A study that observed participants on a three-day trip to New York City found that heart rates synchronized when people were physically close. Synchrony was higher among socially familiar peers, manifested when they were physically close to each other and paying attention to the same things. The paper is PNAS Nexus.
When two people interact with each other, some of their movements and physiological processes tend to synchronize. This synchronization of physiological processes is called interpersonal physiological synchrony. This may include similarities in heart rate, respiration, skin conductance, hormonal activity, brain activity, or other physiological signals.
Synchrony can occur during conversation, sharing emotional experiences, cooperation, conflict, physical contact, or joint activities such as music or dance. This does not necessarily mean that people have identical physiological responses, but rather that responses change in a related pattern over time.
Research measures interpersonal physiological synchrony by continuously recording physiological signals from multiple people and examining whether the variations are statistically related. Greater synchrony may be associated with empathy, emotional connection, trust, cooperation, and relationship satisfaction. However, conformity is not necessarily a sign of a positive bond, as it can also occur during stressful or hostile interactions. Some synchrony may result from shared environmental stimuli, such as both people reacting to the same event, rather than from direct interpersonal influences.
Study author Hanlu He, a researcher at the Technical University of Denmark, and colleagues point out that measuring interpersonal physiological synchrony requires continuous monitoring of physiological processes and has thus far been studied primarily in laboratory conditions. These researchers wanted to examine whether and how interpersonal physiological synchrony emerges during everyday social interactions outside of a laboratory setting.
They also wanted to know whether synchrony depends on social affiliation, that is, how psychologically close individuals are to each other. To study this, they decided to compare the level of heart rate synchronization between two sets of people: those who are socially connected to each other and those who are not, while controlling for physical proximity. Their hypothesis was that heart rates tend to synchronize when people are physically close, and that this synchronization depends on the type of activity they engage in.
The participants in the study were 72 students between the ages of 19 and 32. Of those, 60 were men. The study authors recruited them through three sessions of the Audio Explorer Challenge, a competition sponsored by hearing aid company Oticon. They entered the competition in pre-formed teams of two to five individuals, and the winning team was awarded a place on a study trip to New York City. In total, these participants participated in three 4-day training trips to New York City (23, 24, and 25 participants per trip).
During the trip, participants participated in a combination of pre-planned group activities and unstructured free time, allowing for extensive real-world social interaction. During this time, the study authors continuously collected data about the participants and their environment from 9 a.m. to 10 p.m. every day.
Every 20 seconds, the hearing aid participant wore a recorded estimate of the surrounding sound environment. Participants also wore wristbands that measured their heart rates. In addition, participants carried smartphones that recorded GPS location data, allowing study authors to determine participants’ distances.
Participants’ social intimacy was estimated through group membership. Participants who participated in the Audio Explorer Challenge as members of the same team were considered familiar, whereas participants who were not part of the same team were considered unfamiliar with each other.
They found that across all three trips to New York City, participants’ heart rates tended to sync when they were physically close. The researchers interpreted this to mean that a shared environmental context was sufficient to elicit synchrony. Synchrony tended to be higher among participants in the same group (socially familiar peers), especially when they were jointly paying attention to the same thing during close interactions, such as watching a lecture.
Heart rate synchronization also depends on the sound environment. The study authors reported that synchrony was higher when the noise level in the environment was low to medium and the signal-to-noise ratio was medium to high (when the sound being attended to could be clearly heard). When the environment was very noisy and noisy, synchrony decreased.
“These findings demonstrate that interpersonal physiological synchrony emerges in naturalistic social settings and is modulated by physical proximity, social familiarity, social context, and sound environment, establishing it as a reliable marker of real-world social engagement,” the study authors concluded.
This study contributes to scientific knowledge about physiological synchrony in interpersonal relationships. However, it should be noted that the study authors were unable to determine exactly how each participant interacted when physically close to the other participants. Therefore, it remains unclear whether it was physical proximity itself or similar activities that the participants were engaging in that caused interpersonal physiological synchrony. Additionally, it remains unclear whether the differences in synchrony seen in different sound environments are caused by the sound environment itself or by the types of activities typically performed in different sound environments.
The paper, “Heart rate synchronization as a marker of real-world social engagement,” was authored by Hanlu He, Jeppe H. Christensen, A. Josefine Munch Sørensen, and Ivana Konvalinka.
