How Brain Activity Changes as Strangers Become Friends

Challenging the Synchronization Hypothesis in Social Neuroscience

Conventional neuroscience assumptions have long held that as two individuals grow more familiar and feel a deeper sense of closeness, their brain activity naturally mirrors one another more closely through a process called inter-brain synchrony. To test this, the ETH Zurich researchers equipped participants—divided into same-age groups aged 18 to 35 and 70 to 85, alongside cross-generational pairs—with mobile brain sensors during their drawing sessions. The technology allowed participants to move and interact naturally without distorting the recorded neural signals. The results challenged the standard synchronization model. While same-age pairs started with lower neural synchrony that steadily increased over the six weeks as bonds formed, cross-generational pairs showed the exact opposite trend. According to findings reported by ETH Zurich, cross-generational pairs exhibited higher brain synchrony at the start, which then steadily declined from week to week even as the participants reported feeling closer to one another.

In the cross-generational pairs, brain synchrony was higher at the start—and it decreased from week to week.

Ryssa Moffat, ETH Zurich

Mutual Prediction Versus Common Cognitive Processing

To explain these age-based differences, researchers look past traditional “Common Cognitive Processing” models, which assume that brains synchronize simply because two people react to identical external stimuli. Instead, Moffat’s analysis points toward a “Mutual Prediction” framework. In this model, individuals constantly work to anticipate their partner’s next move, comment, or reaction. Life experience and daily routines heavily influence this predictive effort. Moffat explains that same-age participants, largely consisting of students with shared daily routines, quickly found common topics. In contrast, younger and older pairs faced a steeper learning curve when figuring out what to discuss and how to predict each other’s behavior. Greater initial brain synchrony in intergenerational pairs reflects the intense cognitive effort required early on to bridge generational divides and understand one another.

Role Division and Two-Brain States in Intergenerational Pairs

A secondary analysis published in Acta Psychologica examined the same experimental data from a different perspective to identify recurring patterns of shared brain activity. One specific state lasted significantly longer in intergenerational pairs, characterized by lower overall brain-to-brain synchrony coupled with high internal synchronization within the frontal lobes of individual participants. The researchers interpret this distinct neural state as an indicator of a defined division of roles. In these cross-generational interactions, the younger participant typically adapted their behavior to fit their partner, while the older participant tended to assume a leadership role. This behavioral split also manifested physically in the drawing process: same-age pairs frequently drew simultaneously, whereas intergenerational pairs leaned toward taking turns.