Researchers from the University of Oxford, working under the EU-funded RELNET project, believe that the link between number of friends and the ability to tolerate pain is due to a system in the brain that involves endorphins, pain-killing chemicals that also trigger a sense of wellbeing. Writing in the journal ‘Scientific Reports’, the team led by Professor Robin Dunbar and Katerina Johnson, a doctoral student, aimed to explore the theory that the brain’s endorphin system may have evolved to not only handle our response to physical discomfort and pain but also influence our experience of pleasure from social interactions. In particular, she was interested in the ‘brain opioid theory of social attachment’, where social interactions trigger positive emotions when endorphins bind to opioid receptors in the brain, giving us a ‘feel-good factor’ when seeing close friends. ‘Social behaviour and being attached to other individuals is really important for our survival – whether that is staying close to our parents, or our offspring or collaborating with others to find food or to help defend ourselves,’ commented Johnson. Conducting the experiment To test the theory, the research team examined both the social networks and pain thresholds of 101 adults aged between 18 and 34. Each participant completed a questionnaire that was designed to quiz them on friends they contacted once a week and those they only communicated with once a month. The personality of each participant was also probed, looking at traits such as ‘agreeableness.’ They also had to provide details on their levels of physical fitness and general levels of stress. As greater endorphin activity in the brain is linked to higher pain tolerance, each participant was also asked to squat with their back against a wall and their knees at right-angles to their body, which is a simple yet uncomfortable exercise, for as long as they were able to. This provided the project team with an indirect method of gauging endorphin activity in the brain. Expected & unexpected results The researchers found that equally for both men and women, larger social networks were linked to greater pain tolerance. Most surprisingly, it was the number of friends contacted only monthly, rather than weekly, that appeared to be the most important factor. The results, when controlling for stress, fitness and agreeableness, showed that an increase from seven to 12 friends in this second layer of personal contacts is predicted to boost pain tolerance from 1 minute to 4 four minutes on average. However, the project team did state that it was not clear whether the link was due to greater social activity boosting the release of endorphins that subsequently dampens pain, or whether individuals with a more active endorphin system experience greater rewards from social activity and hence surround themselves with more friends. There were also some other interesting results from the study. Participants who reported higher levels of fitness were able to endure the pain test for longer but generally had smaller friendship groups. This could merely be a question of time – those who exercise more have less time for socialising. However, another possibility is that those who experience higher endorphin levels from exercising do not need to seek a similar feeling from interacting with friends. Additionally, those who reported higher stress levels were also found to have fewer friends, although there was little correlation with pain tolerance. However, it is unclear if stress hinders people from forming meaningful friendships, or whether having more friends allows individuals to better cope with the pressures of modern life. ‘These results are also interesting because recent research suggests that the endorphin system may be disrupted in psychological disorders such as depression. This may be part of the reason why depressed people often suffer from a lack of pleasure and become socially withdrawn,' commented Johnson. Although the research results seem to support previous evidence that endorphin activity in the brain might be linked to social interactions, further research will be needed to ascertain what causes the differences between individuals, with genetic variations that impact the level of endorphin receptors in the brain being a possible avenue for future enquiry.