We describe the Cacioppo Evolutionary Theory of Loneliness (ETL) and its manifestations in contemporary society. The early conceptualizations of loneliness were as an individual difference characterizing a relatively small subset of the population. The ETL characterizes loneliness as not simply addressing an individual difference, but also as addressing the effects of loneliness on people generally. The progression motivated by the ETL to animal models and comparative analyses broadens the focus further to periods long before hominids evolved. The premise underlying our ETL is that an organism’s perception of being socially isolated (i.e., lonely) automatically signals an environment in which the likelihood is low of encountering social behaviors categorized in terms of evolutionary fitness as mutual benefit or altruism. As a result, the likelihood is high of the organism exhibiting behaviors categorized in terms of evolutionary fitness as selfish. This shift in the fitness consequences of behavior is posited to be evolutionarily old and to operate in humans in part through nonconscious processes. The ETL addresses the adaptive functions of loneliness that foster short-term survival but that in the modern world can have deleterious long-term consequences. In doing so, the ETL places the social level of organization front and center in scientific investigations of the human brain and behavior. The centrality of the social world highlighted by the ETL is not attributed to social construction but to social and biological processes, including evolutionary forces operating across social species long before humans walked the earth.
REad also: On Loneliness and Companionship