have increased the perceived
‘richness’ of the device —i.e., the extent to which a medium facilitates the conveyance of immediate feedback and contextual information [
20]—and may therefore have provided more options for emotional expression. Even though it is not reflected in the actual social cognitions, this finding suggests that in order to advance the understanding of the design of affective haptic communication devices, it is indeed valuable to discern between responses that are elicited by the physical qualities of haptic interfaces and those informed by the social meaning of the signals.
6 Conclusions
Based on the results from our studies and meta-analyses, which could not confirm any of the hypotheses, as well as on the doubts and criticism regarding the suggested physical-social warmth link, we conclude that physical warmth does not have clear beneficial effects with regard to feelings of social connectedness, social warmth, or comfort in a setting of CMC. The effects of the physical-social warmth link, if any, seem too small to be of actual practical use. Warmth does, according to the physical-social warmth link, not induce significantly stronger effects than alternative affective haptic communication channels such as mediated touch or heartbeats. If the supposed beneficial effects of the physical-social warmth link actually exist, they may only appear in CMC under very specific circumstances that go beyond the ones that were considered in our studies. In order to advance the development of effective affective haptic interfaces, it is not only important to understand the opportunities that these interfaces offer, but also their limitations. From the latter perspective, the work as presented here thus provides valuable knowledge despite the unsupported hypotheses. Another valuable insight provided by this work is the suggestion that research on affective haptic interfaces in general, not only interfaces that include warmth, should discern between the inherent hedonic qualities of haptic actuators, and stimulation with haptics that have social meaning such as social touch technology. A thorough understanding of the capabilities of different (combinations of) haptic actuators and attributions, and of how these stimuli are cognitively processed, may inform the design of meaningful haptic CMC devices.
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