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No single construct has been more central in the development of the biologic sciences than homeostasis. Although the term homeostasis was coined in the 20th century, its conceptual origin can be traced to the notion of a stable, relatively unchanging internal environment, which was first described by Claude Bernard in the 19th century. Bernard recognized the fragility of life, surrounded as it is by a constantly threatening, aversive, and often pathogenic environment, and he argued that viability in the face of external challenge depends on an organism’s capacity for protecting its internal milieu. In this context, homeostasis is a dynamic, self-regulating process that ensures constancy and permanence in the internal physiologic state through complex, multilevel feedback systems that respond to a deviation in one direction with a countering adjustment in the opposite direction. Thus, the fundamental goal of a homeostatic system is to maintain an inerrant “set point” that assures stable and continuous biologic functioning. The regulation of body temperature, cortisol suppression of adrenocorticotropic hormone (ACTH) secretion, and glycogenolysis during periods of hypoglycemia are all examples of feedback loops that protect the continuity and equilibrium of an organism’s interior.

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Whereas homeostasis governs regulatory strategies within the tissue, cell, or subcellular structures, the closely related concept of adaptation refers to the behavioral and biologic activities that promote the survival of individuals or groups. In evolution, adaptation involves the selective preservation and reproduction of organisms and characteristics of organisms that offer survival benefits in the face of external threats.1 Beyond evolution, however, adaptation has been used to describe complex social and individual developmental processes that respond to specific environmental challenges. For example, daytime continence emerges in a 3-year-old child within a context of growing parental expectations for toilet training; a preschooler clings to a tattered but revered blanket (a so-called transitional object) to calm his or her uncertainties and fears about attending a new childcare center; and a 12-year-old girl exhaustively discusses her first menstrual period with friends as a means to cope with the complications and challenges of sexual maturation. At all stages of development, the capacity to weather, absorb, and find meaning in the vicissitudes of life is one of the defining characteristics of humankind.

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Many novel, challenging life experiences are commonplace and are accessible to a range of homeostatic and adaptive strategies. However, children also encounter circumstances that strain their adaptive capacities and may present acute or chronic stressors that exceed their ability to cope. Indeed, psychosocial stress has been defined as those environmental demands or threats that overtax an individual’s ability to adapt. When such conditions are encountered, a variety of biologic and behavioral responses are evoked; if sufficiently intense or prolonged, such responses can lead to the development of a diagnosable disorder.

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Research in humans suggests that 2 principal, interrelated systems are involved in the neurobiologic response to stress: (1) the corticotropin-releasing hormone system, and (2) the locus ceruleus–norepinephrine system.2

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