People in close proximity to each other synchronize their heart rates

A study observing participants across three multiday trips to New York City found that people’s heart rates become synchronized when they are in close physical proximity. Synchrony was higher among socially familiar peers, and it emerged when they were in close physical proximity to each other and paying attention to the same things. The paper was published in PNAS Nexus.

When two people interact with one another, their movements and some of their physiological processes tend to become synchronized. This synchronization of physiological processes is called interpersonal physiological synchrony. It may involve similarities in heart rate, breathing, skin conductance, hormonal activity, brain activity, or other physiological signals.

Synchrony can occur during conversation, shared emotional experiences, cooperation, conflict, physical contact, or joint activities such as music and dance. It does not necessarily mean that people have identical physiological responses, but rather that their responses change in related patterns over time.

In research, interpersonal physiological synchrony is measured by continuously recording physiological signals from several people and examining whether fluctuations are statistically linked. Greater synchrony has sometimes been associated with empathy, emotional connection, rapport, cooperation, and relationship satisfaction. However, synchrony can also occur during stressful or hostile interactions, so it is not always a sign of positive bonding. Some synchrony may result from shared environmental stimuli, such as both people reacting to the same event, rather than from direct interpersonal influence.

Study author Hanlu He, a researcher at the Technical University of Denmark, and colleagues note that because measuring interpersonal physiological synchrony requires continuous monitoring of physiological processes, it has mainly been studied under laboratory conditions thus far. These researchers wanted to examine whether and how interpersonal physiological synchrony emerges during everyday social interactions outside laboratory settings.

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They also wanted to know whether synchrony depends on social affiliation—how psychologically close individuals are to each other. To study this, they decided to compare heart rate synchrony levels between dyads of people with social ties to each other versus those without, while controlling for their physical proximity. Their hypothesis was that people’s heart rates would tend to become synchronized when they are in close physical proximity and that this synchronization would depend on the type of activity they engage in.

Study participants were 72 students between 19 and 32 years of age. Of those, 60 were men. Study authors recruited them through three editions of the Audio Explorer Challenge, a competition hosted by the hearing aid company Oticon. They entered the competition in preformed teams consisting of 2 to 5 individuals, with the winning teams rewarded with participation in a study trip to New York City. In total, these participants took part in three 4-day study trips to New York City (23, 24, and 25 participants per trip).

During their trips, participants engaged in a combination of preplanned group activities and unstructured free time, enabling a wide range of real-world social interactions. During these times, study authors continuously collected data about the participants and their environment between 9 a.m. and 10 p.m. each day.

Every 20 seconds, the hearing aids participants wore recorded estimates of the surrounding sound environment. Participants also wore wristbands that measured their heart rates. Additionally, participants carried smartphones that recorded their GPS location data, allowing study authors to determine their proximity.

Participants’ social familiarity was estimated through group membership. Participants who participated in the Audio Explorer Challenge as members of the same team were considered familiar, while those who were not on the same team were considered unfamiliar with each other.

Results showed that, across all three trips to New York City, participants’ heart rates tended to become synchronized when they were in close physical proximity. Researchers interpreted this to mean that shared environmental context was sufficient to elicit synchrony. Synchrony was higher among participants from the same group (socially familiar peers) and it tended to emerge during close-proximity interactions, particularly when they were jointly paying attention to the same thing, such as watching a lecture.

Heart rate synchronization also depended on the sound environment. Study authors report that synchrony was higher when sound levels in the environment were low-to-moderate, and when signal-to-noise ratios were moderate-to-high (when people could clearly hear the sounds they were paying attention to). When the environment was very loud and noisy, synchrony was reduced.

“These findings demonstrate that interpersonal physiological synchrony emerges in naturalistic social settings and is modulated by physical proximity, social familiarity, social context, and the sound environment, establishing it as a reliable marker of real-world social engagement,” the study authors concluded.

The study contributes to the scientific knowledge about interpersonal physiological synchrony. However, it should be noted that study authors could not precisely identify the type of interaction each participant was engaged in when in close physical proximity to others. It therefore remains unknown whether it was physical proximity itself or the similar activities participants were performing that drove interpersonal physiological synchrony. It also remains unknown whether the differences in synchrony found in different sound environments were driven by the sound environments themselves or by the types of activities that are typically performed in different sound environments.

The paper, “Heart rate synchrony as a marker of real-world social engagement,” was authored by Hanlu He, Jeppe H. Christensen, A. Josefine Munch Sørensen, and Ivana Konvalinka.