The two-body problem in interpersonal distance:A process model of social space regulation in psychopathology.

Interpersonal distance (IPD), the physical proximity individuals maintain during face-to-face interaction, is disrupted across a range of disorders involving distorted or threat-biased body representation, from eating disorders to social anxiety and schizophrenia. The dominant mechanistic account links this disruption to peripersonal space (PPS), modeling IPD as the output of the perceiver's own defensive boundary. This account is egocentric: it treats the approaching person as a source of threat rather than as a body whose size, kinematics, and spatial requirements must themselves be represented. IPD regulation, however, is a two-body problem that no current single-body model addresses. Drawing on recent evidence that the spatial properties of another person's body enter the same sensorimotor system that codes one's own PPS, the present article extends the body-representation framework of Möllmann et al., (2024) into a process model of IPD regulation. The model specifies how self-body and other-body representations are coupled across action-guiding, metric, and evaluative stages within a continuous control loop, and proposes how this coupling could render otherwise contradictory findings (most notably the inconsistent IPD effects in anorexia nervosa) more tractable. I further argue that markerless multi-person motion capture is a suitable platform for measuring this process, and outline a research agenda that moves beyond static stop distances toward the approach-avoidance kinematics through which body-representation-based IPD dysregulation is most directly expressed.