Watching a friend or relative in a stressful situation can loosely synchronize both of your heart rates, experiments at a fire walking ritual suggest.

In the experiment, when a spectator observed a relative or friend walk across hot coals, both the onlooker and performer's heart rates changed at the same time, though they didn't match each other beat for beat.

Past studies have observed that sports fans' hearts race when their teams score, but no one had yet delved into the physical effects on both spectators and participants, according to study co-author Ivana Konvalinka, a Ph.D. student at the Center of Functionally Integrative Neuroscience at Denmark's Aarhus University.

The finding suggests social bonds between people may manifest themselves even more powerfully than thought, Konvalinka noted.

The results show that "we can find markers of emotional connectedness in bodily measures as well—it's not just a cognitive effect," Konvalinka said.

Konvalinka suspects synchronization may occur between friends or relatives during other stressful or emotional events, such as weddings.

A Fire Walking First

For the experiment, Konvalinka and colleagues attended an annual fire walking ritual in the rural Spanish village of San Pedro Manrique (map).

The team put heart rate monitors on 12 fire walkers, 9 spectators who were relatives or friends of at least one fire walker, and 17 spectators with no connection to the fire walkers.

Advanced statistical analyses revealed that the heart rates of relatives and friends followed similar patterns as those of the performers. No such effect existed in onlookers who did not know the performers.

It's unknown how this mechanism actually occurs, noted Konvalinka, whose study appeared this week in the journal Proceedings of the National Academy of Sciences. For instance, part of the phenomenon may be because related people have similar heart rates. However, this is not enough to explain the entire effect, she said—for example, why similar effects were seen in friends.

Synced Heartbeats "New Avenue" for Bonding

The discovery that people's hearts can harmonize solely on visual or auditory information reinforces a law of nature, according to Michael Richardson, a psychologist at the University of Cincinnati in Ohio.

The natural law of coupled oscillators holds that when two or more rhythms meet, they will become coordinated—a phenomenon seen across the natural world, from fireflies matching their flashes to groups falling into step.

"We like to think, as we move through our world, we're this isolated being," Richardson said. This study, as well as a decade of laboratory work, has "demonstrated this is not the case."

Research has also shown this "social entrainment" helps keep our relationships healthy and may even reduce prejudice, he noted.

Ritual expert Richard Sosis added, "This study is opening up an entirely new avenue of research that will help us understand how people bond.

"Anthropologists have long appreciated that ritual binds people together, but it is unclear how this bonding is achieved," said Sosis, an anthropologist at the University of Connecticut.

"The primary assumption is that [group] activities such as communal dancing and singing and shared body movements would bind people together.