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Falling into physical sync with someone else is widely assumed to encourage generosity and cooperation, yet a new study published in Proceedings of the Royal Society B suggests that matching bodily responses can sometimes point in the opposite direction. When pairs of strangers played an unequal game of financial trust, partners whose sweat responses rose and fell together saw smaller returns sent back to the person who took the initial risk, but only when the two could see each other. The findings indicate that sweat synchrony is not automatically cooperative, and that its social effects depend on both visual contact and a partner’s level of emotional engagement.
During social interactions, two people often begin to mirror each other’s automatic bodily processes, a phenomenon known as physiological synchrony. Without either person noticing, involuntary signals such as heart rate or microscopic sweating can rise and fall on nearly the same schedule. Scientists frequently measure it through skin conductance, which tracks tiny changes in how easily the skin carries a mild electrical current as sweat glands react to emotional arousal and alertness.
“The mechanism behind this is still unknown, but this happens all the time – between mothers and their children, between romantic or friend couples, between therapist and patient,” study lead author Fabiola Diana, who conducted the research at Leiden University and Utrecht University and is now a postdoctoral researcher at ETH Zurich, told PsyPost. “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.”
When strangers have to decide whether to trust each other under uncertainty, they rely on visual impressions and rapid intuitive judgments formed in the moment. Live exchanges supply a continuous stream of bodily signals, and a 2017 meta-analysis combining prior experiments found that falling into behavioral or physiological sync generally promotes helpfulness and cooperation.
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Mutual visibility appears to play an important part in linking sweat synchrony to cooperative choices. In a 2020 experiment co-authored by two of the same researchers, pairs of strangers played a simultaneous cooperation game either looking at one another or separated by a divider. Partners synchronized their sweat responses more closely when they could see each other. That sweat synchrony predicted cooperative success only during face-to-face play.
Nearly all of those earlier games placed both players on equal footing, making identical choices at the same time with matching stakes. Real-world trust is often unequal: one person puts resources at risk first without knowing the outcome, while the second decides whether to return the favor, known as reciprocity, or keep the windfall. Meanwhile, a 2024 review by two of the same authors argued that unconscious mimicry and synchrony evolved primarily to help individuals predict what others might do next under uncertainty, rather than strictly to build social warmth.
That distinction raises the question of how physiological synchrony functions when one partner bears all the vulnerability and the other holds the power to reciprocate. Diana and her colleagues tested whether seeing a partner strengthens physiological synchrony during unequal exchanges, and whether that synchrony predicts 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 (e.g., asking someone to look at your purse while you are going to the bathroom), 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 explained. “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?”
The researchers recruited 122 healthy young adults (56 women, average age about 21) at Leiden University and paired them into 61 same-sex pairs of strangers. Two pairs were excluded because one member showed almost no measurable sweat response, leaving 59 pairs (118 participants) for analysis. Materials, data, and scripts were posted publicly on the Open Science Framework; the paper did not report preregistering its hypotheses, where predictions and analysis plans are locked in beforehand, and distinguished planned tests from unplanned exploratory analyses.
Participants arrived separately and sat across a table divided by a movable overhead barrier, wearing headphones for audio instructions without speaking. Each pair played two six-round blocks of an economic trust game: one with the barrier raised for full face-to-face visibility and another with the barrier lowered to block their view, alternating the starting order across pairs.
In each pair, one player acted as the “trustor” and the other as the “trustee,” with both starting every round with 6 euros. Using a hidden dial, the trustor sent between 0 and 6 euros to the trustee. That amount was tripled, and the trustee used a hidden dial to return a chosen share of the tripled sum. Trustors received no feedback on how much their partner returned, keeping them reliant on live impressions, and players could earn a bonus of up to 2.50 euros.
Sensors on the fingers and torso recorded skin conductance and heart rate, while webcams filmed participants’ faces so software could track 17 facial muscle movements across more than 826,000 video frames. Eye-tracking glasses also recorded pupil size, but head movements during decisions degraded the recordings enough that pupil data had to be excluded. Skipping the first round of each block, the team split the physiological signals into overlapping eight-second windows to measure how closely partners’ responses aligned within a four-second lag.
Trustors invested an average of 4.14 euros out of 6 per round, and trustees returned an average of 61.6 percent of the tripled amount received, in line with other trust-game research. Comparing real pairs against randomly matched strangers showed that heart rate synchrony did not exceed chance in either setting, so analyses focused on skin conductance. Sweat synchrony was slightly higher face-to-face than behind the barrier, a difference the authors describe as smaller than in earlier studies. Partners’ response timing also clustered tightly around simultaneous responses during visual contact while scattering irregularly when blocked.
Video recordings from the remaining pairs, after setting aside nine pairs with camera malfunctions or unusable footage, indicated that visual contact altered facial behavior. Participants displayed more smiling and friendly expressions, conversational eyebrow movements, and mild signs of tension when face-to-face, whereas frowning and negative expressions occurred more often behind the barrier. Even so, overall facial expressivity was not a statistically significant predictor of skin conductance synchrony, and individual facial muscle movements showed minimal association with sweat synchrony.
For trustors, visual contact alone did not produce a statistically significant overall rise in investments, because block order mattered. Pairs starting behind the barrier invested more once the barrier rose and made their partner visible, whereas pairs starting face-to-face kept investing at similar levels after the barrier came down. The authors suggest that once trustors put a face to their partner, that impression carried over into later rounds. Skin conductance synchrony had no statistically significant link with trustors’ investments in either condition, and a follow-up Bayesian analysis, which weighs statistical evidence for a null result, offered moderate support that the two were unrelated.
The pattern for trustees was more complicated. Overall, trustees sent back a larger share of the tripled money when looking at their partner than when separated by the barrier, regardless of which block came first. Yet within the face-to-face block, stronger skin conductance synchrony predicted lower reciprocity, meaning trustees returned a smaller proportion of the pot when their sweat responses closely tracked their partner’s. When the barrier blocked visual contact, the relationship between synchrony and reciprocity was not statistically significant.
The direction of that result caught the research team off guard. “We expected that people whose bodies were more in sync would be more generous toward each other, and we found the opposite,” Diana said. “However, it prompted us to rethink the function of synchrony from a purely ‘affiliative’ tool to something different.”
To examine why sweat synchrony coincided with reduced generosity during visual contact, the researchers conducted an unplanned exploratory analysis looking at overall physiological arousal, indexed by average skin conductance. Trustors, who made blind choices under uncertainty, had higher overall arousal than trustees, who acted with full information. The negative link between face-to-face synchrony and reciprocity was strongest when the trustor showed low arousal, weaker at average trustor arousal, and not statistically significant when the trustor was highly aroused.
The finding that mutual visibility strengthens and stabilizes sweat synchrony echoes an earlier study of familiar pairs, which measured electrical brainwave activity through scalp sensors (electroencephalography) rather than skin conductance. In that work, brainwave synchrony emerged spontaneously whenever partners could see one another, with shared social behaviors such as eye contact and smiling predicting greater neural synchrony.
By contrast, the drop in generosity tied to sweat synchrony stands in tension with an earlier experiment on gratitude between partners. That experiment found that prompting feelings of gratitude increased mutual cooperation, forgiveness after a partner’s betrayal, physical coordination, and synchronized activity in brain areas involved in decision-making and movement. However, that earlier work tracked brain-to-brain synchrony after deliberately inducing a positive emotion, whereas Diana and her colleagues measured involuntary sweat responses during an unequal trust game where emotional states unfolded without prompting.
In the authors’ reading, the exploratory arousal pattern suggests that the state partners tune into matters as much as sweat synchrony itself. The researchers propose that low trustor arousal may reflect boredom or disengagement from making repeated blind choices, and that trustees who visually registered and mirrored that detached state felt less inclined to share. They also suggest that lacking feedback kept trustors from connecting gut feelings to partner fairness, while subtler cues like pupil shifts or slight facial blushing might transmit arousal 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 noted. “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, whether you like or trust them. The take-home message is: synchrony is not good in its nature, but its effect on the interaction depends on what emotional state we synchronize with.”
“The effects are small-to-medium, which is to be expected in this sort of setting,” Diana said of the effect sizes. “In a lab game between strangers, a lot of things shape behavior and physiology at once, such as personality, mood, or even how the person slept, so any single factor, like how in sync two bodies are, will only nudge decisions a little. Small effects are normal in this kind of research, and they are not a sign that nothing is happening, especially if other researchers in other experiments with similar settings have observed a similar pattern. But nobody should read this as a tool that predicts whether a person will be generous or not.”
There are a few things to keep in mind when interpreting these findings. Because physiological synchrony arose naturally rather than being experimentally manipulated, the study cannot establish that matching sweat responses caused trustees to return less money, and future preregistered studies will need to test whether partner disengagement actually accounts for reduced generosity. The study was also sized to detect effects of roughly the magnitude it found, so the reversal will need replication before it can be considered established.
“Please don’t conclude that ‘being in sync is bad’ or that it makes people less cooperative,” Diana cautioned. “Our finding about the role of arousal came from an exploratory analysis we didn’t plan, so it is a lead for future work and not a proven explanation.”
The laboratory setting also constrained natural behavior: strangers sat in silence, risked small bonus sums rather than personal savings, and never learned the outcome of each round. Heart rate synchrony never exceeded chance, and lost eye-tracking data prevented testing whether pupil changes contributed to sweat synchrony. Future studies that allow conversation, vary feedback, and capture cleaner eye-tracking recordings could help determine when shared bodily rhythms support cooperation and when they undermine it.
“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 said. “My next question is whether synchrony helps us predict what others will do next. When we fall into rhythm with someone, we may pick up subtle information about their intentions, emotions, and likely actions, sometimes before they say or do anything explicit.”
“In other words, synchrony could be part of what makes humans good social predictors,” she added. “It may allow us to anticipate whether someone is about to cooperate, withdraw, become aggressive, or offer support. So in my future research I would like to investigate the possibility that synchrony is not just something that happens between people; it may be a bodily mechanism through which we read, understand, and anticipate one another.”
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.
