Stress physiology, as applied to the average vertebrate, is the study of the de-fenses mobilized by the body in response to physical challenges-being chased by a predator when injured or sprinting after a meal when starving. In contrast, humans have the cognitive sophistication to activate the identical stress response habitually for purely psychological or social reasons-worries about mortgages, relationships, and the thinning ozone layer. Although activation of the stress response is critical for surviving pursuit by a lion, it is pathogenic when mobilized chronically, and many Westernized diseases are caused or worsened by overactive stress responses. How do psychological and social factors-such as unhappiness-activate the stress response? Broadly, for the same physical stressor, an organism is more likely to have a stress response if it lacks outlets for frustration, social support, control, or predictability. Social status also modulates the stress response. Many studies of social primates suggest that low-ranking individuals have chronically activated stress responses and are more prone toward stress-related diseases. This likely refl ects their being subject to higher rates of both physical and psychological stressors than are dominant individuals. However, in primates, social subordination is not always associated with such maladaptive physiology; it is not just rank that infl uences physiology but also the sort of society in which the rank occurs, as well as that individual’s experience of rank and society.