How two people's heartbeats become synchronized, even when they are far from each other

In a small Spanish town, people celebrate the summer solstice by walking over beds of hot coals. A team of scientists wondered how this fire-walking ritual could enhance group bonding, and discovered something extraordinary.

Led by Danish neuroscience researcher Ivana Konvalinka, the scientists wanted to measure the excitement levels of participants, so they attached portable heart monitors to several fire-walkers, their family members, and other people in the crowd. When the fire-walkers stepped onto the coals, their heart rates spiked with excitement and fear. This isn't very surprising. What was surprising was that their family members' heart rates spiked at exactly the same time.

People unrelated to the firewalkers didn't experience any change in their heart rates, so the shared heart rate is likely a phenomenon that occurs between people who are already emotionally connected.

What Konvalinka and her colleagues believe, after analyzing a dozen fire-walkers and their loved ones, is that rituals like this promote social bonding via shared biological states. Previous studies attributed feelings of group bonding to shared movements, like those you find in dances and some religious services. But Konvalinka's research suggests that people can share the same physical sensations even when they are not moving as one. Scientists call it "synchronized arousal."

How two people's heartbeats become synchronized, even when they are far from each other

In a paper published yesterday in the Proceedings of the National Academy of Sciences, Konvalinka and her colleagues wrote:

We hypothesized that synchronized arousal even in the absence of synchronized action might be one mechanism responsible for these social effects of collective rituals. These effects have never been quantitatively investigated in a natural setting. The present findings show synchrony over time of bodily arousal between active participants with their related onlookers, a social modulation that is amplified in the ritual itself. A qualitative look at the data showed that the fire-walkers' heart rates had a distinctive "signature," as seen across all 12 firewalkers, with a high peak distributed around the walk itself. The same pattern was seen with related spectators, whose heart rates peaked for the walk of their relatives and friends.

So what causes this peculiar ability to influence our loved one's heart rates? The researchers say it's simply caused by what amounts to sympathy. Knowing a loved one is doing something potentially dangerous causes our hearts to race - just as our loved one's heart is racing in response to facing that danger.

Konvalinka and her fellow researchers were excited to discover this new facet of synchronized arousal because it allows for "the quantification of social effects in human physiology." In other words, we can measure just how bonded you feel to people around you. Now you can look forward to the day when you'll have to wear a heart monitor during trust exercises at work, to make sure you're bonding with everybody properly. Or perhaps, before you get married, you'll have to watch your fiancee do something dangerous and prove with your heart rate that you are fully committed to them.

Read the full scientific article via PNAS

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