You might not think your sweaty palms are your most attractive feature, but they – along with the cadence of your heartbeat – could be a physical manifestation of your true desire, a new study suggests.

In a dating experiment conducted in the Netherlands, researchers recruited heterosexual participants to take part in real-life 'blind dates', in which they would meet a prospective partner in a 'dating cabin', while wearing various sensors designed to measure the physiological dynamics between the pair.

These sensors included eye-tracking glasses with embedded cameras to determine what each participant looked at during the date, as well as recording how they laughed, smiled, and otherwise acted with their partner.

At the same time, each participant also wore sensors to monitor their heart rate, and sensors to track skin conductance (aka electrodermal activity), measuring how perspiration on the skin changes in response to psychological or physiological arousal.

Ultimately, the researchers were looking for signs of physiological synchrony – the hidden choreography of mutual, non-verbal cues that can emerge when people connect with one another, or share an experience together.

"We hypothesize that, if a gut feeling of attraction truly exists… there must be a physical manifestation of interpersonal attraction in the real world of behavior," the researchers, led by first author and psychologist Eliska Prochazkova from Leiden University, explain in their paper.

Here, the team thought those manifestations might show up in a range of overt synchronous behaviors, with attracted participants mimicking or reciprocating their partners' physical expressions, such as smiles, laughter, eye gazing, nodding, and other gestures.

But it didn't turn out that way.

"None of these signals predicted the extent to which one person was attracted to another," Prochazkova explains. "It's the invisible, internal signals such as heart rate and skin conductance that determine this."

In each blind date, the participants sat on opposite sides of a table with a barrier between them, which opened for a few seconds so they could see each other, before closing again, at which point the participants would rate how attracted they felt to the other participant.

After this, the barrier opened again, and the participants got to talk freely for a couple of minutes (followed by another discreet attraction rating), and then were instructed to look at one another without talking for another 2 minutes (and then rating them once more).

All the while, during this series of interactions, the eye-tracking glasses recorded their exchanges, while the electrocardiographic and electrodermal sensors monitored their heart rate and skin conductance.

As it turns out, those latter measures are where all the action is, the researchers say.

"We found that if the dates were attracted to their partner, their heart rate synchronized with that of their date," Prochazkova says.

"If one person's heart rate increased, then the other's did too. And if their heart rate decreased, so too did the other's."

According to the researchers, perspiration changes in the skin followed the same kind of syncing pattern, even while explicit gestures such as laughing and smiling did not, which may highlight the importance of subconscious physiological coupling in the development of romantic attraction, the team suggests.

It's possible, the team says, that more overt forms of physically mimicking one another's gestures and behavior – laughing together at the same joke, for example – only represent a superficial level of synchrony.

By contrast, far subtler 'microexpressions' could convey and reflect a deeper, more emotionally resonant connection between people, although the researchers acknowledge there's much we don't yet understand about this psychological phenomenon – nor whether synchrony causes attraction, or vice versa.

Of course, it bears to keep in mind this sort of blind date in a controlled cabin is an artificial setup, although for studies like these, that's exactly the point. In fact, the team points out that future studies would want to go even stricter.

"Since this is one of the first studies that attempted to detect attraction using real-life eye-tracking and physiological measures, we advise researchers to replicate our findings in even more controlled laboratory setting, ideally with a larger sample, before attempting to use these measures in the field," they write.

For now, all we really know is that when two people come together, much goes unspoken. But on a mysterious wavelength, we sometimes connect with a special someone.

"Crucially, our findings imply that, on the dyad level, the interacting partners' physiological states synchronize into mutual alignment on a moment-by-moment basis," the researchers write.

"During these moments, a joint mental state potentially facilitates the feeling of a 'click' and attraction."

The findings are reported in Nature Human Behaviour.