Amplitude without phase: interpersonal movement coordination grades inter-brain coupling

Interpersonal movement coordination is a foundational feature of human sociality, scaffolding communication, affiliation, and joint action from infancy onward. Whether this behavioral coupling is accompanied by genuine coupling between brains, rather than one manufactured by shared sensory input or motion artifact, has proven difficult to establish. We recorded dual-brain EEG with synchronized head-mounted accelerometry from participant-partner dyads (16 dyads, 155 dyad-condition recordings) across six conditions grading interpersonal coordination from stillness to fully reciprocal joint movement, using improvisational dance to elicit sustained, naturalistic, gradable coordination. Windowed cross-correlation confirmed graded, partner-specific movement coordination, including a lead-lag signature that reversed with who led. Against a pseudo-dyad surrogate, inter-brain synchrony took one specific form: amplitude-envelope coupling in the high-beta and gamma bands, robust to a leakage-corrected measure, with no phase-based or slow-band coupling. Within dyads, movement synchrony predicted the strength of this amplitude coupling (standardized beta = 0.54 in gamma), an association that strengthened monotonically with frequency, held under a dyad fixed-effects estimator, and was undiminished when the amount of movement was entered into the model. Movement synchrony and inter-brain coupling also co-fluctuated within interactions, above surrogate. Two control analyses excluded the leading alternatives: the coupling was not sensorimotor rhythm-tracking, since corticokinematic coherence was present but non-localized and not movement-selective, and not a shared movement-frequency rhythm linking the brains. These results identify interpersonal movement coordination as an organizing variable for inter-brain coupling and specify that coupling as fast-band amplitude co-modulation, not phase synchrony. Brains couple when bodies coordinate, not when they merely move together.