Everyone has felt lonely at some time in his or her life, and everyone knows how painful it can feel. Humans are an essentially social species with the motivation to form and maintain interpersonal relationships as a fundamental organizational principle of behavior. But if loneliness persists, say psychologists and psychiatrists, it can lead to mental illnesses such as depression or anxiety disorders. 

Now, researchers at the University of Haifa in Israel and the Universities of Bonn and Oldenburg in Germany have discovered how loneliness is connected with reduced trust and is reflected in changes in the activity and interaction of various brain structures, especially the insular cortex. The results therefore provide clues for therapeutic options. They published their findings in the journal Advanced Science under the title “Loneliness and the Social Brain: How Perceived Social Isolation Impairs Human Interactions.” 

From an evolutionary perspective, loneliness may have evolved to motivate the formation of new social relationships, in the same way as hunger induces scavenging, they wrote. “However, when the connection with other individuals fails, loneliness impairs inflammatory and immune responses and promotes a phenotypic hypersensitivity to social threats and self-centered behavior. The perception of the social environment as threatening may lead to various negative biases in loneliness.”

For instance, it has been suggested that lonely individuals allocate their attention faster toward threatening social stimuli, anticipate rejection more often and exhibit negative attribution styles, they continued.  Eventually, even positive social interactions might fail to alleviate feelings of loneliness, as lonely individuals show reduced positive ratings of social encounters and attenuated reward-associated brain activity in response to positive social stimuli. “Importantly, however, while the detrimental impact of loneliness on social interactions is well established and theoretical frameworks point to negative biases and selfish behavior as putative mediators, the neurobiological mechanisms that hinder the formation of new, positive relationships and thus the alleviation of loneliness are still elusive.”

Behind the feeling of loneliness is the perceived discrepancy of the need for social relationships and the inadequate fulfilment of such a need, said the researchers. As with hunger that wants to be satisfied, feelings of loneliness can also provide the motivation to connect with other people. “One reason for this keenly felt loneliness may be a lack of trust in fellow human beings,” said psychologist Dr. Dirk Scheele from the Bonn University Hospital. 

Together with Prof. Simone Shamay-Tsoory from the University of Haifa (whose research focuses on the neural basis of emotions and social behavior) and Prof. Dr. Dr. René Hurlemann from the University of Oldenburg, the team team therefore investigated the relationship between trust and loneliness in more detail. Using an online questionnaire, the researchers selected 42 people from 3678 adults who were affected by severe loneliness but did not suffer from a mental illness or were receiving psychotherapy. The control group consisted of 40 people who did not suffer from persistent loneliness. 

“It was important to us that our findings could be attributed to the loneliness experienced and that any influence of mental illness could be ruled out as far as possible,” explained lead author Jana Lieberz. 

Participants first completed tasks in the brain scanner. Among other things, they played a trust game. Here they were given ten euros in start-up capital. Based on portrait photos displayed on a screen, they were asked to decide how much of the money they were willing to share with each of the people shown. They knew that making a profit beyond their start-up capital was only possible if they shared their start-up capital with others. At the same time, however, they had to trust that their gambling partners would not keep the money they had staked for themselves. “Participants with pronounced feelings of loneliness shared less with others than the control group,” the researchers said. “We interpret that as a lower level of trust.”

The team also found processing deviations in brain areas involved in trust formation compared to the control group. This was particularly evident in the anterior insular cortex, which was less active in lonely individuals and did not connect as prominently with other brain areas. “An important function of the insular cortex is to perceive and interpret one’s own body signals, such as the heartbeat,” they added. “It also helps to correctly interpret other people’s reactions, such as facial expressions or mood – or trustworthiness.”

After the trust game, the experimenters also simulated a standardized conversation situation with the respective participant, which dealt with emotionally positive content: What would you do with a lottery win? What are your hobbies? Afterwards, the team asked the participants about their mood. The researchers also collected blood and saliva samples to examine, among other things, an increase in the bonding hormone oxytocin in response to the conversation and measured the distance in centimeters that the subjects maintained from the experimenter.

It was found that those affected by severe loneliness were in a less-positive mood after small talk than the control group. Levels of the bonding hormone oxytocin also changed less. Furthermore, lonely people maintained a spatial distance from the experimenter that was about ten centimeters greater than that of those hardly affected by loneliness. “Overall, the results show across tasks that chronic loneliness is associated with reduced trust in fellow human beings,” said Scheele. “This can mean that interactions with others are experienced as less positive, which makes it harder to connect with others and exacerbates the loneliness spiral.”

The research team believe that their findings could serve as a starting point for interventions. “The reduced trust of lonely people could be given greater focus in therapies by making it a topic of discussion and thus making those affected aware of it. It would then also be possible to look at strategies on how affected individuals can strengthen their trust in other people, the concluded. In a study currently underway at Bonn University Hospital, the researchers, together with colleagues from Haifa and Oldenburg, are investigating whether psychotherapeutic group interventions can reduce these negative mental biases.