We tested the hypothesis that social laughter elevates pain thresholds both in the laboratory and under naturalistic conditions. In both cases, the results confirmed that when laughter is elicited, pain thresholds are significantly increased, whereas when subjects watched something that does not naturally elicit laughter, pain thresholds do not change (and are often lower). These results can best be explained by the action of endorphins released by laughter.
An important distinction is drawn between Duchenne laughter (relaxed, unforced laughter that is stimulus-driven and emotionally valent, involving involuntary contraction of the orbicularis oculi
muscles) and non-Duchenne laughter (context-driven and emotionless, with no orbicularis oculi
]. Neuroimaging evidence suggests that these two types of laughter involve different neural pathways [42
]. The involuntary nature of Duchenne laughter is largely responsible for the well-known contagion effect whereby we are stimulated to laugh just by others laughing. Precisely because Duchenne laughter is intensely social and contagious [1
], it is likely that the endorphin effect is limited to this form of laughter. Indeed, only Duchenne laughter has the capacity to mitigate negative emotions and stress [40
Most of the phenomena that trigger endorphin release involve physical exercise (running, circuit-training, rowing, etc. [18
]) or other forms of pressure on the body surface (e.g. grooming and massage [46
]). In the case of laughter, we assume that the functional mechanism is the muscular exertion involved in sustained laughter. As the sonograms in Davila Ross et al.
] illustrate, ape laughter typically consists of a series of alternating exhalations and inhalations, whereas that of humans typically consists of a sustained series of exhalations without drawing breath (see also [1
]). (This capacity to maintain a long series of exhalations is crucial to speech [1
].) It is this long series of exhalations that appears to be exhausting (hence triggering endorphin release), and this might be either because the physical effort involved is itself significant or because emptying the lungs in an uninterrupted series of exhalations is taxing.
Although it has been argued that positive affect plays an important role in the bonding of groups of individuals [49
], experiment 5 suggests that affect alone may be insufficient to create a significant endorphin surge. Given that neuroimaging studies have demonstrated a direct relationship between endorphin uptake at receptor sites and perceptions of affect [21
], our results suggest that the sense of heightened affect in this context probably derives from the way laughter triggers endorphin uptake.
Although laughter plays an important role in regulating conversation in humans [1
], it may also play a significant role in facilitating social bonding among groups of individuals [2
]. In both primates and humans, for example, laughter plays an important signalling role during social play [1
]. The capacity to sustain laughter for periods of several minutes at a time may exaggerate the opioid effects, thus ramping up the sense of heightened affect that humans experience in these contexts. A key aspect of this may be that social (or Duchenne) laughter is highly socially synchronized [1
]. In a study of physical exercise (rowing), synchronized activity ramped up endorphin production (as indexed by change in pain threshold) by a factor of two over that generated by exercise alone [33
]. If the opiate effects of endorphins create a sense of wellbeing, synchronized activity might then lead to enhanced prosociality, and hence group bonding and cooperation [50
]. Indeed, even simple behavioural synchrony is sufficient to enhance cooperative behaviour in subjects [51
]. As we might anticipate a similar effect arising from social laughter, a promising future development would be to test whether sustained laughter in groups enhances prosociality or altruistic behaviour.
Laughter contrasts with many more conventional aspects of non-verbal communication in one important respect: it seems to create euphoric states in the performer similar to those experienced in communal music-making, dancing and some of the rituals of religion [52
]. There is some evidence to suggest that these euphoric states are also associated with the release of endorphins [11
]. Singing, dancing and rituals have long been recognized as important components in the process of bonding whole communities in traditional societies, a process referred to variously in the anthropological literature as ‘effervescence’ [54
] and ‘communitas’ [55
]. An obvious hypothesis is that all these activities exploit the same psychopharmacological mechanism (the release of endorphins) as social grooming does in primates [25
], and so provide a bridging mechanism (i.e. a form of grooming at a distance) that enables humans to bond social communities that are much larger than those that primates can bond by social grooming alone [12
]. This possibility awaits detailed testing.