Three adults with developmental disabilities and severe, chronic self-injurious behavior (SIB) were exposed in separate studies to multi-element functional analog experimental and control conditions. Participants' physiological status, primarily cardiac function, was recorded with a sophisticated computerized ambulatory monitoring garment (VivoMetrics 1 LifeShirt system). Results revealed a reliable and consistent heart rate (HR) pattern in all conditions, consisting of an escalating HR immediately before SIB occurrence, and quickly followed by a temporary drop in HR level occasioned by SIB action. This waveform was not an artifact of movement, HR level, respiration, posture, or SIB topography. This discovery suggests that the fundamental function of SIB is an escape from autonomic arousal. This function is consistent with other physiological and clinical findings.