Evolutionary theory and cross-species comparisons are explored to shed new insights into behavioural responses to traumatic entrapment, examining their relationships to the Stockholm syndrome (a specific response to traumatic entrapment) and complex post-traumatic stress disorder (PTSD). A selective literature review is undertaken examining responses to traumatic entrapment (including hostage, domestic abuse and similar situations) and the Stockholm syndrome, before examining mammalian, reptilian and other defensive responses to relevant threats. Chimpanzees, the closest relatives of humans, are closely examined from this perspective and commonalities in behavioural responses are highlighted. The neurobiological basis of defensive behaviours underlying PTSD is explored with reference to the triune brain model. Victims of protracted traumatic entrapment under certain circumstances may display the Stockholm syndrome, which involves paradoxically positive relationships with their oppressors that may persist beyond release. Similar responses are observed in many mammalian species, especially primates. Ethological concepts including dominance hierarchies, reverted escape, de-escalation and conditional reconciliation appear relevant and are illustrated. These phenomena are commonly encountered in victims of severe abuse and understanding these concepts may assist clinical management. Appeasement is the mammalian defence most relevant to the survival challenge presented by traumatic entrapment and appears to be the foundation of complex PTSD. Evolutionary perspectives have considerable potential to bridge and integrate neurobiology and the social sciences with respect to traumatic stress responses.