Social brain research offers new insights into how strangers become friends

In brief 

• With the help of portable brain sensors, ETH Zurich researchers tracked, week by week, how the brain activity of two people changed as they got to know one another. This is the first study in this research field to include older adults aged between 70 and 85 years. 

• When younger and older adults draw shared pictures together over several weeks, they get to know each other better. Their brain activity, however, does not necessarily become more similar as a result. 

• The results suggest that the extent to which two people’s brain activity becomes more similar depends on how fluently they can predict and anticipate each other’s behaviour. 

What happens in the brain when two people engage in joint creative activity and build a new relationship with one another in the process? ETH cognitive scientist, Ryssa Moffat investigated this question. She is a postdoctoral researcher at the Social Brain Sciences Lab within the Chair of Cognitive and Social Neuroscience. 

Her study has now been published in the journal PLOS Biology. The setting was as follows: over a period of six weeks, 61 pairs met six times to engage in drawing together. The participants came from a younger age group (18 to 35 years) and an older age group (70 to 85 years) and were divided into 31 cross-generational pairs and 30 pairs of the same age. 

When two minds engage with one another 

At the outset, the participants did not know each other and first had to become familiar with one another. At each session, the pairs first drew individually on separate sheets of paper, then drew together twice on a single sheet of paper. They were free to decide whether to take turns or draw simultaneously. They were allowed to converse throughout the session but were instructed to remain silent while drawing.

At each session, the researchers recorded participants’ brain activity with the help of wearable brain sensors. This enabled the researchers to investigate how the process of getting to know one another was reflected in the pairs’ brain activity. At the same time, the mobile sensors ensured that participants could move and respond to each other naturally, as in everyday life, without distorting the recorded signals. 

The similarity of the brain activity was particularly crucial for the ETH researchers: to what extent do the brains of two people synchronise during an encounter? And how does this synchronisation change over the course of repeated sessions? The focus was not on whether a pattern of brain activity corresponds to a specific subjective feeling such as liking or irritation. 

Stronger feelings of closeness, more similar brain activity? 

Moffat was interested in what researchers call ‘inter-brain synchrony’: when people perform a shared task such as drawing, each person's brain reacts continuously to the other person's actions – and the brains can be said to be 'synchronised' when similar changes in brain activity occur simultaneously in both people 

Previous studies on inter-brain synchrony suggested that the activity patterns of two brains become increasingly similar as people grow more familiar with one another. Accordingly, the ETH researchers led by Ryssa Moffat initially expected that synchrony would increase over time, as the pairs adjusted to one another while engaging in drawing.  

But the researchers were surprised: although the participants in cross-generational pairs felt closer to one another over the course of weeks, their patterns of brain activity did not become increasingly similar. 

An unexpected age difference 

“In the cross-generational pairs, brain synchrony was higher at the start – and it decreased from week to week,” as researcher Moffat shares. “In the same-age pairs, it was the other way round: they started with lower synchrony, which increased over the course of the six weeks.” Regardless of this trend, however, one thing became clear over the course of the weeks: in both groups, brain synchrony was higher when the couples drew together than when the individuals drew separately. 

Consequently, collaboration led to greater synchronisation of brain activity – initially in a similar manner for both groups. It was only over the course of the six weeks that the difference became apparent: while the brains of the same-age pairs showed increasingly similar activity, this synchronisation tended to decline in among the intergenerational pairs. 

Anticipating rather than being in sync 

How can this difference be explained? A definitive answer is still pending. Thanks to the portable brain-scanning technology, Moffat can state that when two people come to feel closer, it’s not a given that their brains ‘tick’ alike.  

Moffat’s study takes a different approach here from many earlier studies on brain synchrony. These assumed that two people experiencing something together process similar stimuli – and that their brains therefore react in similar ways. Researchers refer to this model as ‘Common Cognitive Processing’. 

By contrast, Moffat’s explanatory approach assumes that two people learn to better anticipate their counterpart’s behaviour and adapt to it: What will the other person say next? How will they react to my suggestion? In research, this model is known as ‘Mutual Prediction’. 

This model may explain the age differences that emerged in the study. This is because, as Moffat explains, the participants’ life experiences also play a role in mutual prediction: “The participants of the same age were mostly students with similar daily routines and quickly found topics they had in common. Younger and older people, on the other hand, first had to work out what they could actually talk about.”  

For Moffat, this means: “Greater brain synchrony can also mean that two people are making a particularly strong effort to understand one another.” 

Who leads, who follows? 

These efforts are particularly called for at the outset of a relationship: when younger and older people meet for the first time, they initially find it harder to predict their counterpart’s behaviour, explains Moffat. “Peers often find common ground more quickly or already have it. Over time, however, they become more playful, surprising each other in their conversations and thereby making it harder for their counterpart to predict their behaviour.” 

In a second study published in the journal Acta Psychologica the researchers provide insights into why brain synchrony differs between younger and older people.  

In this study, they examined the same encounters and measurement data from a different perspective: they investigated which recurring patterns of shared brain activity emerged during the encounters. The researchers identified seven so-called ‘two-brain states’ that remained remarkably stable throughout the entire six-week study period. 

One state was particularly noteworthy and lasted significantly longer in the intergenerational pairs than in the same-age pairs. In this state, the synchronisation between the two brains was comparatively low. What stood out instead was a particularly strong connection within a single person’s brain – an internal synchronisation between two areas in their own frontal lobe. 

The researchers cautiously interpret this as an indication of a division of roles: the person with the stronger internal synchronisation was more likely to adapt their behaviour to that of the other. In most cases, this was the younger person – while the older person tended to take a leadership role. The researchers also found that same-age pairs were more likely to draw at the same time, whereas intergenerational pairs took turns more often. 

New ground for social brain research 

“Our study is the first to show, over a period of several weeks, how brain activity develops in younger and older people,” concludes Ryssa Moffat. Previous studies have focused on parent-child, teacher-pupil, doctor-patient and romantic relationships. Older people have so far been largely overlooked in social brain research. Over the long term, Moffat’s research could also reveal “how strangers become acquaintances and perhaps even friends”.

References

Moffat R, Dumas G, Cross ES: Social interactions between people of same and different generations shape longitudinal changes in interpersonal neural synchrony, loneliness, and social connection. PLOS Biol 24(8) 2026. DOI: external page 10.1371/journal.pbio.3003899

Naudszus, LA, Moffat, R, Cross ES: Two brain states characterise within- and between-brain connectivity during intergenerational collaborative drawing. Acta Psychologica, Vol. 269, 2026, 107252. DOI: external page 10.1016/j.actpsy.2026.107252