In 1992, G. P. Chrousos and P. W. Gold defined stress as a state of threatened homeostasis, or instability to which a person (or animal) reacts with an adaptive response to preserve its internal equilibrium. Mild, brief, and controllable challenges could be perceived as pleasant or exciting and could be a positive input for emotional and intellectual development. However, as defined by H. Selye in 1976, more intense, persistent, and uncontrollable situations of threat or perceived threat may lead to maladaptive responses. In humans, these maladaptive responses include the development of mood disorders.