Nonlinear coupling dynamics within and between brains during romantic kissing as compared to joint passive tasks

Abstract Interpersonal interaction shapes brain dynamics in ways that often elude traditional linear analyses. Using electroencephalography (EEG) hyperscanning and nonlinear dynamical methods, we investigated how neural complexity and coupling within and between brains differ between joint and solitary conditions across four ecologically valid contexts: romantic kissing, resting state, pendulum observation, and music listening. Fourteen romantic couples completed each task while EEG was recorded simultaneously. Linear mixed-effects models showed that joint conditions generally enhanced inter-brain coupling, with kissing producing the strongest effects—combining elevated coupling with increased complexity. Even in resting state, with eyes open or closed, coupling increased in the absence of external stimuli, suggesting internally driven partner-oriented synchrony. Chaotic pendulum motion selectively boosted mutual synchrony, likely via heightened attentional engagement, while music listening effects varied with musical style, sex, and the frequency range of spectral estimates of nonlinear measures. Psychological ratings of romantic bond, relational dominance, and kiss quality exhibited nonlinear, often U- or V-shaped, associations with neural measures. These findings extend social neuroscience by demonstrating that nonlinear hyperscanning approaches capture flexible “hyper-brain” network dynamics tuned to the affective, cognitive, and sensorimotor demands of real-world interaction.