Hans Selye, who became one of the world's leading experts on stress, made a foundational discovery in the 1930s while working at McGill University. His research, originally focused on sex hormones in rats, took an unexpected turn when he observed that prolonged exposure to various negative stimuli—including extreme cold, surgical injury, excessive exercise, and shock—produced a consistent set of physiological changes. These changes included adrenal enlargement, shrinkage of the thymus and lymph nodes, and stomach ulceration. This accidental finding led him to identify and develop the theory of the General Adaptation Syndrome.