Symmetry Breaking in Human Motion

Collective human movement is a hallmark of complex systems, exhibiting emergent order across contexts from pedestrian flows to biological collectives. In high-speed, directional settings, alignment ensures efficient navigation, whereas in low-speed, non-directional, socially engaged contexts, alignment arises from interpersonal interaction rather than locomotion goals. Using high-resolution spatial and orientation data from preschool classrooms, we uncover a sharp distance-dependent transition in pairwise alignment that reflects a spontaneous symmetry breaking between behavioral phases: below a threshold of ~0.65 m, side-by-side orientations dominate, while face-to-face orientations prevail at larger distances. This transition stems from a distance-dependent competition among three alignment mechanisms—parallelization, opposition, and reciprocation—whose interplay generates a bifurcation in the effective interaction landscape. Fourier decomposition of orientation distributions reveals these mechanisms, enabling a minimal pseudo-potential model that captures the transition as a non-equilibrium phase change. Monte Carlo simulations using the inferred interaction terms reproduce the empirical patterns. Our findings establish a quantitative framework for social alignment in low-speed human motion, extending active matter theory to a previously unexplored regime of socially mediated orientation dynamics, with implications for coordination and control in both biological collectives and artificial swarms.