Within the Cognitive Activation Theory of Stress (CATS), the stress response occurs whenever there is a discrepancy between what the organism is expecting, and what really exists. It affects the biochemistry of the brain, mobilizes resources, affects performance, and endocrine, vegetative, and immune systems. Initial positive feedback and feed-forward mechanisms are gradually changed by homeostatic mechanisms. Slower reactive hormones such as cortisol seem to dampen the initial response. The time course may depend on psychological mechanisms. Subjects with efficient coping show the fast- and short-lasting catecholamine response, while subjects with high defense mechanisms (related to stimulus expectancies) may show more signs of prolonged activation. Non-coping individuals show a sustained general activation which may develop into somatic disease or illness.