The maintenance, production, and transformation of stable organism-environment systems require an intricate organization of the organism's behavior. In man as in many other species, emotions are an essential part of the behavioral system. They play a crucial role as evaluative classifiers or categorizers of states and events, as signals to oneself or to conspecifics, all in the service of generalized appropriate action on the vagaries of an only partially predictable and manageable environment. Emotions are not just internal states. On the contrary, they should be viewed as internal reactions that an organism has learned to observe or detect within itself and within other organisms, in principle conspecifics and related species. Thus, the outward expression of emotions has important signal value. Especially in social species, where the maintenance of a successful organism-environment system depends on the regulation of action among members of a social group, emotional expressions are vital signals of the internal states of individuals.

A particular and biologically extremely important social bond is that between a young infant and its primary caregivers, simply because most neonates cannot survive without the care of their elders, and – in our own and related species – the care of the mother in particular.