The current study sought to examine the role of touch and pain in inter-partner heart rate and respiration coupling.

The statistical analysis was based on the CLO model20, which allows estimating interpersonal bi-directional coupling between one partner’s signal exchange and a shift in the signal exchange in the interacting partner. Our findings confirm that interpersonal touch as compared to no-touch is associated with increased respiration coupling, during both pain and no-pain conditions. In line with these findings, interpersonal touch has been reported to increase coupling of electrodermal activity between partners35. Moreover, researchers have shown that touch can communicate emotions in a way that the receiver is able to recognize the emotional states communicated by the toucher40,41,42. Thus, in the current study the partners may have communicated their emotions via touch, as evidenced by an increase in physiological coupling.

The finding of an increased pattern of heart rate coupling during the pain and touch condition indicates that touch may allow communication between the participants but only during pain. This indicates that coupling in heart rate is evident only when the target experience pain and empathy in the observer is possibly provoked. Indeed, the powerful effect of social touch has been shown to affect our emotional well-being and diminish distress or pain in various settings28, 37, 43,44,45,46,47,48. For example, it has been reported that skin-to-skin touch may have an analgesic effect on human babies undergoing minor medical procedures45 and may even therapeutically reduce pain in cancer patients44, 49 and those with chronic pain50. Previously, Coan et al. (2006) reported that greater pain reduction is observed following the touch of a partner as compared to a stranger37. Thus, it could be suggested that inter-personal physiological coupling underlie touch-related analgesia.

In contrast to our original hypothesis, pain did not increase physiological coupling in the absence of the partner’s touch. This effect was consistent across both physiological signals and for both model directions (female-male, male-female). The decrease in coupling when the women experienced pain and their partners observed their pain partially contradicts the findings of Konvalinka et al.17. This study reports that during a fire-walking ritual, the cardiac rhythms of active participants were synchronized with those of related bystanders. However, one should take into account major differences between this study and the current study. While the sample studied by Konvalinka et al., 2011 consisted of experienced fire-walkers who were well trained to manage their pain and did not feel the pain51, our participants, who represented a normal population, did not have such experience. As a result they probably experienced higher levels of pain and distress during the pain condition, which may disrupt the physiological coupling. In addition, a possible explanation for these contradictory findings may rely on the different definitions of synchronization used in the two studies. Konvalinka (2011) defines synchronization as a similarity between the heart rate of a firewalker and that of a related spectator, while we defined interpersonal coupling as “one partner’s signal changing from increasing to decreasing while the other partner’s signal decreases”.

Here we show that in the absence of touch, the experience of pain disrupted the physiological coupling as the targets probably were focusing on their own pain experience. Indeed, previous research showed that pain interrupts the attention, that the suffering person pays to the external world52. It is thus possible that the women may have focused on her own pain and engaged in self-based strategies to cope with pain53, 54, which may explain their physiological “disconnection” from the male partners. In line with this idea, it was shown that social touch increases our attention to social stimuli55. However, self-based strategies seem to be less effective than those involving physical touch between partners, as we and others found that touch is associated with a greater analgesic effect than no-touch36, 38.

Indeed, the moderation analysis provides additional evidence for the notion that touch-related analgesia is related to the empathy of the toucher. The results show that both empathic accuracy and trait empathy moderated inter-partner physiological coupling, so that dyads with highly empathic male partners demonstrated increased coupling. That is, we observed a shift from exhalation to inhalation in a participant when their partner inhales (the heart rate shifts from a decrease to an increase when the partner exhibited an increase in heart rate).Consistent with this, researchers have shown that greater empathy is associated with better physiological linkage between romantic partners56 and greater touch-related coupling of electrodermal activity35. Moreover, in the couples where the female partners reported greater touch-related analgesia, we observed enhanced physiological coupling. Thus, the effects of touch on cardio-respiratory inter-partner coupling may be associated with pain analgesia. It is thus possible that cardio-respiratory changes, where the observer of pain affect’s the target of pain, is associated with the level of empathy of the observer. That is, the empathic response of the observer is communicated to the target of pain. This idea is in line with Hertestein et al. (2006) who demonstrated that people can identify various emotions including love and sympathy from the experience of being touched41.

Similarly, it is possible that the target of pain communicates back the analgesic effect of touch to the observer. Thus, the use of touch may improve the quality of non-verbal physiological communication between partners, especially when one of them feels pain, enabling the toucher to better project his empathy to the female partner and consequently have an analgesic effect.

It is important to note that the CLO model enables the identification of associations in both directions (i.e. the male partner signal change predicts the shift in signal change of the female partner and vice versa). However, in this study, both directions showed the same pattern of results supporting the idea of physiological bi-directional communication. These findings of physiological coupling may also explain recent findings of partners being able to influence women during labor. For example, it was reported that massage and breathing coaching from partners can decrease negative affect, as expressed by a depressed mood, anxiety, and pain as well as enhance positive affect, shorten labors and hospital stays, and decrease postpartum depression57.

Apparently, skin to skin touch is important for pain reduction, which may explain people’s preference for social touch58. Moreover, touch activates reward circuits in the brain59, 60. Indeed, skin-to-skin touch has been shown to activate the reward system, which results in pain reduction both in animals and in humans47. It seems that this phenomenon has evolutionary roots. For example, non-human primates devote much more time to grooming than they actually need for hygiene reasons, resulting in endogenous opioid release61, 62, as well as pain and stress reduction63, 64.

It is still not clear exactly how inter-personal physiological resonance is related to touch-related analgesia. Observing the pain of others can trigger emotional resonance in the observer, activating brain mechanisms similar to those of the suffering person (e.g., anterior cingulate and insula cortices) and areas that are classified as the “mirror neuron system” (e.g., inferior parietal cortex)23. As the parietal lobe integrates sensory information among various modalities, the assumption is that multisensory integration of both visual and tactile stimuli may facilitate the emotion resonance with the observed target, as also expressed in associated autonomic and somatic responses65. Moreover, tactile-induced analgesia66 correlates with activations in brain areas related to multimodal neural activity67 and emotional processes68,69,70,71,72. The partner’s touch may also enhance inter-partner brain synchronization in areas related to the pain matrix, a hypothesis that should be tested in future research using novel hyperscanning techniques73. In addition, the anterior cingulate cortex (ACC) has also been found to be relevant in the context of pain perception, empathy for pain22, touch60 and reward systems74. The ACC appears to play a role in a variety of autonomic functions, such as regulating heart rate and blood pressure75, 76. This complicated physiological mechanism may underlie the observed coupling between the partners.

Recent research shows that the neuropeptide oxytocin may play a key role in synchrony77 as well as in touch analgesia via physiological coupling. It was demonstrated that warm touch can increase the levels of plasma oxytocin, and reduce stress78 and depression79. Moreover, recent studies have highlighted the role of oxytocin in inter-personal coupling, increasing touch interaction synchronization while reducing stress80, 81 and enhancing brain-to brain coupling in alpha rhythms82. Thus, future research should test the role of oxytocin in touch-related analgesia.

It is important to note that our findings of interpersonal coupling during touch can also be explained by the phenomenon of Huygens synchronization of two connected pendulum clocks83. According to this explanation, the mere connection between objects creates synchrony between these objects. Nevertheless, the moderation effects of empathic accuracy and pain analgesia reduce the probability of this explanation. Interestingly, the difference in pain between the conditions with and without touch was higher in females than in males, who estimated their partners’ pain. These findings can be explained by the fact that female participants experienced the real pain and their male partners only guessed the females’ pain level. It is thus possible that males did not realize the magnitude of the effectiveness of their touch and therefore their pain ratings did not decrease in the touch condition.

Although the current study used a controlled design with several balanced conditions, it has several limitations that need to be acknowledged. First, only the female participants underwent pain stimuli while the male participants did not, so that the generalizability of our results is restricted to the male-to-female direction. Therefore, future research should test the effect of touch and pain in both men and women as well as in homosexual and heterosexual participants. This could also include parent-child, sibling, and best-friend interactions, in comparison to the interactions between strangers. Second, this study used a single subjectively adjusted degree of heat pain. Future research should test physiological coupling on varying pain intensities and also cold pain stimuli. Third, the subjects were asked for a static handholding, without squeezing, stroking or rubbing following the paradigm proposed by Goldstein et al. (2016). However, comparing between these different types of touch is of high interest and would benefit from future investigation. Fourth, partners could use visual information in all interacting conditions. The visual and tactile sources of information may interact in the brain and therefore future research should estimate these effects. Finally, reporting pain for both emotional and intensity components may shed more light on our findings.

To conclude, we show here that touch regulates physiological coupling during pain, suggesting that interpersonal coupling is affected by various contextual social cues. Yet the prevalent approach in testing perception and behavior is to split one complex system (e.g., the delivery of pain) into several subparts and to explore each of them independently. Although this simplified approach allows analyses of human responses, it lacks the sensitivity to capture elements involved in real-life social interactions. Considering that human behavior is fundamentally different when we are interacting with others rather than merely observing ourselves, here we investigate physiological response using a paradigm that also consider social contexts. Since physiological resonance has important evolutionary significance for animals and humans84,85,86, investigation of inter-personal coupling provides an interesting opportunity to understand our behavior in the natural social environment.