Hypothalamic-pituitary-adrenal responses to brief social separation.

Involuntary separation from close social companions is widely held to lead to pathophysiological outcomes. Presumably, the relationship with, or category of, the separated individual determines the nature of the physiological response. Here, experiments examining the consequences of brief involuntary separation on the activity of the stress-responsive hypothalamic-pituitary-adrenal (HPA) system are reviewed. Only those studies designed specifically to assess the effect of the absence of the social partner are considered. Evidence for HPA activation in response to social separation has been obtained in a number of species; yet, many studies find no effect of separation of affiliative partners on HPA activity. The occurrence of an HPA response to separation does not appear to be related to the phylogenetic position or cognitive capacity of the species studied, nor is it a universal response to mother-infant separation. Rather, it is suggested that the pattern of results can be largely understood in the context of attachment. Separation of partners exhibiting signs of emotional attachment leads to an immediate and persistent HPA response, whereas separation of partners that are affiliative, but do not exhibit attachment, has little or no effect on HPA activity.