By far the most important figure in the history of the concept of stress as applied to human behavior is Hans H. B. Selye (1907-1982), who is credited with starting modern research into the concept itself and its physiopsychological manifestations. In 1950, in addressing the American Psychological Association convention, Selye introduced his theory of stress reaction, which has since become the standard model of stress. In 1956, he published The Stress of Life, in which he elaborated his stress theory and perfected its definition of physiopsychological stress as “the nonspecific response of the body to any demand made upon it,” which could be a real or perceived threat, challenge, danger or change that requires the body and the mind to adapt.
Selye’s research made important contributions to psychology, biology and medicine. He had begun his work in 1926 when, as a second year medical student, Selye noticed many similarities among patients who, in spite of suffering from very disparate diseases, all seemed to have many symptoms in common. He would later describe this constellation of common symptoms as a syndrome, ‘‘the syndrome of just being sick.”
His discovery of the stress response was a byproduct of his research on the effect of hormone injections in rats. Noticing that many of the rats became sick after receiving the injections and that the same sickness struck a control group of rats injected only with a neutral solution containing no hormones, he had the intuition that the rats could be having a reaction to the trauma of being injected rather than to the hormones. Selye surmised that being handled and injected caused high levels of sympathetic nervous system arousal in the rats, which eventually developed health problems such as ulcers. Selye coined the term "stressor" to label the traumatic stimulus that acted psychologically on the mind while at the same time producing a physical effect.
In refining his theory of the stress reaction, which he dubbed the General Adaptation Syndrome, Selye identified three distinct stages. The alarm reaction is the first stage, which occurs when the organism first becomes aware of the stressor. In the second stage of recovery or resistance, the organism mounts a response to the stressor, by mobilizing resources, using energy and repairing itself. The third stage of exhaustion occurs if the organism is unable to put an end to the stressor. This third stage also signals the onset of chronic stress.
With remarkable insight, Selye sliced the concept of stress into four variants. These he called eustress (the good stress caused by positive and exciting stressors), distress (the harmful stress caused by unpleasant or negative stressors), hyperstress (caused by stressors so overwhelming that they overcome all abilities of the body to adapt), and hypostress (the mildest stressors that barely cause a physical and psychological reaction, while still being noticeable).