What happens inside the brain when two strangers begin creating something together? A new study from ETH Zurich suggests that the answer is more complex than the popular idea that people who grow closer must eventually develop increasingly similar brain activity. Over six weeks, researchers followed 61 pairs as they met repeatedly to draw together, tracking not only how their social connection changed but also how their brains interacted in real time. The participants included adults aged 18 to 35 and older adults aged 70 to 85. Thirty-one pairs combined younger and older people, while 30 pairs were made up of participants from the same age group. The results, published in PLOS Biology, reveal that stronger social bonds do not necessarily produce greater neural synchrony. In some relationships, increasing familiarity was accompanied by decreasing similarity between the partners’ brain signals.
The experiment was designed to capture social interaction as naturally as possible. At every meeting, participants first drew alone on separate sheets of paper and then completed two collaborative drawings on the same sheet. They could decide whether to draw simultaneously or take turns, and they were allowed to talk during the session, although they had to remain silent while drawing. Their brain activity was measured using functional near-infrared spectroscopy, or fNIRS, a portable scanning method that uses light to detect changes in blood oxygenation near the surface of the brain. Unlike conventional scanners, fNIRS does not require participants to remain inside a rigid machine. The lightweight caps allowed the pairs to move their arms, respond to one another and behave more like people engaged in an ordinary shared activity. Researchers also recorded body movements, creating a detailed picture of how social behavior and brain activity unfolded together.
The central concept was inter-brain synchrony, a measure of how similarly the activity of two people changes over time. When two individuals perform a shared task, their brains are continuously exposed to one another’s movements, decisions, pauses and reactions. If comparable fluctuations in neural activity occur at approximately the same moment, researchers describe the brains as synchronized. This does not mean that the two brains become identical, nor does it prove that the participants are experiencing the same emotion. Instead, synchrony is a statistical relationship between signals recorded from two nervous systems. The ETH Zurich team was particularly interested in how this relationship changed as strangers became more familiar. Earlier research had often suggested that repeated interaction should make partners’ brain activity increasingly alike, especially when they learn to coordinate their behavior and understand one another more easily.
The findings challenged that expectation. In the cross-generational pairs, participants reported feeling closer to one another as the weeks passed, but their overall brain synchrony declined. Their neural signals were most similar at the beginning of the relationship and became less alike over subsequent meetings. Same-age pairs showed the opposite trajectory: they began with relatively low synchrony, which increased throughout the six-week period. Yet the researchers found an important pattern common to both groups. During collaborative drawing, the participants’ brains were more synchronized than when they drew separately. The shared task clearly brought their brain activity into a stronger temporal relationship, but the long-term direction of that relationship depended on the age composition of the pair. The result suggests that momentary coordination and the gradual development of social familiarity may involve different neural processes.
One possible explanation comes from two competing models of interpersonal brain activity. The first, often called Common Cognitive Processing, proposes that people’s brains become synchronized because they are receiving and interpreting similar stimuli at the same time. If two participants watch the same movement, hear the same sentence or focus on the same drawing, their brains may respond in comparable ways. The second model, Mutual Prediction, emphasizes the way people learn to anticipate one another. Rather than simply processing the same information, partners gradually develop expectations about what the other person will say, do or suggest next. In this framework, reduced synchrony can sometimes reflect successful adaptation: one person may be anticipating the other’s behavior and preparing an appropriate response before the same external event reaches both individuals in the same way.
Age-related differences in experience may have influenced this process. Participants of similar ages were often students or people with comparable daily routines, giving them an immediate supply of shared topics and cultural references. Their early interactions may therefore have involved relatively little effort to establish common ground. As the meetings continued, they may have become more spontaneous and playful, introducing surprising ideas and making one another’s behavior harder to predict. Cross-generational pairs, by contrast, initially had to discover what they could discuss and how to interpret one another’s perspectives. Their higher synchrony at the beginning may have reflected intense efforts to understand an unfamiliar partner. As the relationship developed, the need for such effort may have decreased, even while their reported sense of closeness increased. In this interpretation, greater synchrony is not automatically a marker of intimacy; it can also signal the mental work required to bridge a social or experiential gap.
A second analysis of the same encounters, published in Acta Psychologica, provided further clues about how the pairs coordinated their behavior. Researchers identified seven recurring “two-brain states,” or stable patterns describing how activity was distributed within and between the participants’ brains. These states remained remarkably consistent across the six-week period, suggesting that social encounters may settle into recognizable modes of coordination. One state lasted significantly longer in the cross-generational pairs. It was characterized by comparatively low synchronization between the two brains but unusually strong synchronization between two regions within one individual’s frontal lobe. The frontal lobe supports functions including planning, decision-making, attention and the regulation of behavior. The researchers cautiously interpreted this pattern as a possible division of roles: the participant with stronger internal frontal synchronization may have been more actively monitoring and adjusting their own behavior in response to the partner.
That pattern was most often associated with the younger participant, while the older participant tended to occupy a more leading role. The behavioral data supported this possibility. Same-age pairs were more likely to draw at the same time, whereas cross-generational pairs more frequently took turns. Taking turns can create a clearer leader-follower structure, with one person acting and the other observing, interpreting and preparing a response. At the same time, the researchers emphasize that these findings should not be treated as a universal rule about age. The study identifies tendencies within a particular experimental setting, not fixed characteristics of younger or older adults. Nor can fNIRS alone reveal precisely what a participant was thinking. The technology measures changes in blood flow associated with neural activity, and the signals are limited largely to the outer layers of the brain. Neural synchrony is therefore best understood alongside behavior, self-reported social connection and the structure of the interaction itself.
The study’s broader importance lies in its timescale and its inclusion of older adults, groups that have often been underrepresented in social neuroscience. Previous investigations of interpersonal synchrony have examined relationships such as parent and child, teacher and student, doctor and patient, or romantic partners. By observing strangers as they met repeatedly over several weeks, the ETH Zurich researchers captured a transitional period in which an unfamiliar person gradually becomes an acquaintance and perhaps a friend. Their results suggest that social closeness cannot be reduced to a simple increase in matching brain waves. Two people may become more connected while developing complementary patterns of attention, prediction and action. The brain may coordinate socially not only by moving in lockstep with another person, but also by learning when to lead, when to follow and when to anticipate what the other person will do next. That finding could reshape how scientists interpret neural synchrony in everyday relationships, teamwork and communication across generations.
Subject of Research: Interpersonal brain synchrony, social connection, loneliness, collaborative creativity and relationships between same-age and cross-generational pairs.
Article Title: Social interactions between people of same and different generations shape longitudinal changes in interpersonal neural synchrony, loneliness, and social connection
Web References: https://doi.org/10.1371/journal.pbio.3003899
References: PLOS Biology; DOI: 10.1371/journal.pbio.3003899. A related study was published in Acta Psychologica.
Image Credits: PLOS Biology / Social Brain Sciences Lab
Keywords: inter-brain synchrony, fNIRS, social neuroscience, collaborative drawing, brain activity, aging, cross-generational relationships, neural coordination, mutual prediction, interpersonal connection
