A crazy thing happens when you put two strangers in a room and ask them to complete a task that requires trust: Their hearts start to beat as in sync.

That’s what Danish researchers found in a recent study. They randomly paired up undergraduate students and asked them to build cars out of Lego together.

Some of the pairs were also asked to complete what’s called a “public goods game.” Each was given the equivalent of about $12, and they were each given the option of putting all, some, or none of that money in a collective pot, without knowing what the other would do. The money in the pot would then be multiplied by a factor of more than one but less than two, and then split between the two players.

The point of this game is that both players are better off if they both put all their money in, but a player working in his own self-interest would be better off betting nothing. You would only put your money in the pot if you’re fairly sure your partner is going to do the same.

The research found that when students were asked to put money on the line and trust each other, their heart rates sped up — and synced up. They trusted each other: They put more than 80 percent of their money in the communal pool on average, and believed that their partner would do the same.

Would you trust a stranger?

The finding came as a surprise to the researchers. “This is the first time that anyone has shown that trust between two people can be seen in heart rhythms and we have no idea why it happens,” co-author Panagiotis Mitkidis told ScienceNordic.

What could possibly explain it? Hormones like oxytocin have a role to play in regulating bonding and trust, the researchers suggest, although their function is poorly understood.

Do you trust me?

But there’s evidence that when our bodies act together, our brains do, too. Some research suggests that group singing, clapping, and dancing makes individuals more likely to self-sacrifice for the greater communal good. These synchronized activities can weaken psychological borders between the self and the group, resulting in a sort of hive mind.

Perhaps our synchronized hearts are just a subconscious extension of the same phenomenon? In moments when we require trust from another human, our heart literally yells out to the other. “Do you trust me?”

“I’m here. I trust you. We can do this, together.”