Summary
An investment game, a positive conversation and a distance task reveal group differences that may help explain how loneliness persists.
At a glance
In a study comparing 82 participants selected by loneliness level, the high-loneliness group invested less in a trust game, preferred greater interpersonal distance and showed weaker responses to positive interaction. Brain imaging also found lower activation in three trust-related regions. These findings suggest a possible self-reinforcing loneliness spiral; they do not establish a longitudinal causal sequence or show that every lonely person responds this way.
How Loneliness Can Harm Us
Introduction
People often say that successful people are lonely. Yet research suggests that when the need to belong goes unmet for a long time, the resulting loneliness can harm physical and mental health and may even increase the risk of death. To explore loneliness and its underlying neural mechanisms, Jana Lieberz and colleagues from Germany, Israel and Oldenburg conducted a study published in Advanced Science.
One possible mechanism involves trust. Cooperation is one of the most common ways to build positive, friendly connections, and it depends on mutual trust. Previous research has found that people with high levels of loneliness cooperate less with others, while brain imaging has shown substantial overlap between the neural circuits involved in loneliness and trust. These findings caught the researchers' attention. Dr. Dirk Scheele of the University of Bonn suggested that a lack of trust in others could be one reason loneliness develops.
How the Study Worked
The researchers used a preliminary questionnaire to screen 3,678 participants, selecting 82 who met the study's requirements. They then divided these participants according to their loneliness levels: 42 formed the high-loneliness group (HL), and 40 formed the low-loneliness group (LL).
The Trust Game
Participants first completed a trust game involving two roles: an investor and a trustee. In each round, the participant acted as the investor, starting with €10 and deciding whether—and how much—to invest with an unfamiliar trustee. An investment of €A was tripled to €3A through simulated stock trading, with that sum going to the trustee. The trustee could keep the entire amount or return €2A to the investor. If the trustee shared the proceeds, the investor would finish with €10 + A.
Participants in the HL group received no feedback about the investment outcome after making their decisions. Those in the LL group played against a computer, which randomly decided whether to share the proceeds. All participants underwent functional magnetic resonance imaging (fMRI) throughout the game.
Positive Social Interaction
After the trust game, participants had a 10-minute semi-structured interview with a researcher of the same sex. The conversation followed three positive questions in order: “If you won the lottery, what would you do with the money?” “What happy experiences did you have during childhood?” “What are your interests and hobbies?”
The researchers continuously recorded electrodermal activity (EDA) and electrocardiography (ECG) from five minutes before the interaction until it ended. They also collected saliva samples before the interaction, immediately afterward and 15 minutes afterward.
The Stop-Distance Task
The researchers also used a stop-distance task to measure interpersonal distance as an indirect indicator of trust in strangers. Participants completed the task twice, once before and once after the positive interaction, with two trials each time. The task involved the same researcher who conducted the interview.
In the first trial, participants approached the researcher from two meters away and stopped at a distance they considered appropriate. In the second trial, they stopped at a distance where they felt slightly uncomfortable. For each trial, the researchers recorded the distance between the participant's chin position at the start and at the stopping point.
What the Researchers Found
Compared with the LL group, participants in the HL group invested less in the trust game. They also preferred significantly greater interpersonal distance in the stop-distance task. Scheele interpreted these behaviors as signs of lower trust in others among participants with high levels of loneliness.
The saliva samples revealed a further difference. After the positive interaction, the LL group showed a significant increase in the area under the curve (AUC) for salivary oxytocin, whereas the HL group showed no difference from baseline. The groups' post-interaction AUC values also differed significantly. This pattern was interpreted as a weaker response to positive social interaction in the HL group.
Brain imaging added another piece to the picture. During the trust game, the HL group showed significantly lower activation than the LL group in three regions associated with trust: the left anterior insula, the amygdala and the right nucleus accumbens. Lieberz explained that an important function of the anterior insula is to perceive and interpret signals from within the body, such as the heartbeat. It also plays an important role in correctly interpreting other people's facial expressions and assessing their trustworthiness.
The Loneliness Spiral
Taken together, the findings suggest a possible explanation for how loneliness can reinforce itself. Higher loneliness was associated with lower trust and greater interpersonal distance, alongside a weaker response to positive interaction and reduced activation in brain regions related to trust. These patterns can make it harder to gain warmth and energy from positive encounters, potentially deepening loneliness and creating a self-reinforcing spiral.
The wider concern is one of public health. In 2018, the BBC reported that loneliness was becoming a public-health challenge for the future, with prevalence across different age groups reaching as high as 33%. Lieberz closed with a call for greater attention to loneliness and its negative effects, so that timely, effective interventions can follow.
Frequently asked questions
Does this study prove that distrust causes loneliness?
No. The study compared groups selected by loneliness level. Scheele proposed distrust as one possible reason loneliness develops, while the loneliness spiral is a synthesis of the findings rather than a directly demonstrated longitudinal causal sequence.
What happened to salivary oxytocin after positive interaction?
The low-loneliness group showed a significant increase in salivary oxytocin area under the curve (AUC), whereas the high-loneliness group showed no difference from baseline. The groups also differed significantly in their post-interaction AUC values.
Did the experiment directly measure illness or death?
No. Physical-health and mortality risks provide background in the article. The described experiment reports behavioral, hormonal and neural differences between groups.
