A new study challenges the long-held assumption that falling into bodily synchrony with a stranger automatically promotes generosity. Researchers report that matching sweat responses can, in certain social contexts, correlate with less generosity, particularly when the participants can see one another. The work focuses on how physiological synchrony functions in face-to-face, unequal exchanges of trust between strangers.
Physiological synchrony refers to two people’ s bodies moving in concert—signals such as heart rate or tiny changes in sweat levels rising and falling on similar schedules without conscious effort. Skin conductance, the measure used in this study, tracks how easily the skin conducts a mild electrical current as sweat glands respond to arousal and attention.
Study lead author Fabiola Diana, a postdoctoral researcher who has worked at Leiden University, Utrecht University and is now at ETH Zurich, explained that the mechanism behind synchrony remains unclear, but its repetition is pervasive: “Years of research on this topic have shown that synchrony is supposed to make us feel ‘more connected’ with others, and more attuned to one another.”
Historically, social interactions have suggested that synchrony builds trust and cooperation. A 2017 meta-analysis found that behavioural or physiological synchrony generally fosters helpfulness and cooperation, while a 2020 experiment emphasised that visible faces enhanced synchrony and its predictive power for cooperative success. A 2024 review argued that unconscious mimicry and synchrony evolved partly to anticipate others’ actions under uncertainty, rather than merely to cultivate warmth.
The new research asks what happens to this dynamic when one participant bears the risk and the other decides whether to reciprocate. The scientists wanted to know whether visual contact strengthens physiological synchrony during unequal exchanges and whether that synchrony translates into greater trust and generosity.
“Since trust is the very backbone of human society, and many daily life situations require us to put our trust in complete strangers… our primary objective was to research whether this synchrony could also be one of the reasons why people who do not know each other trust one another,” Diana said. “Another thing we were interested in is how people synchronize. How does it happen? Is there maybe something in the faces of people that makes us synchronize more?”
A total of 122 healthy young adults (56 women, average age about 21) from Leiden University were paired into 61 same-sex stranger pairs. After exclusions, 59 pairs remained for analysis. Each pair took part in two six-round blocks of a economic trust game, one with full face-to-face visibility and another with a barrier obscuring visual contact, with starting order varied between pairs.
In each round, one player acted as the “trustor” and the other as the “trustee,” each beginning with 6 euros. The trustor could transfer between 0 and 6 euros to the trustee; the amount was tripled, and the trustee could decide what share to return. Trustors received no feedback on how much was returned, keeping decisions dependent on live impressions, with a potential bonus of up to 2.50 euros.
Physiological data were collected from the fingers and torso to gauge skin conductance and heart rate, while webcams tracked 17 facial muscle movements across more than 826,000 video frames. Eye-tracking glasses measured pupil size, but data loss from head movements meant pupil data could not be used. The researchers analysed eight-second windows with a four-second lag to see how closely partners’ responses aligned.
Trustors invested an average of 4.14 euros per round, and trustees returned an average of 61.6 percent of the tripled amount, consistent with prior trust-game findings. When compared with randomly matched strangers, heart-rate synchrony did not exceed chance in either the face-to-face or barrier condition, so the analysis focused on skin conductance. Sweat synchrony was marginally higher when the partners could see each other, but the difference was small, and response timing tended to align more closely during visible contact.
Video analysis showed that face-to-face interaction altered facial behaviour: people smiled more, used more eyebrow movements, and displayed signs of mild tension, whereas behind the barrier negative expressions were more common. Yet overall facial expressivity did not predict skin conductance synchrony in a statistically significant way, and individual facial muscle movements had little association with sweat synchrony.
For trustors, merely seeing the partner did not produce a clear rise in investments on average, with the order of blocks influencing behaviour. Trustors tended to invest more when the barrier rose to allow face-to-face contact, but those who began face-to-face generally kept up similar investment levels once the barrier was lowered. The authors suggest a lasting impression from initial face-to-face contact may carry into later rounds.
Overall, skin conductance synchrony did not show a significant link with trustors’ investments in either condition, and a follow-up Bayesian analysis supported the interpretation that the two measures were unrelated in this study.
Trustees behaved differently. They returned a larger share of the pot when looking at their partner, regardless of block order, but within the face-to-face block, stronger sweat synchrony predicted lower reciprocity: trustees who tracked their partner’s sweat more closely returned a smaller proportion of the money. When visual contact was blocked, the relationship between synchrony and reciprocity was not statistically significant.
“We expected that people whose bodies were more in sync would be more generous toward each other, and we found the opposite,” Diana confessed, noting the result was unexpected and prompted a rethink of synchrony’s function beyond mere affiliative warmth.
In a post-hoc exploratory analysis, the researchers looked at overall arousal, indexed by average skin conductance. They found that trustors generally showed higher arousal than trustees, and the negative link between face-to-face synchrony and reciprocity was strongest when the trustor showed low arousal, weaker at average arousal, and not significant when arousal was high.
The study’s authors linked the finding to earlier work on familiar pairs, where brainwave synchrony emerged when partners could see one another and engage in social cues such as eye contact and smiling. They contrasted this with a prior experiment on gratitude, which found that positive emotion could increase mutual cooperation and brain-to-brain synchrony, noting that the current results involved involuntary sweat responses during an unequal trust game rather than induced positive emotion.
From their perspective, low arousal in the trustor may reflect disengagement or boredom with repetitive blind choices, prompting trustees who perceive that state to share less. The lack of feedback, they suggest, may inhibit linking gut feelings with partner fairness, while more subtle cues—like shifts in pupil size or slight blushes—might carry arousal information when faces are visible.
“Feeling ‘in sync’ and ‘more connected’ with other people does not automatically mean the interaction we have will benefit from it,” Diana cautioned. “We may not always feel closer or more affiliated. For instance, when you sync with someone in a negative state, it could resonate with you and put you in a negative emotional state, and that, in turn, may unconsciously affect your decision toward them.”
She emphasised that the observed effects are small to medium in size and that many factors influence decisions in a laboratory setting, including personality, mood, or even how well the participant slept. The researchers stressed that a single factor like sweat synchrony cannot predict whether a person will be generous.
Limitations of the study include its observational nature and the fact that the laboratory constraints may have restricted natural behaviour. The team also noted that heart-rate synchrony did not exceed chance and that some eye-tracking data were unusable. They called for future preregistered studies with opportunities for conversation, varied feedback, and cleaner eye-tracking to determine when shared bodily rhythms support cooperation and when they do not.
“If synchrony is not simply a force for connection, but a way of bodily tuning into another person, then it may also be one of the tools we use to understand them,” Diana concluded. “My next question is whether synchrony helps us predict what others will do next… synchrony could be part of what makes humans good social predictors.”
The study, Physiological synchrony predicts reduced reciprocity only in face-to-face interactions, was authored by Fabiola Diana, Julia Folz, Elio Sjak-Shie, Ruud Hortensius, and Mariska E. Kret.
