Why and how does psychological trauma harm people? The traditional assumption in trauma research has been that extreme fear is at the core of post traumatic responses to events like war and natural disasters. Is terror the only cause of harm? Some patterns of events (such as sexual abuse by a parent, acquaintance rape, or government mistreatment of citizens) generate strong symptoms of trauma even absent intense fear, perhaps because they involve social betrayal. Betrayal trauma theory, drawing on developmental, cognitive, and evolutionary psychology, posits that a) there is sometimes a social utility in remaining blind to betrayal and b) betrayal traumas can be particularly toxic. The talk will describe betrayal trauma theory and our research on the relationship of betrayal trauma to gender, memory, and physical and mental health. This research suggests that betrayal and social relationships must be part of a unified theory of traumatic stress.