Social defeat is a concept used in the study of the physiological and behavioral effects of hostile interactions among either conspecific animals, or humans, in either a dyadic or in a group-individual context, potentially generating very significant consequences in terms of control over resources, access to mates and social positions. Background Research on social stress has accumulated a useful body of knowledge, providing perspective on the effects of detrimental social and environmental interaction on the brain. Research and experimentation suffer from many methodological difficulties: usually a lack of ecological validity (similarity with natural conditions and stressors) or are not amenable to scientific investigation (difficult to test and verify). Social psychology approaches to human aggression have developed a multitude of perspectives, based on observations of human phenomena like bullying, mobbing, physical and verbal abuse, relational and indirect aggression, etc. Despite the richness of theories developed, the body of knowledge generated has not satisfied scientific requirements of testability and verifiability. Animal studies of within-species aggression developed in 2 main branches: A) approaches based on laboratory experiments, on controlled conditions, allowing the measurement of behavioral, endocrine and neurological variables, but with the shortcoming of applying unnatural stressors (such as foot-shocks and restraint stress) in unnatural conditions (laboratory cages rarely approximate native habitats); B) approaches based on observations of animals in naturalistic settings, which avoided artificial environments and unnatural stresses, but usually not allowing the measurement of physiological effects or the manipulation of relevant variables. In real life situations, animals (including humans) have to cope with stresses generated within their own species, during their interactions with conspecifics, especially due to recurrent struggles over the control of limited resources, mates and social positions (Bjorkqvist, 2001; Rohde, 2001; Allen & Badcock, 2003). Social defeat is a source of chronic stress in animals and humans, capable of causing significant changes in behaviour, brain functioning, physiology, neurotransmitter and hormone levels, and health (Bjorkqvist, 2001; Rohde, 2001; Allen & Badcock, 2003). History The social defeat approach was originated from animal experiments, using the "resident-intruder" paradigm, in which an animal was placed in the cage of another animal or group of animals of the same species, in a manner that allowed a non-lethal conflict. It has been documented to produce anxiety-like and depressive-like behavioral declines in susceptible mice, for instance. If animals are allowed to fight on a single occasion only, it is usually regarded as a model of acute