Physical proximity, social familiarity, and acoustic environment modulate heart rate synchrony in real-world social interactions

Human social behaviour 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 (IPS) 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 IPS, we continuously measured heart rate, GPS, and the acoustic environment from 72 participants across three multi-day trips to New York City, capturing naturalistic social behaviour. Across all three trips, heart rates reliably synchronized when participants were in close physical proximity, indicating that shared environmental context was sufficient to elicit IPS. IPS was stronger among socially familiar peers, and context dependent, emerging during close-proximity interactions and joint attention to shared stimuli, but not dispersed interactions. IPS was also modulated by acoustic context, with low-to-moderate sound pressure levels and moderate-to-high signal-to-noise ratios enhancing synchrony, while excessive environmental noise reduced it to levels comparable to non-interactive settings, which may reflect lower levels of joint engagement in noisier environments. These findings demonstrate that IPS emerges in naturalistic social settings and is modulated by physical proximity, social familiarity, social context, and the acoustic environment, establishing IPS as a reliable marker of real-world social engagement.