When people talk about love, they often describe it in terms of “two hearts beating as one.” Although you might be tempted to think of this as nothing other than a cute saying, research suggests that when we love someone, our hearts literally do beat to the same rhythm.

Consider this: in a 2011 study published in the Proceedings of the National Academy of Sciences, researchers examined what happened to the heart rates of both observers and performers during a fire-walking ritual [1]. During rituals of this nature, people walk over a firey bed of coals one at a time while a crowd watches on, often as a test of faith or courage.

This study was conducted during the annual fire-walking ritual of a small Spanish village, in which a total of 12 fire-walkers participated, along with 28 spectators (9 of whom were either related to or friends with one of the fire-walkers, and 17 of whom had no relation to any of the fire-walkers). All participants wore heart rate monitors for the duration of the event. Results revealed that the heart rhythms of people walking over hot coals synchronized with those of the onlookers; however, this was only the case when the fire walker was a close friend or relative of the observer. In contrast, when the performer was unknown to the observer, their heart rates did not mirror one another.

How do we explain this pattern of results? Given that heart rate changes occurred among both relatives and friends in this study, it seems clear that genetics cannot fully account for the observed changes. In light of this, there would appear to be something deeper going on here.

As some additional evidence of this, consider the results of a 2012 study published in the journal Emotion, which examined what happened to the heartbeats of romantic partners who are gazing into one another's eyes [2]. In this study, 32 heterosexual couples were hooked up to heart and respiration monitors while they sat a few feet apart looking into each other's eyes without having any physical contact. During this task, both partners' heart and breathing rates because more similar. However, no such effect was observed when participants were asked to gaze into the eyes of a stranger.

The results of these studies suggest that, among people who share deep emotional attachments, they tend to share physiological states while in each other's presence. Although we cannot say with certainty why this occurs, the preferred theory is that we have evolved a tendency to mimic the body rhythms of close others in order to facilitate bonding. To the extent that this is true, it suggests that the heart may indeed deserve its status as the ultimate symbol of love after all.

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[1] Konvalinka, I., Xygalatas, D., Bulbulia, J., Schjodt, U., Jegindo, E. M., Wallot, S., … Roepstorff, A. (2011). Synchronized arousal between performers and related spectators in a fire-walking ritual. Proceedings of the National Academy of Sciences of the United States of America, 108(20), 8514-8519.

[2] Helm, J. L., Sbarra, D., & Ferrer, E. (2012). Assessing cross-partner associations in physiological responses via coupled oscillator models. Emotion, 12(4), 748-762.

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