Heart rate synchrony as a marker of real-world social engagement.

Human social behavior unfolds in complex real-world environments influenced by social and environmental factors, yet reliable markers of social engagement and connection remain elusive. Interpersonal physiological synchrony has been proposed as one such marker, but its occurrence in everyday settings is not well established. To investigate the social and environmental factors that influence physiological synchrony, we continuously measured heart rate (HR), GPS, and acoustic features of the sound environment from 72 participants across three multiday trips to New York City, capturing naturalistic social behavior. Across all three trips, HRs reliably synchronized when participants were in close physical proximity, indicating that shared environmental context was sufficient to elicit synchrony. Synchrony was higher among socially familiar peers, and context dependent, emerging during close-proximity interactions and joint attention to shared stimuli, but not dispersed interactions. It was also modulated by the sound environment. Periods with low-to-moderate sound pressure levels and moderate-to-high signal-to-noise ratios were associated with increased synchrony, while periods with excessive environmental noise were related to reduced levels comparable to noninteractive settings, which may reflect lower levels of joint engagement in noisier environments. 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.