Ethological strategies for defence in animals and humans are expressed as either aggression or flight behaviour. Aggression is employed by animals during intraspecific competition for resources, mate, territory and acquiring and maintaining social status. It also disperses individuals throughout the biotope. Flight behaviour is used to avoid a source of danger or harm, has both dynamic and static forms, is phylogenetically very old and takes precedence over all other activities including social behaviour. Animals exposed to inescapable threats or attacks exhibit a characteristic defensive strategy, arrested flight, which consists of gaze-avoidance or cut-offs, cryptic postures such as immobility and covert surveillance of their surroundings. Arrested flight also occurs in social encounters when submission fails to reduce attacks, and in prey animals when escape from a predator is hampered. Ethological studies show that during interviews, depressed patients exhibit a pattern of non-verbal behaviour having all the hallmarks of arrested flight. Cut-off behaviour, which seeks to reduce the input of flight-evoking stimuli is especially evident in these patients but takes an extreme form, i.e. eye closure, in the gaze-profiles of paranoid patients. It is proposed that cut-offs always denote the presence of incipient flight and that arrested flight is a ‘last measure’ defensive strategy in response to inescapable proximal threat. It can arise in humans whenever their escape routes are hampered and characterizes the behaviour of patients suffering from depression. As in animals, different pathways may lead to arrested flight in humans. In humans, defensive mechanisms also operate at the mental level through putative ego defences, the psychological function of which is to preserve self-esteem by hindering the access of disturbing emotional material into awareness. It is suggested that they function ethologically as mental cut-offs analogous to the behavioural cut-offs in animals.