Primates have unusually large brains in relation to body size compared to all other vertebrates. The reason for this outstanding anatomical feature is broadly accepted to be the complex social interactions primates have developed. This theory is known as the “social brain hypothesis”, and, simply put, it postulates that the demands of sociality are responsible for the evolution of brain size.

The development of societies is indeed regarded as one of the major transitions in human evolution. The human brain has evolved to thrive in social environments, providing us with the cognitive skills needed to deal with our elaborate personal and social relationships. Still, the size of personal social networks can vary extensively between different people – some of us are just more sociable than others. The biological factors that determine these differences in sociability are still largely unexplored, but it is clear that it is highly influenced by physical and mental health status.

There’s a theory – the brain opioid theory of social attachment – that states that the opioid system is a fundamental element in social bonding. Of particular influence is the neuropeptide beta-endorphin which binds to mu-opioid receptors in the central nervous system.

The opioid system and sociability

Beta-endorphin is an endogenous opioid neuropeptide found in both the central and the peripheral nervous system. In the central nervous system, beta-endorphin acts to induce analgesia and a general feeling of well-being; it is also released in response to exercise, being responsible for what is commonly known as “runner’s high.” Beta-endorphin is also being increasingly regarded as a key molecule in social bonding.

In fact, endogenous opioids seem to be molecular mediators of hedonic experiences; the interaction of mu-opioid receptors with dopamine systems underlies the rewarding nature of social interactions, and opioids have been associated with different social bonding activities such as music-making or dancing.

Endorphins can modulate social motivation and feelings of social connection – this is highlighted by the fact that individuals whose mu-opioid activity is blocked with an antagonist experience reduced feelings of social connection. Reduced mu-opioid receptor expression in humans has also been associated with increased social withdrawal and greater avoidance of social attachment. Furthermore, impaired mu-opioid receptor signaling has been reported in patients suffering from depression.

Sociability, endogenous opioids and pain

With these actions of endogenous opioids in mind, a new study published in Scientific Reports aimed at determining if there is an association between the activity of the mu-opioid system and the number of social relationships one maintains. Given the analgesic properties of beta-endorphin, the authors tested this hypothesis by assessing pain tolerance as an index of activation of the endogenous mu-opioid system; the size of the participants’ two innermost social network layers (friends contacted at least once a week or at least once a month) was determined.

What this study showed was that pain tolerance correlates with social network size in humans, indicating that the release of beta-endorphin and mu-opioid receptor signaling may indeed play an important role in social interactions, and that inter-individual differences in the mu-opioid system may underlie differences in sociality.

Interestingly, laughter, which is a well-known inducer of feelings of well-being and a central element in social bonding, has already been associated with the release of endorphins. It has been shown that laughter can increase tolerance to pain, most likely through endorphin-mediated opioid activity.

The relationship between other variables and social network size were also assessed in this study. Stress, for example, was shown to be inversely associated with social network size, indicating a beneficial effect of social support in dealing with stressful situations. Beta-endorphin is known to alleviate the stress response, so it is actually possible that this is also the underlying mechanism of the stress relieving effect of socializing.

Another interesting observation reported in this article was that there is also an inverse association between physical fitness (which was assessed to account for its influence on pain tolerance) and social network size. This may be interpreted as a need to choose between leading a socially active versus a physically active life. Although this may be simply due to time restraints, it can also be interpreted as choice between different feel-good activities: you can get your beta-endorphin fix from exercise or you can get it from spending time with your friends.

Is sociability a cause or a consequence?

What remains to be determined is the causal relationship between sociability and opioid activity. On one hand, it is possible that individuals with naturally higher mu-opioid neurotransmission obtain greater reward from social interactions, and therefore actively search for social bonding; one the other hand, it is possible that it is the active engagement in social activities that induces the release of higher levels of endogenous opioids.

Since it has been shown that asking healthy subjects to sustain a sad mood for 30 minutes can induce a reduction in mu-opioid receptor activation, it is possible that being in a good mood while socializing and laughing with friends may actively induce the release of beta-endorphin creating a feeling of well-being while also increasing the tolerance to pain, as measured in the study mentioned above.

So, just like exercise is recommended as a treatment for depression, maybe so should be hanging out and laughing with your friends. I actually prefer the latter.

References

Akil H, et al (1984). Endogenous opioids: biology and function. Annu Rev Neurosci, 7:223-55. doi: 10.1146/annurev.ne.07.030184.001255

Bali A, et al (2015). Stress and opioids: role of opioids in modulating stress-related behavior and effect of stress on morphine conditioned place preference. Neurosci Biobehav Rev, 51:138-50. doi: 10.1016/j.neubiorev.2014.12.018

Dunbar RI (2009). The social brain hypothesis and its implications for social evolution. Ann Hum Biol, 36(5):562-72. doi: 10.1080/03014460902960289

Dunbar RI, et al (2012). Social laughter is correlated with an elevated pain threshold. Proc Biol Sci, 279(1731):1161-7. doi: 10.1098/rspb.2011.1373

Inagaki TK, et al (2015). Blocking opioids attenuates physical warmth-induced feelings of social connection. Emotion, 15(4):494-500. doi: 10.1037/emo0000088

Johnson KV, Dunbar RI (2016). Pain tolerance predicts human social network size. Sci Rep, 6:25267. doi: 10.1038/srep25267

Nummenmaa L, et al (2015). Adult attachment style is associated with cerebral mu-opioid receptor availability in humans. Hum Brain Mapp, 36(9):3621-8. doi: 10.1002/hbm.22866

Uchino BN, et al (1996). The relationship between social support and physiological processes: a review with emphasis on underlying mechanisms and implications for health. Psychol Bull, 119(3):488-531. doi:  10.1037/0033-2909.119.3.488